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JDS Labs Assembled Objective2 Headphone Amplifier

100% Positive Reviews
Rated #1 in Desktop Amps


Pros: Excellent sound quality, great price/performance ratio, battery and/or AC powered, adjustable gain options, no background hiss, packs plenty of power

Cons: Everything is on the front-panel, low-volume channel imbalance, too bulky to be used as a portable amp, finicky with TRRS jacks



Without referring to "He-Who-Must-Not-Be-Named's" (A.K.A. NwAvGuy) blog, and speaking from my own listening experiences, I really do think the Objective 2 is a great headphone amplifier. No, I don't think this is the amp to end all amps, but it's a dang good one and it only cost me $156 USD (which includes the Triad brand WAU12-200 AC adaptor). In fact, it can be found even cheaper nowadays from the same vendor, JDS Labs, for $140 (including the same adaptor).





What's in the Package?

  • JDS Labs Objective 2 headphone amplifier, wrapped in an anti-static bag
  • 2 removable, rechargeable Tenergy 9 V nickel-metal hydride (Ni-MH) batteries (already installed in the O2)
  • 4 adhesive clear-rubber feet
  • 3-foot long Monoprice mini-USB to USB cable, with gold-plated connectors and a ferrite bead
  • User Manual
  • JDS Labs business card/contact information
  • 2-year warranty free of part/manufacturing defects
  • Triad WAU12-200 AC adaptor (if purchased)





Custom Orders

There are also 3 things you can get customised when your order your O2; all you need to do is fill out the "order notes" section before you make your purchase:






This is the only place where I have major complaints about the O2.

  • Everything is on the front-panel, and the front-panel only
    • 12 VAC AC adaptor plug
    • Power button
    • 3.5 mm output
    • Red LED power indicator
    • Volume potentiometer
    • Gain button
    • 3.5 mm output

Yeah...there's a lot stuff going-on on the front-panel, and the cable management is a mess.


Adding to the mess of stuff on the front-panel, the amp itself is too big (for me) to be considered a portable amplifier. Perhaps for those people who carry around a "portable" bomb rig that is 6-inches thick with 4 different components and paired with a 10 pound open-back Audeze LCD-2 headphone with $500 platinum-coated silver interconnect cables (slight exaggeration here), this amp is trivial in size; but I am not one of those people and I prefer to have a tidy, pocketable portable rig instead.


Kidding aside, the other thing that bothers me with the O2 is the channel imbalance encountered at low volume-levels with the potentiometer. When past the channel imbalance region (~8:30 o'clock position on the potentiometer), the output is kind of loud even with 1.0x gain, especially with sensitive headphones. Because of this, I don't really recommend the O2 for such headphones unless you have a custom gain of less than 1.0, or you use an impedance adaptor.


As a small note, at least with the V-MODA Crossfade M-100's cables, my O2 doesn't seem to like TRRS jacks too much. If I plug it in all the way, I get a mono-like sound coming out of the O2. Pulling out the jack slightly seems to remedy this issue and all is good.


All things considered, the layout of the O2's front-panel is messy and the potentiometer's channel imbalance can be a bit of annoyance for some headphones.


The O2 outside of its case:







From switching between different amps, headphones, and sources I have at hand, the O2 has a very clean-sounding signature as a whole.

  • Bass extension is good with decent texture
  • Midrange is a bit laid back in presentation, but not lacking in detail
  • Treble is well-extended with no roll-off from what I can detect
  • Soundstage is pretty spacious, having both good width and depth
  • Related, but not necessarily correlated to the soundstage, imaging is also very good and I can easily imagine where instruments are in my head
  • Instrument separation is excellent; instruments are well-defined and never muddled together
  • Related, but not necessarily correlated to the instrument separation, instruments have very good detail and nothing sounds muffled
  • There is no background hiss from what I can hear with 1.0x and 2.5x gain options at reasonable volume levels


Q: Is this what neutral sounds like?

A: I have no idea. I'm not a musician, nor do I play any instruments, so I can't say if it sounds coloured or not.


Q: Is this what a "wire with gain" sounds like?

A: Maybe. This relates to the previous question.


Q: Is this what "boring" or "sterile" sounds like, as some have suggested?

A: Not in my book. Different genres of music of all different kinds of mastering sound fine with me. I have yet to encounter a track and exclaim: "wow this amp is so boring! my music sounds so dull and lifeless!" All I can say is that music in general sounds spacious, well-defined, and clear. Though "involvement" in music is purely subjective, I never find myself being uninvolved in my music unless I'm listening to music as background noise.


