Headphoneus Supremus
Reference all the way
Pros: Superb balance. Fantastic details. Looks beautiful. Comfortable enough
Cons: Not for bass lovers
Ollo Audio, Not many are aware of this brand, they are not a huge brand with a lot of wave behind them but they are something. Originating from the beautiful country of Slovenia, surrounded by the Alps, Ollo Audio is the home to one of the best headphones for under $500. Ollo have been in the market for a few years but have not caught a lot of attention, they do not ride the hype waves. Their headphones are not aimed towards the mass market with a thumpy or fun oriented sound. All of the Ollo audio headphones are aimed at studio use and for more critical listening. Ollo as a brand do not make a lot of models in a variety of price ranges, they only have 3 models and they pay a lot of attention to quality control and details. Their headphones look aesthetically classy with a retro kind of Vibe. Interestingly one can buy whatever spare parts they desire from their website and.. The headphones themselves have 5 years of warreanty!! 5, that is one year, five times.

Ollo has only one Headphone on sale, the S4X reference headphones, which I am reviewing here. It does not have any options for the wood but hey, the stock wood looks classy too. Priced at 399 Euros these headphones face some competition from another growing brand, Austrian Audio HI-X55 and other headphones in its price range like Hifiman Sundara, HD600 and a few more.

Get one for yourself from here:


All other models like S4 and S4R can be bought in B-stock options with a sizeable discount.

P.S. I reached out to OLLO Audio for the S4X review and they responded kindly.



There isn't a lot of stuff inside the box, a cable, a 3.5mm to 6.5mm adapter, a carry pouch and a few documents rounds up the list of accessories.



The Ollo audio ships with a 200cm/78inch long basic looking cable, with 3.5mm TRS and dual 2.5mm TRS terminations, nothing fancy here. It has external braiding from the 3.5mm jack till the Y splitter, it protects the wire from accidental damage and wear too. But due to this braiding the cable ends up being slightly on the stiffer side. The splitter is small and barely has any weight to it. The cable above the splitter is kind of flashy with red rubbery coating over the wire which gives it a bouncy feel but doesn't result into any microphonics thankfully.

Even when the cable has L/R markings, the ear cups don't have anything like that. Thanks to frequency matched drivers plug any on any side and it is good to go.



I already have a very similar looking headphone in my inventory with a slightly different type of wood. Aesthetically the S4X looks more premium with a slightly darker but natural and sober looking wood. The drivers are held in place with two screws on both sides of the cups. The open style back plates give S4X a retro classic vibe. The stainless steel head bend feels a lot sturdier and stable. The spring loaded leather headband does not have any cushioning but due to the wider design feels very comfortable. The headband is held in place with simple screw which can be changed without any problem.

Ear pads have a hybrid design with velour on the face and artificial leather on the sides. With an outer diameter of 90mm and inner diameter of 55mm the Level of comfort is fairly good for a few hours but have to be adjusted after that. I would have liked a slightly bigger pad. The size is somewhere in between on and over ear sizes.

In their words:


"Timelessness of stainless steel coupled with luxury nautical leather and velour for maximum comfort. Paired with a smell of a fresh sustainably sourced wood."




"Custom designed velour earpads fit around your head snuggly while the self-adjusting strap provides for a perfect fit. Combined with a balanced sound, it will enable you to work on headphones for hours."

Ollo S4X's refreshingly simple yet intriguing deign has been awarded with German Design council recently.



Since i have bought a few LG devices with dedicated DACs they have become my first source of music. The S4X sounds kind of edgy with mobile phones, be it the Vivo V19 or LG G7. The S4X demands a bit of power, even when the rated impedance is at just 32ohm it is not the easiest headphone in the market and doesn't comply excellently with weaker sources. Not that the S4X doesn't get loud, it gets fairly loud with the mobile devices but the sound is not at its prime, it kind of feels being dragged, notes are not full enough and doesn't have the best tonality either.

Driving it out of dedicated DAC/Amps is much more desirable. It is an open back headphone after all and is not meant of outdoor use. Driving out of the Burson Audio Playmate is a pleasant affair. It starts to behave well with excellent details and control over notes. It does not feel like being forcefully pushed any more.

Driving the S4X out of a dedicated dap like Shanling M6 yields excellent resolution and details. It starts sounding a bit more mature and perfectly reflects the true characters of the DAP without breaking a sweat.


Ollo uses neodymium drivers which has 2 layered coils with a 25u PET membrane and are precisely matched.

First statement about the S4X's sound attributes on its official product page is "Natural and Brutally honest" The burning question is.. How does this beauty sound? First thing first, these are reference type headphones and do not have a lot of lower end grunt to them (Maybe the closed back plates can change the sound signature but I can't confirm that). Mid range is agile and dynamic with plenty of details, same goes for the treble region.

S4X's sound signature is slightly on the brighter and crispier side with balanced notes presentation.

Ollo audio dont compromise when it comes to sound quality. In their words:


OLLO headphones are individually tested and measured and come with the frequency response measurements chart in the package."

"Designed and measured using IEC 60318-1 standard."



Even when the graph shows a bit of an elevated lower end the S4X cannot be labeled as a bassy headphone by a long shot. The precisely tuned dynamic driver has a controlled and accurate lower end with nice sub-bass extension with required amount of rumble to it. It does not move a lot of air though. Doesn't matter how it is driven, due to the reference tuning and open back design the sub-bass region doesn't go boom boom. It does not vanish into thin air but it is just a few DB more than what the ER-4P delivers. The Mid-bass is a bit more voluminous with much better body and slam. It still doesn't qualify as a "wholesome" but has good amount of body and slam which keeps the party going. The punch is not hard and the rumble is not comparable to other dynamic driver headphones like Sennheiser HD6 mix or even the HD598/599 but it excels with decay speed while maintaining very good amount of texture. The S4X delivers a cleaner and faster response with a slight bit or precipitation. The level of accuracy and precision offered by it is simply class leading.

Should you look at the S4X for its bass thump? Not really, it is tight and precise without much excitement, its has a more analytical tuning to it.

P.S. Equalizing helps (if needed) without messing around. It responds to EQs without any problem.


The Frequency graph shows a slight bit of decrease in volume for the entire mid range but thanks to the open back design the S4X's bass doesn't feel heavy, letting the mid range express itself without any restriction. The transition from upper bass to lower mid range delivers plenty of details without losing any energy. The mid range has plenty of micro details with one of the most accurate and precise notes presentation. It manages to find a sweet spot which balances everything while delivering plenty of details and transparency with the juiciness intact.

The vocal region sounds vivid with excellent accuracy while maintaining appropriate amount of fullness. Both male and female vocals have class leading amount of transparency, definition and resolution with a lot of texture. The tonality is accurate and very organic (the wooden housing might be playing a part in this) making the vocals very enjoyable. Male vocals sound throaty while female vocals are aptly sharp with right amount of energy. Instruments have nice shine to them with perfect depth and bite. A bit more might have pushed it into uncomfortable territory. It impresses me by not favoring any part of the spectrum. There isn't a single instrument that feels dull or over energized. The upper mid is in line with the rest of the spectrum and doesn't have any anomalies while maintaining equally good amount of energy and transparency.

Ollo S4X separates itself from the crowd with its technicality, the way it manages to deliver micro details of background instruments without tempering with the foreground is praise worthy and the contrasty nature makes the whole experience enjoyable.


The S4X has precise notes presentation across the spectrum and it holds true for the treble section too. It does not have the most liveliest or sparkly treble region. What it has is an aptly energetic presentation filled with good amount of details and transparency, transparency which is not the best in its price range. Some headphones like the HI-X55 exhibit better transparency and clarity with deeper and more energetic notes but the S4X doesn't belong to that bunch, the open back design helps a lot by dampening most of the shine and sparkle. It delivers the good amount of energy and spark staying clear of any uncomfortable notes. It has very good amount of air between instruments. Thanks to its slightly bright nature it resolves very good cleanliness.

The transition phase from upper mid range to lower treble region is excellent with transparency and clarity. The extension is very good but it starts to lose energy as it goes deeper into the spectrum. Level of details retrieval is very good, it doesn't miss out on much while keeping the timber close to natural. Cymbals and pianos have good transparency with an organic feel to it. Needless to say that separation and layering is up to the mark with good amount of air and space between instruments. The treble stage is well spread and has good density to it. The bigger stage helps a lot with instrument placements and density.

Is this the best treble? Short answer is no, but If you are coming from a Planar headphone, yes this is better.


In general an open back headphone has to have a wider, deeper and open field of sound compared to most of the closed back headphones and that holds true here. The open back design grants a lot of dynamism and sonicalities. Depending on the track the stage can stretch, if the stage feels a bit narrow near the head with one track, with the right track it feels wider. Overall the stage is well extended in every direction, one might complain that it does not have the height some other headphones like the HD598 but it is much more deeper, wider sonically superior than that. It can place cues much further and precisely. All of the vocals are placed inside the head and most of the instruments are placed outside.



VS Fischer M12s:

The M12s used to be my favorite headphone under 500 euros. It too has a wooden enclosure with a open back design. It has only one in and cannot be sued with balanced cables. The 5mm larger cups are a bit more comfortable on the head as it doesn't press on my ears.

Sound wise it has a slightly more rumble with similar body, the decay is slightly slower. Unlike the S4X the M12s is a bit less colored and more accurate. The S4X has brightness to it across the spectrum and feels a bit colored where the M12s is much more neutral. M12's notes feels a bit sharper with finishing and less forgiving. It too is a lot precise and accurate, what it does better is the tingling feel. It has excellent contrast and the shine of the instruments are more perceivable. Where the S4X doesn't have anything sharp, the M12s doesn't try to hide them which adds a bit of extra character.

Sonically it is equally good but has overall taller and less deep stage.

Which is better.. If you like tingling instruments the M12s will rock, if you like a bit more overall attacking sound with everything sounding eager and forward the S4X is excellent.

VS Austrian Audio HI-X55:

How dare I compare a closed back headphone with open back? I can, because both are a lot similar than different. Both are from Europe and have a balanced sound with excellent build quality. The X55 has slightly bigger and thinner ear pads which are more comfortable on the ears.

