Fosi Audio i5 Planar Magnetic Headphones

Fosi Audio i5



Introduction & Disclaimer
The i5 is Fosi Audio’s very first headphone which is a full-size open-back featuring a 97mm planar magnetic driver.
Many thanks to Fosi Audio for sending over a review sample for the purpose of this review.

For the viewers:


2 cables included - 3.5mm and 4.4mm balanced

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Build, Design & Ergonomics

Having just mentioned these are full-size headphones feels dishonest; the reality is that the i5 is huge.
Everything feels scaled up.
At 527g (without the cable) he’s a chunky boy, but I wouldn’t let that be a deciding negative factor. My old HE-500 at 500g is dwarfed by the i5, so there’s clearly some clever weight saving being done here.

The very top the headband is thinned out, yet it retains a good high clamping force.
The genuine suede leather suspension strap is very soft, thick and conveniently retains it’s shape.
The sliders use resistive silicone to hold the strap in place with friction.
When I first read about that I was sceptical about its functional quality, but I’m happy to report that it works much better than expected and no doubt helps keep the mechanical complexity down, which in turn might help reduce costs and increase longevity.

So it’s great to see some innovation and not just another copy and paste headphone.
Talking about mechanical complexity, or lack thereof, the ear cups swivel around infinitely, but there is mechanical damping in the hinge, so it’s not awkward and doesn’t feel cheap.


The ear pads are hybrid fenestrated pleather and mesh fabric with some very plushy feeling foam which behaves and feels very similarly to memory foam.
The pads are huge with an internal depth of 3.5cm, 8cm in height and 5.5cm of width. Not only that, but the amount of surface area these pads have that comes into contact with your head is also vast.

So to mention again the high clamping force of the headband, these are just about one of the biggest head-hugging headphones around today and despite the weight are very comfortable to wear.
After an hour or two I get a small hot spot where the suspension strap touches my head, but it’s more of a warm spot as it doesn’t get painful, just noticeable; realistically you can’t beat gravity, over half a kilo of headphone has to be felt somewhere.
I think only a handful of headphones ever gave me this kind of head-hugging feeling, like the HEDD Audio HEDDphone, albeit to a smaller degree.

The ear pads are held on with what looks like Velcro.
When I first saw this my heart sank a little. Any isodynamic, orthodynamic, flat-drive, magnetostatic, magneplanar, planar-magnetic enthusiast will tell you that Velcro is no way to create a consistent bass seal.
After taking a measurement on the miniDSP EARS, I wrapped the mounting area with tape and re-measured.

There was no difference.


So, this was the first big indication that this was no ordinary planar.
Removing the pads reveals 85mm of active driver area and the next indication of being a bit different.
Technically, the ideal planar driver diaphragm is designed to have as close to an even pistonic driver motion as possible, yet here, in order to connect the end of the trace to the terminal, the driver has been pop-riveted at the edge of the active area, which also leaves an actual hole through the diaphragm, making these similar to Sennheiser HD800s!
Seriously though, I’m not sure why they’ve done this and I hope that it’s something they fix in their future projects.

Blowing air onto the driver reveals the third big difference – you can hear the diaphragm resonate like a drum skin as air passes around it and between the magnets.
The diaphragm tension is much higher than usual.

To remove the pad mounting plate, you only need to remove these four hex-screws, which reveals that the headphone cups are solid, single machined pieces of walnut that have been fabulously finished on the outside to a high gloss.
The aluminium grills are held by another set of 4 screws.


On either side of the driver are foam gaskets, ensuring a proper mechanical and acoustic seal and now we can get a good look at the driver and how everything comes together, and finally at the bottom of the cups where the 3.5mm stereo sockets reside.

The cable is mostly great.
The main length is very thick and pliable and is a shorter length than average.
It does however have one flaw.
Unlike the JVC HA-DX1000 cable which is even thicker and longer than the i5 cable, after the Y-split, the girth is greatly reduced to ensure flexibility and wearing comfort.
The i5 cables are only incrementally thinner, and the combined girth of both cables is thicker than the main cable which just doesn’t feel right and it a bit unwieldy.