Q: Will it power X headphone adequately?

A: Yeah most likely. I really enjoyed the LCD-2 with the O2 over other amps I've heard, even though the LCD-2 isn't my favourite headphone of choice. From a pure synergy perspective, I didn't like the HE-500 with the O2 as it had a pretty tizzy treble that made rock tracks not enjoyable for me to listen to. That might be due to the HE-500 itself though, not the O2.


Q: Does it sound bright?

A: Not at all. I don't know where people got that idea from.


 bright, brilliant The most often misused terms in audio, these describe the degree to which reproduced sound has a hard, crisp edge to it. Brightness relates to the energy content in the 4kHz-8kHz band. It is not related to output in the extreme-high-frequency range. All live sound has brightness; it is a problem only when it is excessive.

Comparing different headphones with the O2 and comparing the O2 to different amps, the O2 is definitely not bright to me. It's definitely not dark either for that matter. Maybe with an HD800 or K 701 they sound bright, but the those headphones are bright by nature, so it's not the O2 sounding bright.






All in all, if you can get past the messy front-panel and the channel imbalance at low volume-levels, then I truly think the Objective 2 is a terrific-sounding amp. I like its sound more than other amps I've tried, both desktop and portable amps, and everything just sounds so dang clear and clean to me. As such, the O2 is my current reference amplifier. For $140 USD, I think the O2 is a no-brainer if you're looking for a clean-sounding amplifier and you don't want to spend OVER 9000!!!!! (read: a lot of) dollars on an amp. You might not like the sound of some headphones out of it, but I think it's due to the headphones themselves and not necessarily the O2.





Thank You!

Thank you for taking the time to read or glance over my review! I hope this review helps you in some way or another and if you have any questions, feel free to leave a comment below.


Happy Listening!



Pros: Sounds Great

Cons: Not super small

I read about this device online and the reviews were very positive - so positive that I jumped in.  It was my first headphone amp — I purchased my second, the fiio a5, shortly afterwards.


Take into consideration I am an amateur audio geek...


It is just what everyone says about it; zero background noise; clean sound; no extra color.  I did also read that it was a little on the large size to be considered portable, and I agree that it is.  However it will fit in the back pocket of my Levi's so I'm still considering it portable.


After comparing it to the fiio a5 for a week I must say I prefer the Objective2.  The "uncolored" sound is really more my style.  I wasn't sure where I fell on that spectrum...  But after comparing, I get it.  The objective2 is cleaner, more open.  You are not going to find something like a tube amp warmth.  But what you will find is your really great recordings in crystal clarity.


Physically it's a nice solid piece of equipment.  It has a single red light when it's on.  The battery power so far seems to be in line with how it's advertised.  I do like that I can replace the batteries easily if I needed to.  Little things like that make this a special piece of equipment.  The volume control has a wide range so you can adjust it to very specific levels.  JDS labs has responded to my queries in almost real time.  They are very friendly and accessible.


Pros: value, clarity, small footprint, runs cool, reliable, can drive many headphones

Cons: could have more power for harder to drive headphones, could have more modern design, could use few other updates to bring this little Tarzan into 2017

You know that feeling when you wake up in the morning and you can smell the coffee (Columbian, beautiful like their women), bagels (plain but fresh no manager specials) being toasted and birds singing (bunch of them in perfect harmony). It's the feeling of life just being right and problems being thing of past. That's how I feel today so I thought let's write a long overdue review of this little wonder called Objective2 headphone amp. There are many reviews with detailed numbers and measurements but my review is just my opinion based on many hours of listening all type of music with different headphones.


My unit was purchased used for only $55 shipped here at head-fi few months ago. This is my second O2 amp first one was a JDS my current one is Massdrop made in China. Don't let the price tag fool you this little amp can't be beat for the money spent compared to amps costing much more. I had amps up to $2000 and unless your trying to power Hifiman 6 or certain planars your search can end right here right now. Do not waste your money do not fall for the hype because only hype real is the Objective 2 hype and that hype is real backed by measurements and cult like following over past few years.


What makes O2 special is that it's a neutral amp (wire with gain as we call it) and it does not change the flavor of the sound . That is exactly how amps should be and rest of the gear so you can enjoy the soundtrack the way it was recorded in the studio and listen to headphones the way they were designed with their own sound signature. This little Tarzan has good power and is able to make most headphones sing including HD600, HD650, Beyer T1 (600ohm but fairly efficient). 


