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 here. Do note that modding the O2 is done at your own risk, Don't hold me accountable and say you 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. (wow, this Accelerator is such a nagging person :P)
Some prologue of my sound preference (as a reference) (edit 09/2016):
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 DIY kimber's VS series cable as RCAs) to TU8200 tube amp driving Grado SR 125i or T50RP MK3. 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, lesse 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.
Edit 09/2016: So far after more than 3 years, I haven't found a portable amplifier that could rival the sound:price ratio of this opamp modded amp. Given such products' lifespan is usally 1 year or so, it is really quite something. Really good news for my wallet.
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.
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.
Pros: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.
Cons: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!
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.