Sound wise, the X55 has bigger lower end and feel a bit more complete, it still is not bassy but has better slam and rumble. The sub-bass have a bit more heft to it. Mid range is similar to the M12s, it favors vocals a bit while other instruments take a step back, managing to deliver a slightly less aggressive sound. Timber is equally natural but the S4X is a bit more colored thanks to its slight brightness. Treble is a bit more extended with much better energy till the end of the spectrum, the S4X sounds dull in front of the X55. Layering and separation is better than the S4X with treble notes. The x55 has much deeper notes, the S4X feels smaller here, when I A-Bed both headphones the difference with the treble energy and notes depth took me by surprise.

Stage wise, the X55 is much wider and taller than the S4X, one might doubt that this closed back HP might not sound open and grand but the X55 manages to wow me. The depth is slightly lacking but still then it is around 20% bigger than the open back S4X which is surprising.

Which one to pick.. Both are excellent headphones but if you don't want to leak anything and want a bit more flexibility then the X55 is the one to go for as it can deliver a bit more rumble and can take heavy metal without sounding tiring. If the need is for an open back headphone with a reference sound the S4X nails it.


So.. This is the end of review of an excellent sounding headphone which looks gorgeous too. It is a reference headphone which will appeal to those who want details over anything else. It has everything balanced but there is some roll off at both the extreme ends. It does not pump much bass and is kind of small too but that does not stop me from recommending these over any other headphones in its price range. The open back design gives it a big stage and added sonicality. The dynamic feel is intriguing.

S4X is an excellent headphone, but if you want a bit of lower end rumble and don't want to compromise with treble region, even when the x55 is a closed headphone, you can look into that.

Improvements I would like to see:

1. Slightly bigger cups. This 90mm is neither on-ear nor over-ear and feels a bit less comfortable.

2. The mid range can be a bit less forward and the treble can be a bit more forward.

3. A bit more sub-bass will make it more enjoyable.

Before you decide to go..

Make yourself busy with this interesting bit of comparison graphs from Ollo:



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Reviewer at Twister6
Ollo Audio S4X - Reference Headphone
Pros: Boutique brand design and build quality
- Balanced signature with natural reference tuning
- Sub-bass extension
- Mid-bass presence
- Midrange tonality and presentation
- Natural upper-treble extension
- Detail retrieval and easy to drive dynamic driver open-back design
- Good reference tool for music industry professionals as well as audiophile enthusiasts who like and prefer their headphones with natural tonality and timbre of instruments.
Cons: Stock tuning is quite good but Nitpicking - Ear pads are a bit small for over-ear design and bigger ears
- Stainless-steel headband rings loudly when you tap it
- A bit more sub-bass and lesser mid-bass will bring it more in line with Harman Target and enable better neutrality as well as sub-bass rumble
- Perceivable lower-treble dip in 6-9kHz stopping it from absolute neutrality in the range.

About Ollo Audio.

Ollo Audio is a small and young boutique headphone brand from Slovenia. It was started by Rok Gulic’ and Mitja Sajovic. Rok, the CEO of the company is an educated audio engineer and his passion of wanting to make headphones started when he was mixing his band’s record and was tired of working late nights on studio monitors in his home studio, with it being highly disturbing for his family sleeping in the next bedroom. So, like every guy in a home studio with family, he tried mixing on headphones but to no avail as mixing on headphones can be very difficult since you can’t feel the bass like you do with full range monitors with a sub-woofer.

One thing lead to another and they finally started developing proper headphones for professionals, tuning them as close to reference as they could through their research. They have 2 models currently,
  • S4 - Closed back recording and monitoring headphone
  • S4X (€399) - Open back reference grade headphones for mixing and mastering
Rok and Ollo run a very nice blog on their website called OLLO LIFE as well as a YouTube channel that I highly encourage everyone to check out if you want to know more about them. They and especially Rok have a knack for creating nice interesting and entertaining videos which are full of knowledge and behind the scenes insight into the world of Ollo Audio.

Links - Ollo Audio Website | Ollo Life Blog | Ollo Audio YouTube Channel

Ollo S4X Solo 2

I would like to thank Ollo Audio for providing the S4X. Even though the headphones were provided free of charge, I ended up paying almost 100% import duty to import the headphones because of a declaration error. So, it feels like I bought this pair at full price, even though the money went to the wrong people and not Ollo Audio (facepalm). I am not affiliated with the company or any of its sellers and write this review with an unbiased opinion regardless of how the review turns out.

Alert! - This is going to be a long and detailed review of S4X with insight into the world of reference. So, brace yourselves and definitely read the review chronologically.

About S4X.

S4X are Ollo’s Open-Back Reference headphones tuned to be ‘neutral and brutally honest', designed and measured using a GRAS 60318-1 setup.

Technical Specifications.

  • Frequency Response – 20-22kHz
  • Impedance – 32Ω
  • Weight – 350g
HANDPICKED MATCHED SPEAKERS - Ollo measure all speakers in every series and match them together to achieve minimal possible tolerance between left and right channel.
  • Dynamic driver
  • Neodymium magnets
  • 2 layered Coil
  • PET membrane – 25u
NATURAL PREMIUM MATERIALS – They use sustainably sourced woods to make the cups (walnut in my case), stainless steel for headband and luxury nautical leather and velour for their ear pads for maximum comfort.

HIGH PRECISION WOVEN MESH | POWERED BY SATTI - Designed and manufactured to improve acoustics and protect sensitive components.
  • Smooth monofilament fiber
  • Efficient protection from dust, metal particles and liquids
  • Strict airflow control
CUSTOM DESIGNED EARPADS – They are hybrid pleather and velour ear pads with memory foam.
  • Outer diameter: 90 mm
  • Inner diameter: 55mm
  • Hybrid velvet
  • Pleather
  • Memory foam
INDUSTRY LEADING 5-YEAR WARRANTY - The 5 year warranty that Ollo Audio offers on their headphones is class leading and goes on to show how confident Ollo are in their product. 5 worry free years in the asking price of €399 is amazing in my opinion.

Included in the box.

  • S4X Headphone
  • Leather carry pouch
  • 5mm connector cable with 3.5mm jack
  • 5mm to ¼” adapter
  • Graph measurement
  • Warranty card
  • Manual

Ollo S4X Box.jpegOllo S4X Manuals.jpeg

Build Quality.

S4X are designed with sustainability and upgradability in mind. You can see that S4X oozes of boutique brand build quality right away. The walnut wood cups look premium and are cut, sanded and polished very well. They only use a tiny bit of wood sealer on them and aren’t lacquered because you can feel the wood grains quite nicely. The downside to not lacquering the cups is that the wood can chip and dent if one drops the headphones. The outer metal grill though simplistic looks very nice too. The ear pads are hybrids made of pleather and velour, which feel and look quite good for a headphone in this segment. The leather tensioner headband isn’t padded but is very comfortable on the head and after a while you don’t even feel it pressing on the top of your head.

The stainless-steel headband is mounted on the ear cups with two big screws; both feel quite sturdy and built to last. The headband adds a bit of raw industrial design element to the headphone but sadly, rings loudly when you tap it. Now, this isn’t a problem when you’re listening to music but can be quite off-putting otherwise. I think this is one of the things Ollo should definitely think about fixing in the future.

Ollo S4X Build Quality.jpg

Cable – Stock cable has L&R 2.5mm TRS terminations for the ear cups and a 3.5mm TRS jack. It has a cloth sheath which splits to red rubbery L&R wires. The choice of red has a very OnePlus feel to it since my DASH charger’s USB cable is exactly the same colour. I do wish they had paired a better UP-OCC cable with the S4X but the stock cable is no slouch. It is better than stock cables of Hifiman and Sennheiser headphones in this price segment, however I would’ve loved it if they would've offered a 4.4mm balanced cable as stock or at least as an option.

Ollo S4X Cable

Case - S4X come with a soft leather carry pouch which fits the headphones and cable easily though it would've been great if they would've included a hard case to carry and protect the beautiful wood finish from chipping or accidentally drops while travelling or carrying it around.

Fit and Comfort.

S4X are designed to be worn over the ear but the ear pads merely manage to encompass my ears because of the small 55mm inner diameter of the ear pads. That said, even though the hybrid pads and clamping force is rather comfortable and I've been using the S4X for production work for several hours without much discomfort, my ears sometimes do start feeling the inner circumference of the ear pads and it does get a bit irritating at times. The headband tensioner on the other hand does not exert a lot of force on the top of my head and I stop feeling it pressing on my head after a while, not because my head goes numb, but because it is well designed. Haha. I'm not the biggest fan of separate headband to attach the ear cups and a tensioner headband for fit, but this does work and enables for an easy and mostly comfortable experience. Even though I can use S4X for hours, I would love to see Ollo workout an even better fit mechanism than the current one. Something along the lines of HifiMan Sundara maybe?

Ollo S4X Solo Earpads

Upgradability and serviceability at minimal cost.

Now, this is one thing that I think differentiates Ollo Audio from all the other headphone manufacturers around the world. Here is what Ollo offer as a brand,
  • The whole headphone is designed in a way that you can open it up and service it at the comfort of your home within a couple of minutes. All the parts you'd need can be ordered from Ollo's Spare Parts Store individually. The parts are also very reasonably priced with a pair of ear pads being €25 and a pair of matched drivers being €40. That's amazing in my opinion! They can however repair the headphone for you too if you would like to send them in and have them do it instead.
  • Ollo Headphones are upgradable to new models at a fraction of the cost of the new model. So, if Ollo release a new model and your model is eligible for the upgrade, you can just send in your headphones and they'll upgrade your headphone with the modifications and ship it back to you. For example, the upgrade from S4 and S4R to S4X is €149 which includes DHL pickup and return and is just €109 if you opt for standard post shipping.

Here's a video of Rok Gulic' explaining the same -

Measurements and tuning as per 60318-1 standard vs what others use.

Ollo Audio use a G.R.A.S 45CC using IEC 60318-1 ear simulator with a RA0039 coupler and Dewesoft AD conversion Sirius system. Here is their measurements page where they've measured the S4X along with a ton of other headphones - Ollo Audio Measurements

Here is the measurement of my particular S4X unit.

Ollo FR graph

Now some of you who are aware and familiar with headphone measurements and RAW graphs might be wondering why S4X or none of the headphones on Ollo’s measurements page look like the RAW graphs you’re used to seeing with ear/pinna gain. Others might be wondering what these measurement standards are, how and why they differ and the rest of you probably don’t care for any of this. If you’re of the last kind, I envy you for the peace of mind you have in your life not fussing about any of this. Jokes apart, for those who do, let me quickly give you some insight. I’ll try to keep it short and simple without going too deep or digging a hole for myself.