In any case, the build quality is sublime and I fully expect to be using this cable for the next 50 years.

Considering the price of these headphones, especially the discount for backing it early, you get SO much headphone for the money, in quantity of physical material and engineering quality.

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The Sound

Now let’s talk about the sound.

I want to start by saying that since these arrived in my lap I haven’t wanted to listen to music with anything else. So while the i5 and I have been enjoying a fairly long honeymoon phase I can already tell you that it’s not just a phase, and I do have data to back this up!

First of all, it’s mostly clear these are planar headphones from the first moment you hear them. There are certain aspects of the presentation that are inescapable, like the relatively conservative width in the soundstage, compared to dynamics, which is found in almost all planars.
Fortunately, simply due to the physical scale of these headphones, they have a bigger than average height in the staging which has a nice effect of scaling things up.

This is something that also translates directly to a conservative level of imaging in 3D games, so the imaging combined with their physical weight makes these less than ideal for long gaming sessions, although after a week since release I’ve racked up 24 hours on Oblivion Remastered with these on my head, so read into that however you like.

Transient response and detail are both great.
Going back and forth between these and the Hifiman HE-500 and it’s clear that the detail and treble performance on the i5 is superior.
It’s also slightly more aggressive, but definitely steps closer to the level of the old Hifiman HE-6.


While exchanging emails with Fosi Audio about this headphone, I had to ask them – “Why does the i5 measure and sound like a STAX SR-Lambda?”

Apparently, this was one of the goals in the development of the i5 – “to get as close as possible to the performance of electrostatic headphones by utilizing an ultra-thin diaphragm and coating.”

They also shared that the engineers at Fosi Audio deeply admire and tried to learn from the technology and sound of STAX headphones.
So, going back to my initial question to Fosi Audio – yes, the i5 sounds remarkably like a STAX SR-Lambda (normal bias).

The scale and height of the sound.
The detail.
The extra vivid timbre.
And of course the tuning.

Now one thing is obvious, the tuning isn’t perfect.



To reach this level of technical performance, it was decided to prioritize a faster, more detailed treble sound with very high diaphragm tension at the expense of sub-bass performance and overall tonal linearity.
At the very least, the little midrange roller-coaster mostly plays within an 8dB range at the extremes, which should be within what most people think sounds correct, following most standard curves.

These won’t play audibly below 20hz, which is a bit of a shame, but what looks like a bump at 80hz sounds more linear than it looks when performing manual tone sweeps, and adds a very good, percussive thump to any music that requires it.

But this is where the i5 differs from the SR-Lambda, as with a good bass-seal, the SR-Lambda will measure flat down to 10hz, while the i5 measures like a SR-Lambda with a break in it’s seal.



Fortunately for some people, this will be exactly how they experience the sound of any SR-Lambda or any of STAX’s Lambda series and directly related siblings, so mileage may vary depending on your experience, tastes or even your head shape.

Looking at the treble, the graph shows a peak at 8KHz, but when performing tonal sweeps, to my ears it sounds more like a treble shelf between 8.5 and 12Khz.
At no point did I find it fatiguing but I think it’s something to be aware of: the i5 is not treble shy.



The i5 has only one noticeable flaw.

So far I’ve only discovered 3 unique instances where the high tension of the diaphragm actually affects bass sounds in a negative way, and now I’m getting kind of sick of listening to any and all kinds of electronic music in a quest to find more.
The point is, when you hear it, you’ll know, because it sounds like that one particular bass note has a kind of tight drum-skin damped-timbre.

The most obvious example I found was when listening to Thunder by the Prodigy, which to be fair, is underperformed by most headphones anyway, so in practice over 99.9% of music sounds perfectly fine on the i5, and if you don’t even listen to electronic music, then it should be closer to 100% of your library.

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Amping

I would recommend using a powerful amp with these.
Incredibly, these will actually play competently out of a smartphone at around 85% volume, but going up from my phone to the Fosi Audio SK01 made a bit of a difference.

Going from the SK01 to the audio-gd NFB11.28 on low-gain mode made a surprisingly obvious step up too, and going from low-gain to high-gain was another step up, each time gaining just a little bit more control and dynamics.
So throw as much power at them as you can, but weaker amps, DAPs or your phone with a dongle DAC may be sufficient, so don’t let amping be a deciding factor.