Now is there a better amp out there? Yes, of course but requires you to spend at least six times of O2 price tag. While powerful, this little Tarzan could use slightly more power since I find some of my headphones slightly lacking in the lower frequencies but we are talking about very small improvements that takes good ear to evaluate.


For money O2 is a slam dunk and one of most popular amps of all time for a good and well deserved reason. If you are trying to get your feet wet or just trying to find your final amp O2 will not disappoint most of audiophiles.


Now back to last sip of coffee....  refill time. Let's load up on some more bagels. Oh snap, I'm out of cream cheese.


If you find my review informative and fun please take quick moment and sub to my youtube channel:




Pros: Sound Quality, Build Quality, Customization, Value

Cons: Portability Issues

The Objective2 is my first amp along with my beloved HD600 headphones. Here is my review after my experience with it for several months.



Intro : I am an 21 year old Engineering student living in a small town in India. There is not much audiophile-community presence in India, Though lot of potential remains to be tapped. Auditioning an amplifier before purchase is almost impossible in India.


I would like to call myself an music enthusiast, rather than an audiophile. I mostly listen to Bollywood and Indian Classical Music.


I had bought off the O2 amp solely based upon its reputation, and also because of not being left with enough funds buy an more feasible amp.



The O2 amp is designed by an famous American electrical engineer. It is well documented and measured amplifier, leaving no second thoughts on its performance. The goal of O2 amp is to achieve transparent sound with good measurements and at a reasonable price point, So that every enthusiast could afford an decent amplifier.


The O2 can also be customized easily. It is available as a kit, or readily assembled, with/without case. It is cost effective and has many more add-on options.



Design and Build : It is an very well designed and built amp. All components are mounted on an single PCB, with no manual wiring, which ensures longer life. It consists high quality double sided PCB, passive components, semiconductors and an Alps potentiometer. Exterior case has equally good build, with finished anodized and brushed aluminium case. Front and back panels are mounted by 4 screws, which can be removed easily to slide out the O2 board. All controls and jacks are mounted on front panel of O2.



Overall,The O2 amp has Excellent build quality and design. Cant ask for more from an DIY amp.


Sound :  O2 is very transparent. It has no ‘sound signature’ of its own. The output from the O2 depends very much on the source and ultimately on the headphone you are listening; it’s a wire with gain. It is an reference amp in sub 200$ price category, and comfortably competes with amps which costs much more.


The O2 amplifies the input signals without adding any flavour of its own, which results in improved soundstage, dynamics and imaging. This transparent sonic character makes the O2 amp suitable for any genre of music. O2 has zero audible background noise and pitch black background. It has a very low EMI pickup, which is an advantage for Smartphone users.


Output Power: Specifications state that O2’s output power is sufficient for most of the headphones. O2 amp can drive both dynamic and planar-magnetic headphones, though the latter seems much difficult to drive; The O2 takes the dynamic HD600 and planar magnetic LCD2  to loud listening levels, but ultimately lacks dynamics and power to ‘drive’ them properly.



We know that the O2 can have custom gain settings. Though by default, the O2 comes with 2.5X/6.5X gain. Once the gain is flipped to 6.5X, background noise and distortion increases. I feel the gain switch is best set at 2.5X. But this again limits the O2’s output power for demanding headphones.


The O2 amp has low output impedance and is hence is also suitable for IEM’s and earphones. One can hear an small ‘pop’ noise from headphones when O2 is switched on/off.


Portability : The O2 amp is not sleek or light as the Cayin C5 or FiiO E12. The O2 is more of an transportable amp than truly portable.The O2 amp can work either on Internal Rechargeable 2X 9V Batteries or on External 15-20 VAC Adapter.


Once fully charged, The O2 works on battery power for around 6-8 hours, which is quite decent. It doesn’t have any audible differences when running  on batteries or on AC Adapter.

Slow recharging process in implemented in-order to avoid over-charging the batteries. Once the AC Adapter is connected to the amp, the battery recharging process is initiated. It takes around 12 hours to fully recharge the O2. There is no indication to the level of charge in the batteries; we have to manually estimate the approximate battery life remaining.


Most of the portable amplifiers use 5V USB Charging. Whereas O2's AC Adapter is heavy, and is cumbersome to carry around. This further limits the O2’s portability.


Though O2 is an excellent amp, but It's portability side suffers. If one wants to sacrifice the transparency of O2 for better portability, CAYIN C5 and FiiO E12  are pretty good amps, which have higher output power and are truly portable, along with very good sonic quality.




Conclusion :


The Objective2 is an fantastic amp, It is hard to beat O2 amp in its own category. O2 is an easy recommendation for an budget amp. Though not very portable, the Objective2 does its job and does it very well.