The current commonly used standard is 60318-4 which is also known as IEC 60711 or simply IEC 711. Now technically, there are no hard and fast ‘industry standards’ that everyone adheres to or follows but 60318-4 standard is one of the more commonly adopted and accurate ways of measuring IEMs and headphones, except until the recent development of Brüel & Kjær’s Type 5128 which measures even more accurately but isn't as widely adopted or used yet.

Oh, before that let me make you aware of a couple of things you might need to know first.
  • ERP = Ear Reference Point
  • DRP = Drum Reference Point
  • Ear Gain (head/pinna gain) = Gain added by the human ear as the sound travels through the outer ear, into the canal and to the ear drum.
If you refer to the Figure 1, you can see that ERP has a microphone measuring the headphone outside the ear canal whereas DRP has the microphone measuring at the ear drum. Since ERP measures the headphone outside the canal, it shows measurements without ear/pinna gain. On the other hand, since DRP measures the sound at the ear drums where the sound has travelled through the canal and has been affected by the ear anatomy's gain factor, it shows measurements with ear/pinna gain.

Figure 1

Figure 1.

The reason why none of the headphones on Ollo’s measurements page have ear/pinna gain is because the RA0039 coupler that Ollo uses is an ERP coupler which measures the headphones outside the ear canal.

Now, you might be wondering why Ollo Audio use the 60318-1standard/RA0039 coupler which is an ERP coupler and not the 60318-4standard/RA0045 coupler or one of the newer RA0401/RA0402 couplers which are all DRP couplers. I had the same questions and emailed Rok for his take. Here is a summary of our exchange -

After having long discussions with Bang and Olufsen engineers, then GRAS and Dewesoft about the best option as per their requirements, they decided on the 60318-1 setup to measure their headphones mainly because DJs and artists are a major of chunk of their customers and they like to see flat line graphs without ear/pinna gain. Unless people are familiar with and know how to read RAW graphs with ear/pinna gain, it can be difficult to comprehend - which is true. Flatline graphs helps make stuff easier for the layman but are not the best for people like us who like 60318-4 style RAW graphs and want to focus on the details that are available in those 60318-4 style RAW graphs.

Another thing is that Ollo Audio don't want to use compensation curves either. So, a RA0039 coupler, which is an ERP coupler, is the best way to measure when you don't want ear/pinna gain in your graphs. Also, Rok says that 60318-1 is way easier for them to handle on the production floor, is more consistent with quick back to back measurements and doesn't have as big a learning curve. They need ease of use and consistency because they measure every headphone and put its individual graph printout in the box with each headphone, which I think is very cool for a headphone that sells for just €399.

My personal take - I personally think the best way to do flat line measurements ideally is to use the industry standard 60318-4 (711) DRP setup or even better, the new Type 5128 (more accurate but also more expensive), to measure a RAW graph and then compensate with popular target curves like Diffuse Field or Harman Target and specifically specify the compensation publicly. This way you can offer both graphs for people to check out. But I can imagine it taking more time, having a bigger learning curve to keep the measurements consistent one after the other at the production floor and confusing people who don’t know how to read 60318-4 style RAW graphs.

Another observation of Ollo’s headphone measurements I had was that their 60318-1 graphs of some headphones on their measurements page weren’t consistent with 60318-4 (711) graphs or my experience of having tested them in real life, particularly in the upper-midrange and lower treble. Now, that could be because of a number of reasons, with setup and software differences being the primary one and driver differences between samples and all being secondary. Rok too switched to REW software instead of Dewesoft after me having pointed this out to him and noticed that S4X with REW measured quite differently from Dewesoft even with the same measuring setup.

This is why you see a lot of people around stressing on having an industry standard way of measuring headphones so that everyone can cross-refer measurements between different brands, companies and hobbyists easily and without confusion.

Recently, Brüel & Kjær have come out with a newer and more accurate measurement standard called Type 5128 but it hasn’t been adopted by the masses yet. Also, it is quite expensive and many enthusiasts wouldn't want to spend so much money for a hobby. But frankly, we do need an industry standard for consistency and cross-referencing and I can only hope that one day we can have an industry standard measurement setup that doesn’t cost an arm and leg so that every brand and enthusiast can own one. #wishfulthinking

Nevertheless, Ollo Audio’s method works well for their intended purpose and their target audience, and that graph does translate mostly in case of their own headphones S4X. Also, as you get to know more and more about Ollo Audio as a brand and team, you'll see how knowledgeable they are and how aware they are of what they are doing or wanting to do. It definitely shows in the product they've made as well as the enthusiasm, ideas and insight they share in their YouTube videos and blog.

Sound Section.

Prelude - Concept of Reference, Studio monitors and Flat Frequency Response Calibration.

As most of you may know, musicians, producers and audio engineers like to use studio monitors with a frequency response as close to flat in their studios. The main problem one faces when using studio monitors in a room is generally the problematic acoustics of the room heavily influencing the studio monitor's sound. So, professional studios measure their rooms, design, construct and acoustically treat their monitoring rooms to sound as flat and neutral as possible. There are a lot of techniques and solutions to solve room acoustic problems, like basic construction of the room, use of absorbers, diffusors, bass traps, etc. But even after all that, studio monitors might still not sound perfectly flat, which can be because of the studio monitor’s own frequency response or leftover influence of the room even after all the treatment.

That’s where calibration softwares like Sonarworks Reference 4, IK Multimedia ARC System 3 and DIRAC Live come in. They come with measurement microphones and software which helps you measure your studio monitors at various points in the room and then show you the frequency response of how your studio monitors sound in your room. They then enable you to compensate the measured FR of your studio monitors for proper flat frequency response. Check them out if you want to know more about them.

I personally use Sonarworks Reference 4 Studio Edition in my production room to calibrate my studio monitors for absolute flat response. The Headphone Edition of the software has calibration profiles for a ton of popular headphones and Sonarworks have their own target curve that they use for calibration, which is actually quite close to the Harman Target but with a lower bass shelf and a tiny bit more upper treble. So, loosely their idea of flat response in headphones is basically a slightly modified Harman Target curve, which is also their idea of how flat studio monitors in a treated room are perceived and sound to our human ears. Reference 4 works really well for my use and my flat response compensated studio monitors sound better than they ever did before in my room. I achieve my mixes which translate even better, way quicker now.

Diffuse Field (DF) and Harman Target (HT) are two popular reference target curves used by manufacturers around the world for tuning or as compensation in their graph measurements. I personally quite like both for proper reference tuning; Harman Target a bit more for the bass shelf, which makes mixing bass easier for me personally. So, if I’m looking for a proper ‘reference class headphone', I’d want it tuned close to or in the ballpark of one of these curves to properly translate the music in true reference style.

Ollo S4X & UAD

Ollo Audio S4X as a reference headphone.

Now coming back to the S4X, it isn't tuned to follow any of the theoretical target curves like Diffuse Field or Harman Target per se, but is designed and tuned to measure as close to a flat line as possible on their G.R.A.S. 60318-1 setup, which as I explained earlier, does not show the ear/pinna gain peak because of it using an ERP coupler instead of a DRP coupler. So loosely, a headphone measuring close to flat on an ERP setup will be closer to the Diffuse Field than the Harman Target (which has a 7-8dB bass shelf at 100Hz).

S4X does measure very close to a flat line but with a minor low-shelf of around 4dBs around 600Hz and one prominent lower treble dip in the 6-9kHz range. I don't know why they have a dip at 3kHz in their graphs because S4X in fact has very good ear/pinna gain, quite in line with the Harman Target style upper-mids peak and gain.

Sound Analysis - By now you all might eagerly be wanting to know if S4X really does sound ‘brutally honest and neutral’ as Ollo Audio markets it to be, or if it is ‘musical’ enough to enjoy music, or if it really can work as a ‘true reference’ grade headphone or not. The short answer is – Yes, it does and can, but it can do much more than just work as a reference headphone for professionals.

S4X, I would say is a ‘musical reference’, neutral and an honest headphone that first and foremost sounds tonally correct; has good instrument tonality, timbre and definition, and is mostly linear throughout its frequency response with good end to end extension.

Contrary to the belief that I see quite a lot on the internet, overly bright and thin sounding headphones aren’t what true monitoring or reference headphones are supposed to sound like if you really want them to mimic or work as a substitute for good flat response studio monitors. I mean, you can use headphones like Beyerdynamic DT770 Pro (which are quite bright with peaky treble and not the best upper midrange) to monitor if you really have to but mixing or cross referencing on such headphones can be a pain, especially if you can’t get used to its overly bright nature. Prominent treble boosting gives an illusion of more clarity, resolution and details at the cost of coming off as treble peaky, fatiguing and even intolerable for some. But frankly, that shouldn't be the way to go about tuning a proper reference headphone.

Ideally, an honest reference headphone should in fact bring out all the details and clarity but by maintaining good balance and without over-emphasising any particular frequency range, and particularly maintaining the correct tonality and timbre of instruments. This is what the S4X basically does. I personally want reference headphones to sound as close to or at least in the ballpark of my studio monitors and not something that sounds way off or different from proper reference sound. Hence, I pick up the S4X not only to use as a reference headphone for my production work but also when I want to listen to or cross-reference my favourite/new records because of its natural tonality, transparency, balance and musicality without over-emphasising any frequency band for the same.

Let's dive in deeper...

Ollo S4X & PRS

Bass – S4X's sub-bass is boosted by around 3dBs above flat which kinda puts it midway between DF-neutral and Harman Target-neutral. Mid-bass is a little more prominent than sub-bass and is around 3-4dBs above neutral. I quite like the overall bass presentation because it is tight, sharp and translates the bass in songs with good neutral presence and sub-bass extension without any of them sounding too boosted or prominent. The bass character and timbre is very clean and clear which enables you to focus on the characteristic traits of different types of bass models and tones played in different songs nicely. Because of it being an open-back, the sub-bass rumble in Hans Zimmer's 'Why so Serious' at 3:27 is heard more cleanly than rumbly. I personally would've loved if the sub-bass was boosted by at least 2-3dB more, more in line with the Harman Target curve because S4X would've then been able to portray sub-bass with a bit more rumble and that would've been quite cool for an open back headphone. But it does deserve praise for the stock tuning because a lot of other open-back headphones in this range actually have a sub-bass roll-off instead. The mid-bass dominant bass playing in Dua Lipa's 'Pretty Please' comes across very nicely, with good attack, definition and punch. It is never too prominent or overpowering but is technically very slightly north of neutral than how I hear it on my monitors or DF/HT-neutral headphones/IEMs. But overall, because of S4X's relatively neutral bass character, I actually found it quite easy to use when it came to balancing the bass in the mix while doing my rough mixes. I was never inclined to over-mix bass in my songs and bass in my rough mixes was translating quite well to other speakers without me feeling the need for a Sonarworks compensation for the S4X in order to do the same.