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Comparisons

Just as it is with the SR-Lambda series, what this particular tuning does is something like increasing contrast and saturation, to use a photography analogy.
Everything is just more vivid, more exciting, more articulate, more punchy, more percussive, more dynamic and done so in a very tasteful way as proven by STAX over 40 years ago.

Switching from the i5 back to the STAX SR-Lambda (normal bias version) the similarities become just as stark as the differences. The i5 doesn’t have the same ethereal, effortless electrostatic timbre and sense of air, but the i5 does give a small but welcome dose of bass and a touch of warmth that is sometimes missing in the STAX, but damn it’s very close! Closer than I think most people would believe, but don’t mistake these for an alternative, as the i5 still does feel a touch heavy handed and one-dimensional in some percussive moments that require more delicate finesse (which could also be heard as being more punchy and fun) – but let’s keep things realistic, I’m comparing them directly to an electrostatic headphone!

Switching from the i5 back to my trusty old Hifiman HE-500, it’s clear that these are more neutral, they bring you back down to earth a bit like cleansing the palate back to normal levels of saturation and a more common or average soundstage size.
It also goes without saying that the HE-500’s sub-bass performance is superior and is easily preferable for electronic and heavy metal music.

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Modding

Pad Rolling
I don’t own a single ear pad that is a fitting replacement, and holding a few other pads in place didn’t yield any positive results that I could hear, but I don’t want to go into any details there since none of them are viable replacements by size anyway.
I think most people will find the performance of the stock ear pads should get very close to their full potential anyway.

Plugging the Hole

Fosi Audio i5 ‘Unveiled’

Breaking the Seal
With most planar headphones, when you break the seal between the ear pad and your head, or even make a large gap, floating the headphones over your ears, one thing many people notice is that the bass suddenly gets boosted.
In a graph, this usually looks like a hump, very much like the i5’s graph but not always at the same frequency.

Performing this ‘modification’ with the i5 does practically nothing, as has already been demonstrated by my earlier attempt at sealing the Velcro-gap with tape.
These are not your average planar.

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Conclusions

If you’re quite sensitive to treble in general then you might want to look elsewhere, the weight will automatically rule these out for a fraction of you too and another small fraction of measurebators won’t even be reading this review at this point.

If you’re looking for a genuinely brand new planar flavour, where music just sounds so lively and vivid, I can see these becoming a companion to almost any other neutral-type planar on the market today.

No other planar sounds like this.

They’re just a joy to listen to and might even be a gateway drug for people who want to try out the closest thing to the STAX Lambda sound without going down the electrostatic rabbit hole in both cost and quirks, or for those who did love the sound but wished for a slightly smoother presentation.

I want to congratulate Fosi Audio on making a great first headphone and they should be very proud of their achievement.
They dared to make something different and I’m all for it.

Since I was vacationing in Shenzhen, I paid a visit Fosi Audio office, self-collected the i5, and saved them the trouble of shipping the headphones to me.


The Fosi Audio i5 comes in a massive box with a magnetic flap cover. Thick sponge protects the headphones from rough shipment. Included in my pre-launch review unit are 3.5mm and 4.4mm cables of 1.8m length in a cloth bag, as well as a 6.35mm adapter. The pre-order version in Kickstarter will include an additional 4.4mm upgraded cable and extra pair of earpads depending on the bundle.


At 550 grams, the headphones are rather heavy and slightly tight, but the weight and pressure is alleviated with the suede leather headband. The earpads are thick, using fabric on the sides in contact with the head, and perforated leather on the outer areas. I totally love fabric earcups as they are more breathable and lasts longer, though I would last longer if they are lighter.


Encasing the 97mm planar magnetic drivers is the aluminium frame encircled with walnut wood finishing. The ear cups practically tilt and swivel in all directions without restrictions. The overall design is pretty much similar inspired by headphones like Hifiman, but Fosi Audio manages to distinguish their creation starkly with the S-shaped grille accent.

