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Desktop Amps item created by TheOneInYellow, Jan 7, 2012
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 Nighthawk, Beyerdynamic Amiron Home, Meze 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.
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 - Size, performance, facilities(custom model)
Cons - None
I bought this to listen to my vinyl on my Hifiman HE-400 phones. It does a great job of driving them with a very neutral and I can hear detail that my £5500 worth of amp and speakers don't pick up. The JDS Labs would cost $191 shipped to the UK. I bought a really nice version from a UK based eBay supplier (macwidow2011) - Swiss made board, neutrik connections, ALPS pot, RCA inputs and outputs with 1/4" jack - all for £109 shipped.
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 - Ability to build yourself, modification options, case engraving, customer support
Cons - Would love for the LED to indicate low battery/charging
You've already heard that the sound quality of this amp does not match its (low) asking price. You already know that it's one of the best SS desktop headphone amps you can get for your cans at any price point, really. So why am I writing another review on an amp that you KNOW is going to be awesome? Well, my review is for those who like to build their own amps and JDS labs is a company that "gets" the DIY movement and have made NwAvGuy's design really easy to build for those of us who like to roll our own gear. My review is meant for those who might be interested in ordering a DIY kit but who might not be sure of what they're going to get:
Remember back in the day with the O2 was new and there were people scrambling to put together Group Buys for PBCs and components? Well, those days are OVER, thanks to JDS Labs!
All the components come in their own plastic baggies, labeled clearly so you know where they go on the PCB. The PCB itself is of the highest quality you can expect as are the components themselves. All op amps and mosfets come in sealed anti static bags. For me, this kind of kit is a dream - I don't have a lot of time (or patience) to wait for people to put together Group Buys for an amp like this to save a few bucks. You would think that JDS labs would be marking up the cost of the DIY kit for this convenience, but frankly, they deserve a lot of credit for being able to provide the DIY kit for this amp for almost as much as it would cost you to go and buy the parts at a local electronics store, so the small "up charge" that you're going to pay isn't much at all when you consider how much time you're saving if you were to decide to spend a few days gathering the components yourself (and it might actually cost you more considering shipping charges and/or gas).
I built mine using Cardas quad eutectic solder (the ones JDS sells are built using lead-free solder). I'm sure lead-free solder is fine, it's just my own personal preference to use quad eutectic solder for all my builds (and that's the nice thing, isn't it? You can use whatever solder or equivalent components of your choice for builds like these if the ones included aren't to your liking). It took me about 3.5 hours to put together the kit (PCB board and components, $60 at the time of this review).
At the time of this writing, there are no detailed instructions on how to put the kit together yourself on JDS Labs' website, but you don't need any really. Everything is labeled on the PCB itself, and everything is separated and labeled in baggies for you. There is however, help on their site for the various mods you can do (gain, RCA connectors, rear power, etc.) You do need to know how to install components for a build like this (like knowing which is the positive lead on a capacitor and which is pin 1 on an op amp, etc.) Adequate soldering skills are also necessary and I highly recommend a variable control soldering station for this kind of build too.
JDS Labs' customer service is excellent. I've received responses to emails within a short amount of time and I've got nothing but great vibes from them with each response. Although I didn't have any questions concerning the build itself, they have been very responsive and helpful. I ordered an enclosure from them which I later decided to get laser engraved and they've been very helpful in answering file format questions, I'm sure if you have any issues with their kits that they'll be just as helpful.
I'm so happy with my kit and interactions with JDS Labs over this Objective 2 that I'm going to be ordering more kits to build soon!
Pros - (in my case I bought the kit)DIY. Perfect. Just the perfect wire-amp with no coloration or issues.
I own the desktop version of the O2. What to say? It is just perfect in every sense. No coloration , no noise, great power, best tecnicalities out there. BEST VALUE EVER. Endgame amp.
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.
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
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 - Flat sound. tight bass.
Cons - Charging mechanism is pathetic.
It lives upto its name. This ne is a gem as far as neutrality is considered most neautral Amp i have had or tried so far. I have tried HiFi man EF5 Fiio E11, E17 ,little dot mk2 etc. and found them clearly sub par comparing the bar this product has raised for its price. The immediate this one can notice is great improvement in both treble and Bass. Bass is quite tight. Treble is cold and accurate. Might sound little harsh for headphones like ATH50 but suits well enough for HD650. Wanted to try out well known tube amps especially Schiit Valhalla, Bravo audio ocean and the HD650 specialist (as per many folks on forums) Bottlehead crack. Unfortunately they are not easy to find in the part of the world i live. and i am not much into DIY either.May be someday i'll get my hands on them and then compare the famous tube sound signature with this little beast's accuracy in terms of SQ and see what suits my ears more, accuracy or the warmish (yet close to accurate) sound. For now i'd safely say this is best in class Amp and a great value.
Let's look at its shortcomings.
1. the charging mechanism has not LED indicator to show whether charge is full or not or whether it's charging or not for that matter. It gets really annoying that i have to keep it off while it's charging and yet i don't know whether it is actually charging. It in fact most of the times is charging unless the heavy adapter has lose connection(it is quite possible as the adapter weighs more than O2 itself.) or you forget to switch the button on (like i have done couple of times )For a 130$ product this wasn't a big feature to ask.
2. Gain switch is useless IMO as it totally screws SQ I never needed it though, HD650 can get enough juice on 2.5X gain setting.
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.
(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.
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:
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 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:
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.
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.
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:
Spoiler: For Opamp rollers (Updated 28Nov15)
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.