Mids – Mids are the name of the game here. S4X is very natural sounding, majorly thanks to how well done the midrange is. Midrange is revealing and very well layered. It has very good separation of instruments and brings out micro-details pretty well. Lower midrange is mostly linear but has very slight emphasis above neutral in the 250-600Hz (around 2dB above neutral) which adds a tiny bit of body and warmth. So, deeper vocals and snare body sound very good, which also enables you to focus and work on them a bit more precisely when working on your mixes.

But upper-midrange definitely takes the MVP title. Upper-mids have a nice accurate revealing forward presentation with good ear/pinna gain at around 3kHz, which I hear to be almost in line with Harman Target curve if I'm hearing it right, even though Ollo's own graph shows a dip here. S4X's upper-mids are revealing but very natural, enabling good strong definition, tonality and timbre of instruments. It is very well layered and micro-detailed without coming off as dry or too clinical. Good big rock guitars and acoustic guitars have the right tone, presence and definition. Vocal are positioned exactly how they are intended to be heard in the mix. S4X does not highlight vocals or brings them unnecessarily forward, just like what a reference headphone is supposed to do. Frankly, my own preferences in the ear/pinna gain is 2dBs or so lesser than what the Harman Target or S4X has. So, if S4X had a tiny bit less ear/pinna gain, I would've been even more happy personally but the stock gain is nowhere a downer by any means.

Treble – S4X's treble can be classified as clear, detailed, layered yet smooth without any harshness or sibilance. It has good resolution and detail retrieval without being overly bright. There is a perceivable dip in 6-9kHz of lower treble which is responsible for keeping sibilance and harshness in check but can take out absolutely neutral representation of cymbals and hi-hats and the harmonic content of other instruments in this range. But to be honest, it's not missing as prominently as I expect it to in songs, though fixing this dip will definitely make S4X's treble response even better. It has a nice natural upper treble extension which I feel is quite in line with Harman Target's upper treble response. This along with easier lower treble allows for a comfortable listen without fatiguing the ears as much over time. My particular S4X unit's graph shows a minor dip around 5kHz too but I'm not really hearing it. Maybe it's so minor that it isn't as perceivable or maybe it's an artefact. I cross-checked with S4X's graph on Ollo's website and that one didn't have this 5kHz dip either. So, I guess we can just ignore this. With a test tone test, I could clearly hear the upper treble extending till 18.8kHz. So, either the headphone rolls off past that or my hearing does. I think it could very well be the latter. Haha. Last time I checked, I could hear till 19.2kH but nevertheless, S4X does most of the treble response in line with my preferences and I don't have any major complaints besides that dip in lower treble, which if corrected would definitely perfect the S4X even further.


S4X is rated 32Ω but sensitivity figures are not listed on the website. It is fairly easy to drive through smartphones, laptops and SE output of DAPs, though needs a tiny bit more juice than other 30-40Ω rated closed-back headphones like AKG K371 (32Ω) and Audio Technica M50X (38Ω).

Soundstage and Imaging.

S4X being an open-back headphone, has soundstage a bit wider than most average closed back headphones and even open-backs like HD6XX but isn't exceptionally wide as such. It is in fact wide enough, with good depth, to enable an out of head experience mostly but generally follows the mix of the song precisely. Imaging is very well done and you can pinpoint every instrument precisely, which adds on to its capabilities as a reference headphone. Again, in general brighter treble helps give an illusion of an airier, more open and wider soundstage, but the real skill is tuning a headphone to do the same without overly boosting the treble. This is where I think S4X does good, staying true to its reference nature and yet giving a decently good out of head experience for the most part for its price.


Comparisons include some cool competing reference tuned headphones around or below S4X's asking price of €400, regardless of the type of headphone or driver technology.

S4X vs AKG K371 ($149) - Price wise, this is not a fair competition but K371 is AKG's relatively new headphone that is tuned to the Harman Target curve plus I heard Rok himself praising its tuning and value in one of the videos, so I thought it'll be a fun comparison to see if it really competes with S4X or not. K371 is a closed-back headphone with a 50mm dynamic driver with titanium coated diaphragms. So, right off the bat, K371 can be used for monitoring purposes but S4X cannot if you are recording and monitoring in the same room. Fit wise, K371 is lighter and covers the whole ear better but the fit isn't as snug as S4X. S4X does have a tighter clamping force than K371 but nevertheless remains comfortable for hours. S4X headband tensioner exerts a bit more force than K371's too but overall, S4X gives me a more reassuring fit than K371. Build quality wise, S4X definitely has the upper hand with its boutique and premium design and build quality.

Sound wise, even though K371 traces the Harman Target almost perfectly, S4X does have the upper hand in a lot of things. K371 has around 4dBs more sub-bass and more neutral mid-bass presentation compared to S4X. So, you can feel the sub-bass rumble and punch better in K371 but mid-bass shines more in S4X. S4X does have the upper hand in bass precision, sharpness and speed. K371 does lower-midrange more linearly neutral throughout whereas S4X has a bit more fullness in the 250-600Hz range. Both have similar upper-midrange presentation but S4X has better resolution, clarity and instrument definition. K371 does not have a lower treble dip in the 6-9kHz range like the S4X, so it does lower treble more evenly but S4X has better upper treble clarity, resolution and extension. S4X is also more open sounding owing to its open-back design and also has a bigger soundstage in comparison.

Ollo S4X vs K371

S4X vs HD6XX with fresh pads ($220) - HD650/HD6XX have been long time reference sweethearts for professionals as well as audiophiles. Both are open-back dynamic driver headphones and even though HD6XX has an impedance rating of 300Ω, it is fairly easy to drive through DAPs but is also known to scale better with good power. Even though HD6XX's build is not bad for its price, S4X's boutique design and build quality just seems more premium and attractive. Fit wise, HD6XX is quite comfortable because of large ear pads that go over the ear properly but the headband can have a stronger clamping force for people with bigger heads. On the other hand, S4X's auto headband tensioner mechanism and clamping force is more comfortable but the ear pads are smaller. S4X has a much better looking and feeling cable too compared to HD6XX.

S4X is easier to drive considering its lower impedance rating of 32Ω. Sound wise, running both through the 3.5mm SE output of Hiby R6 2020 in high gain, S4X has better sub-bass extension whereas HD6XX rolls of around 40Hz. S4X has a bit more mid-bass and lower midrange body in the 250-600Hz whereas HD6XX is more linear and neutral there. Both do upper-midrange very well but S4X has a bit more note weight and definition whereas HD6XX has slightly better clarity. When it comes to lower treble, HD6XX has a minor dip in the 6-10kHz region too but is not as dipped as the S4X and as a result, hi-hats and cymbals are a bit more prominent and better portrayed in the HD6XX than S4X. But as a result, HD6XX comes off brighter in lower treble relatively in back to back comparison too. HD6XX has good upper treble extension with fresh pads but is known to start rolling off as the pads start wearing out. For now with fresh pads, both have good upper treble extension though S4X does it a bit more consistently across the upper treble range. Soundstage wise, S4X is bigger and wider too. HD6XX presents one of the best values at its asking price of $220 but there are quite a few things that S4X does better, justifying its price jump.

Ollo S4X vs HD6XX

HifiMan Sundara pre-2020 revision (~$350) - Sundara is another popular headphone under €400 that is tuned close to reference. It is an open-back planar magnetic headphone which has excellent fit and finish. Sundara is very light and has a very comfortable fit. Its metal headband is click adjustable and the leather hand band is there for top head support and comfort only. This mechanism is the exact opposite of S4X's mechanism. Sundara fits me much better than S4X owing to bigger ear pads and a very nice and easy clamping force. Sundara too has an impedance rating of 32Ω but needs a bit more juice than S4X to shine. Sound wise, S4X has a relatively neutral tuning whereas I'd label Sundara as more neutral bright. Even though I'm not treble sensitive and love well done sparkly treble, Sundara has some treble spiciness which can come off quite bright with some songs. Going deeper, S4X has better sub-bass presentation and extension whereas Sundara rolls-off around 50-60Hz. S4X has a better and nicer mid-bass presentation with a nice subtle boost whereas Sundara's mid-bass is closer to neutral but a little held back in comparison. S4X does rest of the lower-midrange quite linearly and neutrally up till 2kHz whereas Sundara is very linear and neutral till 1kHz but then has a perceivable dip in the 1-2.5kHz range which takes away from absolute natural tonality and timbre of instruments, which S4X does better. Both have a forward upper-midrange presentation but S4X has slightly better tonality and timbre of instruments as well as better instrument note definition. Sundara is a bit more boosted in the 4-5kHz region which can come off as shouty at times with some songs. Sundara is bit more neutral and accurate in the lower treble. Both have good upper treble extension but Sundara is brighter in its upper treble presentation. Sundara, because of its brighter character portrays a wider soundstage but at the cost of it coming off as bright whereas S4X keeps things more organic and natural and enables a decently wide soundstage, which isn't as wide as Sundara but definitely not bad at all in isolation.