The following features make O2 amp unique amongst others :


1) Transparent Sonic Character

2) Battery/Adapter operation

3) DIY and customization

4) Decent output power

5) Easy maintenance

6) Transportable

7) Affordable.



O2 is like an transparent window, which shows us, the view into music.


Pros: Light, Gain Switch, Replaceable Battery

Cons: Not Lithium Batteries for More Power or Run Time

I've been reading so many replies and reviews how the O2, Magni and Vali sound the same. After weeks of listening to all of them, (yes I have them all) they are all not exactly the same . I've been using the same music and all else in the system is the same and have only changed the amps. Started with O2, then went to Magni. Yes the Magni and O2 sound very close. The Magni does sound a tad processed and thin compared to O2 but the Magni does have more authority as it is more powerful in comparison even though they all seem to struggle reproducing the source. The O2 is a really nice amp but when compared to the Magni the O2 is better overall, but sound wise when compared to Vali in sound the Vali is a tad better.

I hope this helps anyone who maybe interested in the O2.


Update: I would recommend the Aune amp over the O2. Way more resolving, less distortion and is a true Class A amp.


Pros: Noticeable improvement in some cases, very little improvement in other cases

Cons: Everything is on front panel


So, this amp makes the signal just a bit cleaner. It was pretty noticeable on my Q701s, noticeable for general clarity, separation, esp cymbals got a bit clearer. For my HD 558 (w/ foam mod), slightly noticeable, but not that much. For my closed-back headphones->very minuscule, you might notice if you listen really hard. When I bought this, I just needed power for my Q701s so I was surprised the quality actually improved, thus I might have exaggerated my initial impressions a tiny bit. Overall the difference minor even for the Q701, but still pretty noticeable. It's not OH WOW I CAN'T BELIEVE THIS, IT'S LIKE DAY AND NIGHT kind of difference.


I bought this amp for my Q701s (Which I won't receive until Friday). When it came I took it out just to make sure it works (All my current headphones are low impedance and don't require an amp). I hooked it up with probably one of the worst possible sources you can imagine: a Nintendo DS (I was in the middle of playing a DS game when it arrived). Of course there was volume, but I also almost immediately noticed that the soundstage got bigger and the instrument separation became better. This was on my AD700s. I thought to myself it must be me imagining things. Just to make sure I switched back to directly plugging into the DS and the DS->Objective2->Ad700 setup. There really was an improvement, not just volume, I adjusted the knob so that the volumes were the same. The improvement is definitely real and doesn't require careful listening to notice, it was almost immediately noticeable. Needless to say, I am impressed as I have always been skeptical of headphone amps, but this one made me believe all the hype.


Pros: Does its Job and Does it Well

Cons: Pricey? Even for what you get . . .

Lets see, a battery powered Amp that runs for hours on hours with a charge, or a desktop Amp that can throw almost a watt out and run forever?


I don't know - its sort of both. It is too big to be taken anywhere, but definitely small enough to travel if you want it to. I ordered mine with a black face plate which I think is a little cooler looking.


There appears to be some "mythology" surrounding this amp, some claim it "ruins" their sound quality while others think its awesome. I think its an amp - that is, it takes a signal, amplifies it, and otherwise has no real impact on what you hear. It has no real noise I can detect (except at max gain of 6.5X with nothing playing at max volume, which is possibly my source, not the amp). Those who adore vacuum tubes might hate this discrete amp, but I hate replacing tubes every 9,000 hours, and like the idea that this thing should run forever


As far as amps go, the sound quality has been awesome, convincingly "clean", and does not seem to distort or struggle even when being pushed hard. You would have to spend at least another $50-100 bucks or so to get to the next level of power (about 1 full watt) with similar features and quality.


Mine came with small stick-on rubber feet, and a blocky 12 V DAC adapter. Having all jacks in the front is not a HUGE deal, but function is definitely following form in this case. Clutter could be reduced if the power and input jacks were in the rear, but you'd have to do these mods yourself. On that front, it would be nice to have digital/RCA/USB inputs as this unit probably isn't going anywhere. Whole unit has a nice solid feel, volume knob is solid feeling. Only gain and power switch come across as a little "cheap". I ordered stock 2.5X gain and 6.5X gain switch - which is about right for my HE-400s. For a more efficient headphone, as you can choose gain level, I would recommend 1X gain and perhaps 2.5-4X for less efficient cans. If you listen to many High Dynamic Range recordings (like DVD movies, for example), you may want that high gain (6.5x) switch - it can help reach the right volume levels with certain headphones. But, in general, the gain settings are not 'volume' buttons consider your main sources.