On specs, the i5 headphones produces more than 120 dB SPL, with 10 – 50,000 Hz frequency response. With 28-ohm impedance and 98dB/mW sensitivity, Fosi Audio recommends more than 500mW of power to listen to them. They drive quite well on normal devices and I don’t need to turn the volume too much, but you definitely get better performance with more premium amps.

Audio Experience​

I feel that as a first pair of headphones designed by Fosi Audio, they have aimed to deliver a sound signature that takes the full advantage of the planar magnetic driver characteristics while tuning to achieve the sonic features of dynamic drivers.

Without the need to turn up the volume too much, I still manage to hear snappier sound, more obvious nuances, more refined percussions. But unlike other headphones, the details are not the result of just making the details louder. The i5 just makes the instruments less veiled. Having a massive driver size of 97mm means the sound is delivered over a larger plane so the music is not cluttered. The sound staging is not too wide and while they sound close enough to let me hear details, they are not too in-your-face.

To my ears, the i5 does not sound excessively bright, because it expertly balances the treble sparkles with the bass and midrange warmth. The low frequencies have sufficient resonance to achieve depth, punchy impact yet they do not linger too long to obstruct the musical presentation. The i5 also delivers more midrange details from instruments like Rhodes piano without the bloom, adding more joy to my easter-egg hunt for new sounds.


The i5 is tuned such a way to avoid sounding harsh to the ears, particularly the upper midrange. It also brings the bass more prominence, the midrange sounds full and clean, while vocals do not stand out in the mix, saxophone and trumpet solos sound warmer and less uncomfortable at loud volumes. Yet, they are not dark-sounding, nor does the treble roll-off. If you pick tracks without much bass, then the i5 can sound quite transparent.

I like that the i5 lets me listen comfortably to albums with heavy and harsh treble mixes like Ayumi Hamasaki. I like that the i5 makes jazz albums like Tsuyoshi Yamamoto Trio’s “A Shade Of Blue” sound more “live” as the headphones bring out the unintended recordings of the performers’ voices and cues. Basically, for albums that sound harsh due to excessive upper midrange peak, I can listen to them at louder volumes and enjoy the overall music arrangements and recordings.

For my review, I paired the i5 with the Creative Sound Blaster X5 running with 4.4mm balanced cables because of a slightly better treble detail due to its dual DAC compared to the Fosi Audio K7 which is a more neutral desktop amp.

Compared To​

Sennheiser HD 800 S: this is superior in delivering a more natural sound staging. Vocals are centre forward, the left-right mix is more discernible, more comfortable to listen to in terms of imaging. Upper midrange is unforgiving, so the mix must be of audiophile quality or else it would sound unpolished. The i5 tonality is more refined, with more bass. Staging is closer, instruments less separated but still uncluttered.

Sennheiser HD 550/505: the i5 delivers better details without turning up the volume, because the 97mm diaphragm covers the entire surface of your ears, while the former is only a measly 38mm. For that, I can definitely hear the instruments and the timbre more clearly. On the HD 550 or HD 505, the overall sound presentation is more midrange-centric, more warm.

Hifiman HE400se: the HE400se sounds comparatively feeble, more laid back, I have to turn up the volume more to get the HE400se to give me more impact. With the i5, I get details a lot easier, filling my ears with glorious music and the finer performance sparks.

Audio-Technica ATH-ADX3000: the treble is more transparent, sparkly, sizzle. The i5 provides more bass, the treble is less forward but remains sparkly at a lower intensity. Imaging-wise, ADX3000 is more wide and separated, while the i5 sounds more naturally cohesive in the instrumental mix. The i5 sounds more full, the ADX3000 more spatial and airy.

Verdict​

The Fosi Audio i5 delivers elevated bass without overpowering the highs, offers sparkling nuances without sibilance, achieves midrange details without muddiness. The headphones make timbre sound more distinct such that I can hear more tonality at moderate levels. It is also one of the few headphones where I can turn up the volume without feeling the pain, and when I do that, I hear even more precise attack and recorded decay.

While the headphones are quite heavy, the plushy earpads offer excellent padding and the headband gives me good support. I am surprised that I can wear them longer than some headphones at half the weight.