Ollo S4X vs Sundara


So there you have it! Ollo Audio S4X in my opinion is not only a well tuned Reference headphone but also oozes of very good boutique brand design, build and quality. Ollo Audio differentiate themselves from other headphone manufacturers by offering a headphone that is upgradable to future models for a fraction of the cost, completely serviceable at home with easily available and well-priced spare parts that can be purchased from their online store as well as having tight QC by carefully hand selecting matched driver pairs for proper channel balance and FR consistency. Sound wise, even though S4X doesn't quite follow any of the established target curves, it comes rather close to Harman Target style of reference tuning. The small tuning changes that take it slightly away from absolute reference neutrality are rather easy to overlook because the rest of it is so well done. In my opinion, if you like me, like and appreciate natural tonality of instruments and want your music to be played very honestly without much added sugar or spice, S4X fits in very well as a musical headphone too. I've been using the S4X for the past month and a half, not only for my production work but also to cross-refer and listen to my favourite records and to be honest, I'm definitely not complaining having paid almost full price of €399 for it in the form of import taxes. Plus the industry and class leading 5-year warranty goes on to show how confident Ollo are in their product. So after all this, if you think the S4X is for you, definitely give it a shot! I definitely highly recommend it, be it for a musician, artist or producer/engineer looking for a reference headphone to mix on or an audiophile looking for a reference headphone to enjoy his music collection with. Also, definitely subscribe to and keep Ollo Audio as a brand to watch out for as I see them going places with products like these. Happy listening!

Gear used for testing and review.

  • DAPs – Hiby R6 2020 | iBasso DX160
  • Audio Interfaces - Universal Audio Apollo | Focusrite Clarett 8PreX
  • Laptop – Apple Macbook Pro 15″
  • Phone – OnePlus 7 Pro

Reference Songs list.

  • Foo Fighters – The Pretender, Best of you, Everlong & Sonic Highway album
  • Coldplay – Paradise, Up in flames & Everglow + Everyday Life Album
  • Biffy Clyro - A Celebration of Endings & Ellipsis albums
  • Ed Sheeran – Thinking out loud, Bloodstream & Galway Girl
  • Dave Matthews Band – Come Tomorrow album
  • Dua Lipa – Future Nostalgia album
  • Chainsmokers – Somebody, Sickboy, This Feeling & Closer
  • John Mayer – Slow dancing in a burning room, Stop this Train, Say & A Face to Call Home
  • Gavin James – Always & Hearts on fire
  • Switchfoot – Meant to live & Dare you to move
  • Porcupine Tree – Sound of Muzak, Blackest Eyes & .3
  • Our Lady Peace – Do You Like It & Innocent
  • Linkin Park – Papercut, Somewhere I belong & Talking to myself
  • Maroon 5 – She will be loved, Payphone & Lost stars
  • Lifehouse – All in all & Come back down
  • Breaking Benjamin – Diary of Jane
  • Karnivool – Simple boy & Goliath
  • Dead Letter Circus – Real you
  • I Am Giant – Purple heart, City limits & Transmission
  • Muse – Panic station
  • James Bay – Hold back the river
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John Massaria
John Massaria
No speaker I know of or headphone that measures flat ever is ever engaging - I like the Ollo’s just because they are so bland - as I state in my review below - it’s not a headphone I would grab for pure musical get away but I would say it’s a a great tool to become a better listener and perhaps make some of the best mixes/mastering at the studio so others can loose themselves in the music latter on down stream. A must own for collectors too.


500+ Head-Fier
A step in the right direction
Pros: Build quality
Comfort (earpads and headband)
Technical success
5-year limited warranty
Easily accessible spare parts
Fairly priced spare parts
Easily replaceable nature of the headphone (can be replaced by the owner at home, doesn't require manufacturer's service)
Scientific explanation behind the product
30-day trial period that lets you try the product for yourself
Cons: Cable
Maybe the absence of a hard carrying case
The size of the earpads can be a problem to people with larger ears.

These are the reasons why I moved the rating down to 4.5 stars. Based on my findings, it's well-deserved 4.5 stars.

Who is Ollo Audio?

Originating from Slovenia, Ollo Audio is a fairly new company in the headphone space. Focusing specifically in the field of professional audio engineering use, Ollo Audio has been manufacturing headphones ever since Ron Gulič - the founder - decided to officially start making headphones under the company name… and let’s say that that was when his wife was disturbed while sleeping. Ollo was founded back in 2015, however, it wasn’t until 2016 that they would create their first ever headphone prototype. Ever since, Rok and his team would go on to release three different headphone models in total - the S4, S4R, and the S4X, but also the product that inspired Rok to start the company - Play2ME, a haptic subwoofer that acted as an extension for the headphones. Play2Me was a product that was aimed to fill in the gap that Rok felt headphones couldn’t - the chest feeling you get from the subwoofers. This product would later be discontinued, although there seems to be an upcoming MKII.

Either way, from what I can see, Ollo Audio seems to be mainly focusing on their headphone line-up, as though they released the claimed “flat out of the box” S4X model - a headphone that is aimed to be used as a reference in the professional space. Ollo Audio listened to the feedback from the existing customers, and they fixed some of the issues that were present in the previous two models. Since the company consists of a small team of just 6 people, I believe they have more time to focus and listen to the feedback. This gives me hope that they will release an even further improved headphone model that is based on the feedback from the S4X. Overall, they are on the right path of figuring it out as a new company to the market.

This is a lengthy review that is looking at the S4X from a very technical perspective. I purposely chose to approach the review with a technical approach instead of my standard music listening approach because of the unique nature of the S4X. A unique headphone deserves a unique approach - I don't think an audiophile approach is suitable or valuable for a headphone like this. The review was made with the target audience (pro audio industry) in mind.

I wrote the review over the course of two months - this also included research.

I formed the review in a way that would let the average reader get a better understanding of judging a headphone of this nature from a technical perspective. A lot of effort was put in to this review, and I hope that it opens a different perspective on headphones, especially "reference" headphones. It was not an easy job, and I hope it was worth it for the reader.

Thank you.

Unboxing experience

Raw and simple. No fancy presentation or anything like that, it’s quite straight to the point. You are greeted with rather organic packaging, environmentally friendly packaging that is composed of cardboard and paper.

Formal format of what’s inside (excluding papers, manuals, warranty, individual FR graph):
1x S4X headphone
1x Cable
1x Carrying pouch
1x 6.3 mm adapter


Build quality & design

It’s not so often that you see a wooden studio headphone. Usually when I hear “studio headphone”, I think of old-school Sony’s, or some Beyer’s (Beyerdynamic).

The Ollo S4X seems to be following a very simple design, but when you look into it, it’s clear that many parts were carefully thought out. I personally think that Mr. Rok definitely had longevity in mind when he was designing his headphones. This is simply due to the fact that every part of this headphone is easily replaceable - not only are the spare parts are widely available (you can order them from Ollo’s official website), but they are also quite accessible in terms of pricing. Every part is easily replaceable and can be replaced by the owner (instead of sending the product to the manufacturer’s service). You can replace every part at home, a rather large advantage if you ask me - sound engineers will probably be using this headphone as a tool, and they don’t have time to waste.

The headband construction is relatively minimalistic - it consists of a solid frame, a “bolt”, a headphone yoke (ear-cup holder) that is hidden inside the wooden enclosure, and that’s it.

The frame is made out of stainless spring steel (AISI 301). The nautical leather (faux leather) headband is attached to the frame with the same bolt that holds together the headphone yoke. It has a self-adjusting suspension system - the headband is stitched to an elastic piece (you cannot see it because it is hidden under the faux leather headband piece). Interestingly enough, this suspension system is quite efficient in doing its job, and I believe it’s mainly due to the lightweight (350g) nature of the S4X, but also due to the right clamp force. Now, what I mean by “efficient” is that I can very easily adjust the “size” and it will stay in place, which means that you can very accurately adjust where you want the ear-cups to sit on your head without worrying that the headphones will slide down. I had my doubts at first, but was proved wrong shortly after.

And of course, the ear-cups. Made out of solid wood, the ear-cups are the accent of all Ollo Audio headphones. It’s worth noting that all headphones from Ollo Audio are handcrafted, which explains some rough edges around the hole cutouts on the ear-cups. Thanks to the bolt that holds the yokes, the ear-cups have full 360˚ of pivot (horizontal rotation). Unfortunately the ear-cups do not have any tilt (vertical rotation), and this is something that I would like to see in the future models.

Smell & Wood

I have worked with wooden headphones before, but I have never encountered a headphone like the S4X. They smell like freshly processed wood, and everybody who is familiar with wood knows how lovely that smell is. For some reason, no other wooden headphones had this authentic smell - which I believe is due to the fact that Ollo headphones are handcrafted and are probably freshly made from real wood.

The finish is quite unique and dependent on lighting. While not in direct light, it has an oak finish - picture this as dark chocolate, and while in direct light, it has a gold finish with a gorgeous sheen.


Perhaps not the best part of the S4X is the cable. There are some good parts about it, and some not so good parts. Let’s start off with the good - the 2.5mm connectors’ housings are made of nice aluminum, while the housing for the 3.5mm jack is made of a different kind of metal (more polished). The jack’s housing also features a screw-on system for the 6.3mm adaptor, and has a spring protector which will keep the cable safe from bending damage.
The bad part starts from the Y-splitter. The Y-splitter itself is made out of rubber, which isn’t a big deal breaker, but the red part of the cable is. It looks much more like a cable you would find on some equipment from a workshop rather than a headphone cable. The braided part of the cable didn’t bother me as much (it’s good thickness and good quality), but I just don’t like the look & the feel of the material. I definitely think Ollo should change out the cable for the next model, perhaps use a full rubber cable instead - but keep the metal housings for the connectors and the jack.
Measuring at around 2 meters, I should mention that it has more than enough length.


With a unique combination of custom made ear-pads and a self-adjustable headband, I am surprised to say that these two result to a very comfortable headphone. As mentioned before, the headband has a very good elastic which keeps it in place. The hybrid ear-pads themselves are very comfortable, and the pleather used is of high quality. I’m glad that Ollo Audio decided for a hybrid design, because I cannot imagine how anything other than velvet would’ve felt on this pair.

The fit of the pads is very snug. I personally really like this as though it is a perfect fit for my ears. Very cozy! However, I am certain that people with larger ears will have a semi on-ear fit. This is definitely something to keep in mind.

All of this being said, I experienced no signs of discomfort after wearing the S4X for over 3 hours. This should mean that you can use them for long-listening sessions.



The whole deal about the S4X is the claimed “flat frequency response” out of the box. Now, the previous models have been marketed similarly, but people found the claim to be incorrect. Unfortunately, this has been assumed for the S4X as well… while nobody actually took the time to take any measurements for the S4X. First of all, there is a great reasoning behind the pricing of this headphone. You are not paying 400 euros for nothing. This time around Ollo Audio entirely lived up to its claim by having a flat frequency response, or as Ollo Audio calls it “brutally honest”.