JDS Labs shipped extremely fast, answered my questions, and will "customize" the unit by shipping it with gain settings and color of your choosing. You can choose your own VAC adapter, or just buy theirs, as I did.


Prodigous power from a small unit - I estimate about 700 mWs @ 50 ohms while plugged in. Otherwise, about 500 mWs reliably from about 32-100 Ohms plugged in, and about 500-400 mWs on battery. Far more power than needed for most headphones. Very low impedance output should be a great match for a very wide range of headphones. This thing specs better than amps several times its price.


That said, it also costs almost as much as very good component amps for cars (that provide 10x the power). It costs as much as some very good headphones. In the world of high-end headphones, it is a fabulous value. But I would probably skip it if you get enough volume from current sources. The HE-400 provides a fairly good threshold perhaps (rated 92dB efficiency). Any phone with efficiency ratings in the 95 dB or greater range probably doesn't want this much power. But if your phones are high impedance and you struggle getting good output from a computer, phone, etc, I am sure the O2 will rock your world.


Pros: Fantastic sound to price ratio after modifying, Very versatile for modding - spacious boards, easy op-amp rolling.

Cons: short battery life (8-9 hours), long charge time (12 hrs using 18V ac, 770mA), audio quality becomes source dependent. Not very portable.

This is my first review submission for head-fi, hope that it could provide some comparison and enough information to let you decide whether to make a buy for it. But do remember this review is from my point of view and the ultimate decision is down to your ears to decide.


Tips for mods and results of modding are listed at the bottom of the page. Do note that modding the O2 is done at your own risk, Don't hold me accountable and say you've blown your capacitors because opamp is inserted wrongly or the rating does not match.

Warning: Spoiler! (Click to show)
(Any opamp that operates at 18V should work fine, including some that states 12,15,18V etc. Basically as long as you see 18V it should pose no threat.) I've not tried opamps that don't operate at 18V yet, so please do so at your own risk (Really not recommended too). I also did not try the OPA 627 because it is expensive and pin configuration of a mono opamp seemed different from a stereo opamp, meaning it is not a direct drop in. Correct me if I am wrong though. Again, do this at your own risk.


PS: My O2 is 4-5 years old, any new revisions to the PCB are not known to me. I'm basing my review on the earlier generation's O2 that does not offer the 1/4" jack yet.


Some prologue of my sound preference (as a reference) (edit 03/2017):

I realised that I'm a tube person after playing this hobby for some time, and my reference sound signature is my Aune T1 tube dac (Using Voskhod rockets 6N23P) to TU8200 tube amp (Using NOS Matsu****a 12AU7 and reissue Goldlion KT88s) driving Grado SR 125i, T50RP MK3 and MDR7506 modded with Kimber cables. Vocals are my favourite along with well balanced overall sound. Full sounding, well detailed, good separation, vocal depth, image depth and staging that is in front of you (not on top of you or so). All audio frequency should complement each other, not fight against each other, forming the gateway to heavens when you close your eyes to indulge. Lastly, I believe music should come to you, instead of you trying to look for music while listening. This point is hard to explain but I hope the meaning went through lol!




First few things to start off with here that I believe will be important:

I view this product more as a portable equipment instead of a desktop appliance. Therefore my comparisons are usually made against portable amplifiers. 


The sound of O2 depends greatly on the source.

Warning: Spoiler! (Click to show)

The fact that O2 is a very neutral amplifier makes it reliant on the DAC and the file formats it is using.

For instance, using the DX50* as line out vs. using DX100* as line out, the difference is greatly noticeable.

In another instance, my friend tried my O2 using his phone as a source. He didn't like the sound he's hearing until I changed the source to DX50, where the overall dynamics were improved.


*DX50 is running Firmware1.2.8, DX100 is running FW 1.2.7. Both DAPs are using the exact same copy of songs, FLAC quality converted from CD.


Comparing O2 with other amplifiers:

Warning: Spoiler! (Click to show)

I have the Aune T1 with modded capacitors at the amp section, so I decided to compare the T1's modded amp section vs the O2.

The O2 has a tiny bit less bass punch, lesser 3D vocal depth and sounds slightly less engaging. The highs have not much noticeable difference, probably due to the headphones i'm using.


Source used in this test is in FLAC format using the Aune T1 as a DAC for both amplifiers. The tube on T1 is a NOS Philips JAN 6DJ8. LO cable silver RCA-3.5mm for the O2. Headphones used is Custom One Pro.