The Approach

Audiophile headphones can be tested through testing tracks and music, but how do you test how "reference-grade” a headphone is? This is a question I asked myself before writing this review. I wanted to find the answer, so I proceeded to do some researching. After asking some community members we all came down to the conclusion that there is no convenient way to test how “reference-grade” a headphone sounds. To simply put it - there is no “true” answer.
I mentioned audiophile headphones before, so what is an “audiophile headphone”? In short, I believe it is a headphone that is intended to be used for music listening. These headphones are usually colored and have specific sound enhancements. In other words, the headphone has a character and an intentionally colored frequency response.

Let’s go back for a moment, what is “coloration”? The simplest way to put it would be to say that it is a method of altering the sound, it has some sort of enhancement or recession in the frequency spectrum. Coloration is used to satisfy a certain signature, a certain goal and user base - elevating lower frequencies is one of the most common colorations, reducing high frequencies is another. This is not the “pure” form of sound, it has been changed thanks to tuning, this is something that you do not want as a sound engineer, the main reason is because you are not hearing sound in its "true" form. What a professional wants is a tool that they can trust, something that is as free of bias as possible.

Coloration is an alteration of sound, that's one thing - but interpretation is another. Alteration of sound is an option, it is a conscious decision the manufacturer makes to tune a product, however, interpretation is something that depends on the person. If you have been in this hobby for long enough, you should be aware of the fact that we all hear differently, and this is due to interpretation of sound that differs from person to person. Interpretation of sound is dependent on quite a few factors, some of them are: pinna, concha, ear canal - these three variables alone can significantly impact sound. Interpretation and perception are both elements that are individual and personal, this is what makes our ears subjective, and in no way can you change this. You cannot change your ears, you are born with them. I have theorized a “solution” that isn’t quite convenient, I will touch on that later.

Why can’t you test how reference-grade a headphone sounds? First of all, you can test how flat the frequency response is technically, this would be with the help of an ear simulating measurement systems… but a frequency response can only tell so much. Due to the fact that our ear structures are so different from each other, and the fact that we perceive sound differently, to achieve a reference-grade sound signature you would have to do it individually. Why is this? Well, it all has to do with the fact that you do not actually hear the original frequency response. What the person hears is a different and altered frequency response than the “original” one, this is what you call HRTF. This frequency response varies from person to person, and this is why it makes it impossible to make sure that we all hear the same. Remember the three factors I named before? Each one of them affect the sound in a unique way. If you were to take the pinna as an example, can you imagine how much the pinna differs with different ethnic groups? This is the limitation of ear simulating measurement systems, because I am pretty sure manufacturers do not tune their headphones differently in different parts of the world. If you search "ear resonance" in Google, you will be able to find how different parts of our ear and body affect sound. You can see that the pinna slightly boosts frequencies from 2kHz - 5kHz, the concha has a sharp boost of 5kHz frequencies, and the ear canal & eardrum boost the 2.5kHz frequencies. The question is, how much do these ear resonances vary from person to person? If the answer is not by a lot, then it means that we can have a good idea of how a headphone will sound. This is only if we simulate these resonances in the tuning phase (basically what an ear simulator does).

This being said, when I say “you can’t test how reference-grade a headphone sounds”, I am just referring to an approach that could be applied on a commercial level, otherwise it might be achievable with personal calibration of headphones. However, companies like GRAS make measurement systems that mimic the human ear, they are called ear simulators. In theory, if you combine an ear simulator measurement system with a headphone that measures flat on it, you would be able to make a headphone that has a flat frequency response at your ear. It's not just about that, flat frequency response is something that you want from a loudspeaker in an anechoic chamber, but is it something that you want at your eardrum? I'm not sure I can answer that. It all goes back to the question above - if the ear resonances do considerably vary from person to person, in that case it would mean that even the ear simulator measurement system would not be accurate and consistent. It's an endless loop, you can quite literally start questioning everything - do you even want a flat frequency response at the eardrum? We know you want a flat frequency response in an anechoic chamber for loudspeakers, but can you just assume that you want the same for a headphone? I don't know.

In other words, you might bet able to make a headphone “reference-grade” on its own - which means that the headphone is technically-wise “uncolored” and has a flat frequency response - but we have yet to confirm how much ear resonances vary. If they vary a lot, this would mean that you wouldn’t be able to achieve a headphone that sounds flat to the listener without individual calibration/tuning - but you can probably get somewhat close with ear simulator measurement systems.

We can conclude two things from this:
1. A technically flat frequency response could be achievable with the help of an ear simulator measurement system. This would only be the case if the ear resonances don't vary significantly from person to person.
2. You cannot achieve a "true" flat frequency response without tuning the headphone individually for the listener. In other words, to achieve a flat frequency response for your ears, the headphone would have to be calibrated and tuned to your ears. This is something that is subjective, because this frequency response would sound flat only to your ears.

If all of this sounds a bit complicated, it’s because it actually is. Our ears and perception of sound are quite complex. To achieve a flat frequency response that a person actually perceives as a flat frequency response you would have to do various tests, make casts, measurements, etc. You can get a very good understanding of this by watching the following video:

This is a very important video, you have to watch it to get a basic idea of how much our perception of sound alters the original sound.. all because of things such as our ear canals, pinna, and concha. All of the subjects use the exact same headphone (Sennheiser HD600) and listened to the exact same sound (pink noise), but the way they heard this pink sound vastly differed. Hopefully you can translate this to a larger scale and imagine how a frequency response would be altered and could also vastly differ from person to person.

Perhaps it’s easier to picture this if you think of our ears as filters. If a headphone is flat on its own (on a measurement system that is not an ear simulator), it is technically-wise correct, it is indeed flat according to the graph and the microphones, but since our ears are not the measurement microphones, we all hear a different and altered frequency response - our very own frequency response. If different shapes of the ear result to similar ear resonances, then it is crucial to simulate these ear resonances when tuning a headphone. If there is any sort of repeating pattern in our ear resonances, then it is essential to use an ear simulator to get accurate results - tuning a headphone on a measurement system without taking our ears into consideration is useless. The headphones will be listened to by humans, not microphones.

The term "flat" needs to be well defined in order to understand what "flat" means in the headphone world, what it means to have a flat or neutral sounding headphone - this is something that we haven't really done yet. Applying the same principals that are present for loudspeakers may not be the correct approach to tuning of headphones. Once we define this term, we can then go on to try and achieve it in practice.

Does Ollo Audio S4X have a flat frequency response?

Yes. That’s the simple answer.

To me, this is the most important aspect of this headphone, it’s the marketing claim, so it should live up to it. The only thing I can test the S4X for is the flat frequency response, otherwise I would be basing it on my subjective hearing or on other subjective factors. Since this is a headphone that is marketed as a reference-grade headphone, there would be no point in telling you how this headphone sounds to me in terms of music listening, it would hold absolutely no value.

One thing is very clear, and it’s that S4X is aimed to be a tool, not a music listening headphone. Most users use headphones to listen to music, which means that the headphone needs to sound pleasurable to their ears, however, sound engineers are looking for accuracy. They are looking for specific characteristics to help them with making music, hence why I consider a “reference” headphone a tool. To me, a “tool” should be something that you can rely on, and fulfilling that in the headphone space is a highly difficult task. This is not to say that you cannot use the S4X as a music listening headphone.

At first you might think that no professional sound engineer will depend on a set of headphones to mix or master a track (or any other type of sound engineering), and you would be correct for the most part. However, there are exceptions. Not everybody has a studio of their own, not everybody has the budget to use a studio, not everybody has a studio that is close to their home, and not everybody has the perfect environment to make their bedroom or living space a “studio”. This is why Mr. Rok started his company, he was working late at night and his wife wasn’t really happy with that… it was at this moment that Ollo Audio was born. There are many people that face the same problem, especially younger people. You should also know that there are no standards when it comes to sound engineer’s preferences. Some prefer to use one studio monitor, others prefer another, some prefer headphones, some don’t - this just tells you that it all has to do what the engineer is comfortable with, it’s not as though you have the absolute standard studio monitor that is “the one”. Just take a look at 10 highly successful sound engineers and what equipment they use, you will find that it highly differs.

I also want to state something that I think is very important. I cannot put Ollo Audio's statement of "neutral" to the test, and it's simply because we don't have a clear definition of what "neutral" is. If there were to be a definition, it would need to be free of listener's preference bias, because preference is something else. Preference is what you personally like to hear from a headphone, it's not what determines if something sounds like it should sound in its "true form". However, I do know that Ollo Audio used the IEC 60318-1 ear simulator, and I know they tuned it according to the IEC 60318-1 standard. They definitely took into consideration how our ears alter sound (ear resonances), and they tuned the S4X with that in mind.

The Challenge of making a reference-grade headphone

At this point you should be able to see that it is a great challenge to create a reference-grade headphone, or just a headphone on its own. What I have said above is a present problem for all headphones because it is a problem that is caused by our complex ear structure. Currently, all of the headphones on the market are universal - meaning they are not custom-made for your ears. Since our ears are vastly different, without custom-tuning the manufacturer cannot know the frequency response at your eardrum. I like the quote from Ollo Audio - “A true flat is different for every individual”, and this is because a flat in the free field (in this case it’s the space between the headphones and your eardrum) is not flat by the time it reaches your eardrum.
The second largest challenge that is completely impossible to avoid is preference. Each individual has a different personal preference, that is why it is crucial to listen to audio product in-person before buying them. Again, in theory, a reference-grade headphone shouldn't be a headphone that depends on listener's preference.

But how do you even create a reference-grade headphone? What is a reference-grade headphone anyway? This is a hard question to answer, mainly due to the fact that companies have used this term as a marketing strategy to influence studio users to buy their products. If you go on and read the details to these so claimed “reference-grade” headphones, you will find nothing but a bunch of words that hold no value, words that do not explain the technical details as to why this headphone is marketed that way. I like the fact that Ollo Audio put in the effort to actually explain what their headphone is trying to achieve and what they mean by “neutral”. Reference, neutral, uncolored, all these terms are often mixed up.. leaving us to have a lot of different ways of wording something without a clear definition of either of the terms.

After carefully researching the existing “reference” headphones, I came to the conclusion that Ollo Audio S4X is among the few that have a clear aim with a clear (scientific) explanation. I think Ollo has a very good approach to make an “uncolored” headphone that is aimed to be mass produced, aka commercial product. I have my own personal reasoning behind this opinion. I theorized that the ideal concept is to have a flat frequency at the eardrum, this would be the most uncolored headphone - however, as stated by Ollo Audio, this is impossible to achieve because it would require personal calibration and tuning… which you simply cannot do by selling a commercial (universal) product. Basically, if a headphone measured flat, it would not be flat by the time it reached your eardrum, this is the challenge... it brings me back to the ear resonances, because if they do not vastly differ from person to person, then the headphone will sound very similar to everybody, and my statement above would be false. You cannot avoid the ear structure that changes the sound by the time it reaches to your eardrum, but perhaps you can study how our ear structure alters sound and implement those changes to the tuning of a headphone, this would ideally let you at least mimic how it would sound at the eardrum - this is already being done with ear simulators.

After giving it some thought, to have a product that is the closest to being technically flat, it would have to have a flat frequency response before the sound (from the headphones) is distorted and altered by our ears. The reasoning behind this is that each person would get an altered version of this very flat frequency response, the level to which this flat frequency response would get altered would vary from person to person. However, it should be the only way to unbiasedly create an uncolored headphone without individually calibrating it to the users ears - basically every user would hear a “slightly” altered version of the original flat frequency response.
If you were to tune the headphones to any other frequency response, the frequency response that the user would hear could greatly differ, which means that the level of coloration would differ from person to person - this would mean that it wouldn’t be as close to an uncolored frequency response as if it was originally tuned to be flat
- or so I thought. The above statement is completely wrong, and it is something that I assumed at the beginning. I should note that in the statement above I was talking about a measurement system that does not mimic our ears. While our ear structure definitely varies from person to person, it (in theory) alters the sound in a similar way. This means that even with different ear anatomy in humans, we can draw an average of how each part of our ear changes sound - it can be observed and averaged, as can be seen in an ear resonances graph. A flat frequency on measurement microphones would be useless because nobody would hear it like that, it would be "uncolored" and flat only for the measurement setup.... and we are humans, not the microphones. That is not how we hear through our ears. What you want to do is implement into the frequency response itself is how our ear changes sound (ear resonances), I believe this is what ear simulators and software do. They mimic the way ear, meaning they try to alter sound in the same way our ears do (e.g. boost in the 3kHz region).

Uncolored is a term that I would associate with a flat frequency response. In theory the aim of a flat frequency response should imply that no frequency response stands out, but the terms like “neutral” and “reference” are much more complex. These two terms are very subjective and weakly defined, it’s not as though you can easily define a truly “neutral” or “reference” headphone. It is much more complicated than a frequency response… which is problematic enough on its own. You would need a very strong and detailed scientific explanation behind these two terms. Since neutrality consists of factors & elements outside of the frequency response (such as soundstage, imaging, probably separation, etc.), factors that you either cannot measure or they are things that are complex to measure, you wouldn’t be able to accurately define these terms on paper.

The Solution and how Ollo Audio approached problem

Now that you are aware of the large challenge of designing a commercial headphone that will be listened to by people with different ear structures, I want to cover Ollo Audio’s approach to “overcoming” this challenge.

Knowing that they cannot design a true flat headphone without personal calibration, they took advantage of the challenge itself. How so? Well, the first step is to acknowledge the problem and study it. By knowing the problem, you can attack it strategically and take advantage of it. Because it affects and alters the sound by the time it reaches our eardrum, the problem in this case is our ear structure.

With a firm base and lots of existing research on this matter, Ollo Audio was able to also use it to develop their headphone around it. Instead of designing a headphone that measures flat on a measurement system, they altered the frequency response of the S4X in a way that a flat response would be altered by our ears.

Here are some of the factors that they looked at:
1. They used the Sennheiser HD600 as a base, which is a very good starting point. It’s a headphone that has been used as in the studio space for quite some time, so at least they could study what the consumers consider as “neutral”.
2. The Harman Curve was also something that the research department from Ollo Audio was studying and taking into consideration.
3. HRTF and how our ear anatomy affects sound

Beyond this, there isn’t much that can be done, perhaps lots of scientific research and studying the existing studies. The best you can do when making a commercial product is to mimic the ear, look at the averages, and implement these factors into the tuning of the headphone itself. That's exactly what Ollo did.

The hard truth

… is that there is no solution that has been found, at least for now. Not only do our ears vary on their on their own, but measurements have their own limitations. Each manufacturer has their own method of measuring, and even if they have the same equipment they will not have the same results. There is no “holy standard” and for this reason sound will continue to be subjective.

This is not something that is specific to this pair of headphones, but all headphones. Each frequency response chart is different, was taken in a different lab, by different people & manufacturer’s. Comparing these charts from different companies is not accurate (especially when you do not know all the details about how they were made, and with what equipment they were made) but at least it might give you a general idea about a product.

The only tool you can trust is your ears, anything beyond that at this point is something that you cannot fully rely on. If it sounds right to you, you will perceive it as right. As life goes on, you will form different sound perceptions, but they will be exclusive to you - it's not something that will sound like that to other people. The anatomy of the ear is complex enough on its own, it alters the sound on its own, however, the neurological side is something that is not often brought into conversation… but if you can imagine how much stuff happens with sound perception on a neurological level, it is something that we have yet to explore and study. The only right and wrong is in your mind, so trust your ears, don’t depend on other people’s opinion.

With speakers the question of how the "neutral" response should look like has been answered (if a speaker measures flat in an anechoic chamber), but the same question remains unanswered for headphones.



After spending weeks researching and reading several studies, I believe Ollo Audio not only lived up to its marketing, but also successfully made a tool for the sound engineers themselves. With limitations from both our ears and measurement equipment, the manufacturer had to be very mindful of how they will make the S4X, this required lots of observing and studying.

This is a statement directly from Mr. Rok:
“As there are so many different standards and approaches in the field of equal loudness contours, we decided to base our product development firmly on the feedback from our customers. We listen and do what we can, to adjust sound, comfort and even the purchase process, to accommodate as many pro audio needs as possible.”

To my ears this proved to be true, as though I found the S4X balanced across all frequencies. From Mr. Rok’s interview, he explained that the S4X is tuning is somewhere between the IEC 60318-1 standard (used by G.R.A.S.) and the Harman Target curves. Since Ollo’s target audience are sound engineers and professional audio industry, they made sure to test their product with them and took their feedback. I think the main reason why they were successful with this headphone is because they neither relied too much on neither measurements nor consumer feedback - they found the perfect balance between the two. They basically combined their own research with feedback from the target consumer market (engineers).

Mr. Rok further explained that the S4X is tuned closer to the IEC 60318-1 than to the Harman Target. This should give you a more direct explanation of tuning nature of this headphone.

All of this being said, I have to say that I believe that this headphone lives up to its asking price. There are a lot of things Ollo Audio provides besides a good headphone - 5-year limited warranty, 1-year of full warranty, widely accessible and affordable spare parts, and the easily replaceable nature of the headphone. These things matter quite a lot to somebody who needs a product that they can rely on, especially for professionals - being able to order spare parts and replace them at home is something that definitely holds high value. And most important of all, they provide a special trust factor. If you are skeptical or just want to try the S4X for yourself, you have a 30 days trial period - on top of that you also have the option to pay in installments (€20 a month for 12 months).

At the end of the day, you have to remember that Ollo Audio is a small company. What makes it a special small company is that it is primarily a direct-to-consumer company. This is important because it lets Ollo Audio use the majority of their budget towards the research department and future product development. It is the factor that is letting Ollo Audio do what they are doing now.

And I want to say that I am highly disappointed in people who continued to compare the S4X to the older models without even trying or measuring the new S4X. While the older models were not perfect, it’s clear that serious improvements have been made with the new X model, making it completely independent of its predecessors. Another thing has greatly disappointed me, is the fact that people who have reviewed this headphone have reviewed it from an audiophile perspective… which virtually holds no value to the target audience that will use this headphone in the professional audio field. I really hope somebody like @Resolve can form a more technical or scientific-based review, because I think that is a crucial approach for a headphone like this (whereas it is not crucial for music oriented headphones that aren’t of such strict technical nature). If there is anybody who should be approaching a review by listening, it’s the sound engineers - the people who these headphones were aimed at.

P.S. Ollo Audio has announced that they are expecting to release a closed back model in 2021, so keep a lookout for that. Mr. Rok said in an interview that he is expecting further improvements in performance.

For anybody who is interested to do the research themselves, these are some useful links that are definitely worth a read:
1. David Griesinger touches on several subjects:

Must-read studies by David Griesinger (these are cached webpages of his work, some of these are presentations, so they are missing images):
Frequency response adaptation in binaural hearing
Binaural Hearing, Ear Canals, and Headphone Equalization David Griesinger
The necessity of headphone equalization
Recent concert hall research findings and a method to equalize headphones to an individual at the eardrum

You will notice that David Griesinger's studies mainly focused on actually getting a speaker experience with headphones (hence why many of his studies revolve around binaural hearing). The majority of my assumptions have been based off of my understanding of Griesinger's work.

Must-watch explanations from @Resolve who did a great job at explaining everything in a very understandable fashion. These videos are great if everything seems very confusing:

Ollo Audio's brief explanation on of the S4X:

Or if it fits your taste, a reality-show-like series where Mr. Rok and his team touch on the S4X:

To get a basic understanding of what HRTF is:

Useful explanations of graphs and measurements: - Oratory1990's brief explanation as to why we haven't solved the question of the ideal frequency response for headphones.

Controversial: - a very well written article explaining the neurological side of things.

To further add on to the link above, you also have a lot of things going on a psychological level. This is where things get way too complex to take everything into consideration. The power of our mind is very strong, more so than people think. I am not sure about the name of this psychological test, but it has been done many times: if you present two identical objects to a person and ask them to find the difference, their brain will make up the difference even though it doesn't exist. If you can think of it the same with the audio equipment, then you can understand that a human mind can be convinced that there is a difference between two cables, two sources, two formats, etc. And this is not even taking into consideration the post-purchase rationalization... which on its own can quite significant.

Here are two important definitions from psychology (hopefully you can apply them in the audio space):
Illusion is the distortion or misinterpretation of a real perception.
Delusional perception is when a correctly sensed and interpreted stimulus is given some additional significance.