I've also bought a second hand DX100 v1.2.7 recently and changed the batteries in them, and did a comparison of DX100 on board amp vs my O2 modded amp. O2 had a noticeably more refined and slightly broader soundstage, slightly darker background, instruments separated much better, more natural vocals though it lost a little vocal depth (still more than acceptable imo) when compared to DX100. Volume is adjusted as equal as possible in the vocal region for fair comparison. Anything else to complain is probably the digital steps of DX100 compared to the easier analog control of O2. Winner here is undecided as it's hard to consider if I should bring a big brick or a bigger stack, whether is it justifiable for the slight improvement in sound. Comparison done using T50RP MK3 and 24/96 FLAC songs.


(Edit 03/2017:) Recently I bought a second hand DX200 at a good price :D and compared the amp1 output to O2. Tried it on T50RP MK3. In short the most obvious difference is that O2 is slightly cleaner and less bassy than the DX200 but much better bass control. Low frequency separation is more noticeable and defined than the DX200's. I will do another update after I have done sufficient comparison. Stay tuned!


Have not tried comparing this with Cayin N5, may do so in near future.


Do not bother to ask me how it sounds compared to my TU8200 Tube amp as it is too unfair to do so. The magical sound pixie inside a vacuum tube is mocking at the opamp transistor now.


Stock O2 sound signature:

Warning: Spoiler! (Click to show)

1) O2 is fast and does not smudge the song when playing fast songs.

2) Neutral, it literally only amplifies the source signal.

3) It is suitable for almost any genre songs IMO and generally presents the music with a better soundstage and imaging without changing how the music sounds like.

4) Due to its neutral nature, some may not like the sound of it as it may make a song sound a little boring and slightly less dynamic compared to portable tube amps.

5) The ability for micro details to be picked up is more source dependent. But O2 does a good job on the micro details, making sure they don't get left out.



Warning: Spoiler! (Click to show)

1) Good volume knob, quite strong and can withstand some abuse. No hiss produced when turning the knob.

2) Strong aluminium case to keep internals protected and at the same time would not be too heavy.

3) All the plugs are located at the front panel, which may be good and bad depending on how you use it.

4) When the amp is switched on, only a tiny pop sound can be heard. (no pop for my modified O2)

5) The stock O2 makes the instrument separation, imaging and staging better without tainting the sound.

6) Only 1 circuit board present in the amp, making the configuration a lot easier to be understood (for people who like to mod)

7) Current limiting stock op amps used at output to protect expensive IEMs.

8) Very powerful (up to 613mW into 33 ohms on battery), stock opamp config. 

9) Battery can be changed out when it runs out of juice. Rechargable batteries can be changed when it ages too.

10) Not very space taking if compared to other desktop amps.

11) Very versatile for modding.

12) Good sound performance to price ratio; you get more than what you pay for.

13) Good amp for starters.

14) Stock O2 has nearly inaudible hiss unless you are using CIEMs. Hissing sounds may come from sources instead. However, the opamp can be swapped out to reduce to almost no hiss at all, even when the volume knob is at max.



Warning: Spoiler! (Click to show)

1) Short battery life - ~8h. ~9.5h after changing to higher capacity batteries.

2) Source dependent. = Need to get a good DAP or DAC

3) Big and bulky, not pocket friendly

4) Stock O2 on high gain sounds horrible. Modded opamps high gain still distorts too greatly till it clips at higher levels. You may want to find NwAvGuy's O2 PCB design to see his design gain stage resistors value and reduce the resistor value for high gain to reduce clipping issues.

5) Certain songs (sources) may sound too neutral and cause it to sound boring

6) Some may not like all the jacks to be at the front panel


Interesting things to share about O2:


For Opamp rollers (Updated 28Nov15) (Click to show)

Suitable OPamp replacements for O2: JRC(NJM) 2068DD can be replaced with NE5532 or LM4562 according to the O2 blog site.


Edit: Performed experiments with a circle of friends for op amp rolling and the final configuration for O2 is 2x OPA2132PA for the power stage, (the 2 identical chips on the PCB), then a choice of OPA2228PA or AD823AN for the gain stage (the chip near to the volume knob).  The OPA2228PA offers a tube like sound while the AD823AN is the best all rounder for musicality. The O2 is more musical with either new configuration. Optionally, 3 OPA2132PA can be used together. This gives the sweetest sound compared to other amplifiers you can find out in the market, and it may or may not be liked by people. Conclusion: ROLL THE O2 OPAMPS!