Having done research and reading serious studies on hallucinations has significantly helped me understand the power of our mind. I'll leave this here: "How is a real itch to be distinguished from a hallucinated one if in neither case the presence of an object in public space is required?"

I want to give special thanks to the people from HiFi Guides and The Headphones Community for helping me out with my research.

Special thanks to MaynardGK (HiFi Guides) and Resolve (The Headphones Community) for leading me to major progress with my research.

If you want to support me, you can order the S4X from here:
Note: I have not received any form of payment from Ollo Audio. I wrote the review beforehand and only after I finished the review did I ask Ollo Audio for a link that can help me out. I have not written the review with the intention to earn any financial benefit from it, I invested my time into making a quality review with an appropriate approach. This being said, Ollo Audio had no influence in my review, I completely independently wrote the review without any form of censorship or restriction from any outside force, including Ollo Audio.
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Wow, that was epic. Must feel great to get published after so much hard work.
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@thehutch Yes, absolutely! This might be the most challenging article that I have published. It took several stressful weeks to complete it. Lots of research, lots of reading professional studies, lots of communication on forums to understand the most objective side of things. This still stands as my most scientific-based article! But after I published it, I felt so relieved... it was one of those "I did it!" moments.

However, it all paid off. Not only am I happy with the quality of the article, but I have gained long-term knowledge from the research that I have done for it. I also hope this benefits any readers with the same long-term knowledge that I got from writing it, if not, I at least hope it opened up a new perspective. One thing is for certain, the whole article is based on a severely complex subject.

Thank you for the kind words! I always appreciate feedback from my readers.
John Massaria
John Massaria
No speaker I know of or headphone that measures flat ever is ever engaging - I like the Ollo’s just because they are so bland - as I state in my review below - it’s not a headphone I would grab for pure musical get away but I would say it’s a a great tool to become a better listener and perhaps make some of the best mixes/mastering at the studio so others can loose themselves in the music latter on down stream. A sort of must own for collectors
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John Massaria

Member of the Trade: JM Audio Editions/Headphone Modifications
Pros: Light for a wood design, comfortable for long listening, lovely faux leather maybe better than sheepskin? beautiful real wood, neutral sound, no bloat, not sibilant, natural curve nearly flat, for sound engineers more than casual listening, very little break in required, good fit, band is auto adjustable and I love it, price for performance is outstanding, excellent with lower power mobile devices (true to source of course)
Cons: wire is light but a little too light for excessive use I would think, they are too neutral perfect (is that a con or a pro?) , they can not play excessively loud (you probably shouldn't do that anyway - not healthy for your ears)

OLLO S4X Studio Reference For Serious Recording?

"Flat out of the box. Neutral and brutally honest." is what the company from Ollo claim for their top of their range over the ear open back headphone. I had to try it for myself- I purchased this pair myself- and am glad for it. I am way over and very tired of the $500 or less Bose, Sony, Audio-Technica , AKG, SHURE, Beats By Dre or is it Apple now? Master Dynamic (although I like their sound/ love their construction (MH30) on ear hp for the price), SkullCandy, Marley headphones, JBL, etc... why am I over it? I don't like mass produced poor sounding factory made junk- $500 is a lot of loot! I want it all- hand made gorgeous wood with amazing measurable performance- I am done with wasting $400 here (you hear?) or $500 here.


I am a huge fan of Etymotic ER4p/s and have found few over the ears that can compete with their brutal honest sound production. It's not that I am into S&M but I can tolerate some honest sound reproduction when I seriously listen- especially when I mix professional videos for my clients- be it for a real DEA story of high crime and corruption (The Last Narc or Rouge Narc) or mixing videos and sound for Grammy award winners like Anglea Hunte. If I am filming Michael K Williams for an interview - I can not afford for my sound engineer or for me when monitoring the shoot to fail- there are no second re-takes. When I set up the mics on location, be it a lavalier, shotgun or supercardioid and set the levels on the external recorder - I can not afford a mistake. Everything needs to be done right the first time.

OK-enough on that- back to the well made OLLO Wooden reference S4X headphones (they were just released in 2020) with super comfy velour and better than leather feeling ear pads (really you have to feel them to know what I am talking about)- do I like them for at home use when I mix at the console? Yes. Would I use them out in the field- yes and no- They are not closed backs- so on set you sometimes get bleed and reverb from headphones back in to the mics- so be careful. If you use them on location- be aware. Also you will hear things around you and they will hear your track- this is of every open back and is a normal thing to be expected.

How do they sound? They sound neutral- exactly what the doctor ordered - you get your medicine for the cure here. Don't expect bloat or excessive treble if there is none on the recording- if you want excitement look else where- these can be eq'd and will take a slight bump up in eq nicely if that's your thing (they are very easy to drive too and work very well with phone type connectors (3.5mm) etc if thats what you want- imagine wearing professional ear phones on the train or plane or at the beach (after Covid 19 is over) and people starting a conversation about them - they are remarkable looking on and off- but you can't expect these headphones to do the "rocking of your world" thing unless you do eq em. Low power amps or internal phone amps should drive these fine- just expect to hear the quality of the amp/dac and music. So its possible for these to do both worlds perfectly- just know what you are getting here- these are precision tools designed for professionals at the mixing board- and that they do very very well. Vocals are very precise- if your actor or singer is terrible or over-modulating - expect to hear it and fix it quickly before continuing- that's what these headphones do exceeding well. They pay off in dividends by making the music mix sound as honest as it should be or not. If your old headphones interject too much bass (like so many on the market for mass appeal sake do) your mix will sound weightless and if your vocals sound too high pitched- how can you correct that if you don't know it in the first place? These headphone may just make you more money as a professional because the final mix will sound good (theoretically) on everything be it speakers, car stereo or headphones (the good ones and the bad ones). And if you are not making money at mixing professional tracks, that's ok, it might just make you a better listener of what sounds well recorded and what sounds off- you will be more picky when selecting a poor MP3 song vs a FLAC song. Got a DAC you need to hear how it really sounds? OLLO. Got some new crazy cable you want to test out? Get the OLLO S4X and you more than likely will hear the difference. Did you just get to your friends house to compare equipment? The OLLO s4x should be in your bag.

Please take note: I did read/listened to some reviews of the other models by OLLO headphones and they were not designed to be as precise as the latests version of the s4x I had bought for myself- so do not think or get confussed here please! I am not refering to all OLLO headphones- just the s4x I am reviewing - I never heard the other OLLO's so I can't talk about what I haven't heard or base my opinion on other peoples reviews. I will say the other reviews got me to buy these and try them for myself. I will never take the word from other people or reviewers as a way for me to explain what I personally hear- they may guide me- but I make the decision after hearing for myself what to think, as I think you should too. The money back guarantee is what sealed the deal for me- that tells me the company OLLO is so confident in their product - that you have no risk. I like that and frown on any company who doesn't offer such a deal- especially internet sales when in person try outs are not possible. Do they really expect me to take a risk and get stuck if I don't like them? Those other companies must be crazy.

I love the cuteness of these wooden headphones compared to my larger wooden Kennerton Magni's or Gjhallahorns- but those cost twice as much (and worth it btw)- in the sense they are heavier in actual weight (lbs) and in actual sound- my Kennertons go down deeper and extend higher - reveal more dynamics at lower and higher sound levels and reveal micro dynamics unheard of even by reference headphones like the Sennheiser HD600. I digress - these Ollos are easy to get the mix right and don't need to be playing at ear splitting levels to do their job.

When your headphones need to be a precision scalpel for sculpting or paint brush for painting the perfect musical portrait for your signer artist, actor, film or production house- the OLLO S4X finally have done it- they are brutally honest as their web site says- they are a tool and have a place in any collectors box of headphones if you want to know what nearly ruler flat headphones sound like with your music. I truly think these are an amazing feat of engineering that we as consumers have been craving for a long long time. You would have had to pay $1000+ for that years ago and even then they were not flat like these OLLO's are - and thanks to OLLO we don't need to pay any where close to that bounty- hand made craftsmanship anyone would be proud to have in their collection. They are truly quality.

The Warranty is an unheard of 5 years! I forgot to mention the box they came in is recyclable and earth friendly- some people could call it a cheap looking box- I call it practical. They also supply a faux leather bag with draw string to store your OLLO's or cables in.

The included wire is a bit light duty but that could be a good thing- they use a 2.5mm in the cup and a 3.5mm jack and 6.35mm (1.4") adapter. I seen one video by OLLO where the owner talks to other sound engineers about the cable being removable and not a permanently attached type that are seen on so many other headphones which are supposedly made for professionals- which could break in busy environments like a studio by pulling down the whole unit off a desk or whatever and break the headphones- where by the breakway type conectors will just disconnect saving the fall of entire unit. I also like the idea that you can upgrade the cable when ever you want or if in time with everyday use, owners can replace the cable if it shorts out from wear and tear.

About the 30 day period- They write: "We know it's difficult to make an educated decision based on tech specs and reviews. The only true way to know if it's for you is to try it out. 30 days money back, no questions." Every single on line company should offer this for their products and if they aren't they should be after they read this.

Ollo Ref Pic1.jpg

Price For OLLO S4X Headphone is 399,00€ or $449.065 US Dollars at the time of this review.
Well worth it if you crave reference sound in a hand made headphone. You will not be sorry and the value here is extraordinary. They even send you a graph with your exact headphones as they were each individually tested (the graph is in the box when you receive them- insuring the flat frequency response for both left and right ears- truly remarkable in every way!

I am not here to sell headphones or make any money from recommending these by the way- I am being honest and sincere- if you are tired of the same old box store type offerings then you will find a home for these precision tools of the trade- music for me is what makes the world go around (that and NASA's explaination found here These headphones are necessary for people who want to hear the truth. Can you handle the truth?


John Massaria
Headphone user, music lover and professional video and audio producer
I wasn't paid by OLLO - I bought these with my own $ - so honesty for me is the best policy!
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Translated, thank you.
Erfan Elahi
Erfan Elahi
@ John Massaria " it's a tool for assessing" - are you implying it's too clinical - harsh/ metalic treble or shouty upper mids ?
John Massaria
John Massaria
no- its just not a very engaging sound signature- its nearly flat - a great tool but hardly one you will want to grab when listening casually- and it does not like volume- its made for one purpose evaluate the tracks and mixing imho- it's not harsh or metallic no