Please do note that battery life is influenced by the type of opamp used too. To my experience the OPA2228PA causes the O2 to die faster than other chips I've used.


EDIT#2 (Nov2015): This configuration was used by me for the longest time so i have decided to finalise my configuration and share it. 1x OPA2132PA for gain stage (the chip closer to volume knob) and 2x AD823AN for output stage (the 2 identical chips seen on PCB). The rationale behind the change is because the AD823AN utilises R2R technology and is able to deliver higher current into headphones more efficiently than standard design op amps. That said, don't expect this thing to drive a pair of Alpha Dogs as some headphones are still a little too hard to drive for this amp. (Edit 09/2016: It drives the T50RP Mk3 quite well). Do note that the PA and AN behind the chip numbers are important, otherwise the price of the chips will skyrocket.

Additionally, it is strongly recommended to change the stock batteries to better quality batteries with a larger capacity. It affects the ability of the amp to deliver the current for improved attack and control so your music don't sound bad. I am using 280mAh batteries made in Japan and they last me 9-12 hours fully charged.


O2 can be used as an pre amplifier as it does not change the sound signature much, but it improves the sound stage. Edit#2: Finalised configuration works superb as a preamp, touching up the musicality without colouring the sound.


I hear audible but negligible hiss using my 1964 V2 CIEM with no source connected to the input and volume knob at MAX. It is definitely going to be an audible hiss, but nobody with the right mind will use this amp at max volume to a precious CIEM. With volume knob at 8 o'clock, the amount of hiss will definitely not affect your music.


It would be better to remove the battery if you are using it as a desktop amp. For some reason the sound is different with and without battery mounted.


If you do not like to use the equalizer, then change the interconnect cables to fine tune the sound.

I hope the above review can help!:) 

Please do feel open to PM me for questions regarding op amp rolling for the O2. I'm still active even though I do not post opinions in forums. Will always add on to this review as and when my new equipment comes in.


Pros: Very accurately reproduces the source signal; I guess it could help if the source's headphone jack isn't strong enough

Cons: Could use more power

The objective facts


Background: I listen to music on a higher-end (purchased for $800) Asus laptop, or my Galaxy Note 2 smartphone. With my SRH940s, the phone sounds better in terms of sound signature, but with my Sennheiser HD800s, the laptop sounds better for some reason. But they're both very close. All of these experiments were done listening to music on Spotify 320kbps; that's how I listen to headphones 99% of the time so that's the only way to test for my purposes.


How I tested this: I connected the O2 amp, with low gain, to my laptop's output jack. I set the volume of the O2 amp to its highest level (in other words, I controlled the volume of the music from my laptop's volume meter). The playback was set at 24bit / 192 khz (basic windows audio lets you pick this option). I plugged the HD800 into the amplifier.


The first thing I noticed was that this setup didn't cause the HD800 to be that much louder than it was when I simply plugged it directly into the laptop. In other words, with the O2 amp's volume set at max, and my laptop volume set at 25%, the headphones had close to the same volume as when I plugged the HD800 directly into my laptop and set the volume to 25%. So my laptop has as much power as this thing. Then I turned up the volume to the maximum and used a decibel meter to see how loud the two could make the headphones, and again, very similar. I repeated the above with the phone and got more or less similar results. 


Subjective observations


Next I cycled through a few songs that are very well recorded, to see if they sound better on one vs. the other. I listened once, twice, three times, again, again, focusing on very specific parts of the song, often playing 10 second portions of a song and repeatedly alternating between the two sources. I could find nothing. I would often focus on one tiny detail in a song, to see if the O2 and computer produce it the same, and they could. When the O2 was driving the phone, it had the phone's less desirable sound signature, and when it was driving the laptop, it had the laptop's sound signature, but the O2 didn't change the sound at all. I thought an amp was supposed to improve the sound of my "power hungry" HD800, but this did nothing. However, commenters say that this is exactly how it's supposed to work. In other words, it's not supposed to change the sound at all. So I guess I can't rate it poorly for doing what it's supposed to do.


Conclusion: If you have a severely underpowered source and need power, this will deliver it. But it could do more in this respect. It's not underpowered like some of those pathetic portable amps you see out there. But in my opinion, a $130 desktop headphone amp should provide boundless power, as much as you could ever want. When you buy a desktop amp, you should no longer be concerned about not having enough power. You should be concerned about keeping the volume low so as not to blow out your ears. To provide technical stats, on its website, it states that the O2 can only deliver a maximum of 88mW at 600 ohms. Well what if you're trying to drive a T1 or another 600 ohm headphone? Is 0.09 of a watt enough? Again, it has power, but you might not need the little bit of extra power this provides, and you might be fine sticking with your laptop or phone's amplifier. In light of the fact that it costs $130 and sits on your desktop, but doesn't have huge bountiful power, I'll subtract 2 stars. I'm just not sure why you would buy this if you want a desktop amp.


Pros: Price-to-performance, transparency, custom options

Cons: A bit "boring" by nature


Meant to be married to the OL DAC, the O2 headphone amp matches it in shape, size and simplicity. What’s neat about the O2 is that JDS Labs allows for some simple customizations—you can choose the headphone jack size (3.5mm or ¼”), input type (RCA or 3.5mm), gain levels, and power jack location (front or rear panel), and whether to have it with or without a built-in DAC and lithium batteries for portable use. I went with a rear-mounted power jack and RCA input, so my front panel is nice and simple with just the power button, headphone jack, volume knob, gain button and a red LED power indicator (why red instead of green to match the OL DAC? I don’t know, but it kind of bothers my OCD).

So how’s the little black box sound? Well, it follows the open source amplifier design from the aforementioned NwAvGuy. In other words, it targets benchmark performance at a budget cost ($129). So, much like the OL DAC, the O2 amp aims for transparency. Or maybe a better way to put it is simplicity. 

For those of you that are used to colored amps, there’s nothing romantic about the O2. In fact, it’s likely to come off as a bit dry, a bit sterile, a bit, well, boring, just like the OL DAC. That is, unless your DAC is colored in another way. After all, all the O2 really does is amplify the signal in front of it, which means a great recording, a nice DAC and a stellar sounding headphone or IEM is what’s needed to put you on the path to #AudioNirvana.

The best thing about the O2, sonically speaking, is its versatility. It has the power and dynamic range to drive anything from a sensitive IEM to a power hungry dynamic headphone with authority. The semi-picky Sennheiser HD650, for example, gets plenty loud and hits with modest authority in high-gain mode. The O2 can also play any genre of music well, because, well, you’re hearing the music and the rest of your equipment for what it is. The caveat here is that you need to plan for this. Meaning, the O2 is going to do absolutely nothing to hide poor recordings, and if you favor a darker, warmer sound signature, you’ll want to choose a headphone with those characteristics rather than something analytical like the reference level AKG K701. Or, you could always dabble with some EQ software to fine tune your setup. 

With the OL/O2 combo, which is what I assume a lot of you reading this review are considering purchasing, you’re getting a pair of neutral performers, nothing more, nothing less. This stack isn’t going to romanticize your music collection. Rather, it’s going to allow you to focus on the music and hone your efforts on choosing the IEM or headphone that best suits your musical taste. 

I’ve mainly been running the rounds with the Audioquest NighthawkBeyerdynamic Amiron HomeMeze Audio 99 Classics and Sennheiser HD650. The OL/O2 drives each of these headphones with ease and enjoyment, although none come off as being quite as airy, lush or three-dimensional as when pushed with a powerful tube amp (just my personal preference), the warm characteristics of each is conveyed cleanly and with good. Although these are all darker sounding headphone, I liked the pairings because it brought some balance to the neutrality of the OL/O2 stack. The consistent instrument separation and detail retrieval of the OL/O2 pairing is solid. The sound stage is modest, extending maybe three to four inches out around the head. Stereo imaging is dead center, although I do selfishly lust for a more holographic presentation. As someone who favors really lush sounding gear, this little stack can sound a bit flat to my ears—bass notes hit with impact, but often lack resonance; mids are clear, but are light on warmth; treble is crisp and detailed, and never too brittle, but airiness and texture seem overly controlled. But all of this is inherent in neutral, transparent solid state amps and DACs. Boring can also be better; the neutrality of this combo makes it an excellent setup for audio purists and gear reviewers because it reveals more accurately what a particular recording, headphone or IEM can and cannot do. 

Overall, I generalize the OL/O2 stack as being crisp, clear, controlled and consistent. Sure, it can be boring for those that favor the ooey gooey goodness of lush and distorted tube gear, but boring isn’t always bad. In fact, these little black boxes are probably one of the best places to start for new budget-minded audiophiles looking to learn just what it is they like and lust for. 

JDS Labs Assembled Objective2 Headphone Amplifier

Taken for item page: "The Objective2 (O2) is an open source headphone amplifier designed by NwAvGuy, with emphasis on benchmark performance and low cost. This item includes a fully assembled and hand tested O2 amplifier, ready for use. You will only need an AC adapter.

Model Name/TypeMPNEAN/UPC
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