Pros: Good sound to price ratio, Very versatile for modding.
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.
First few things to start off with here that i believe will be important:
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, and there goes the poison .
*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, so i decided to compare the T1's amp section vs the O2.
The O2 has a tiny bit less bass punch, less 3D vocals 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.
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 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 microdetails, 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.
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 op amps used at output to protect expensive IEMs.
8) Very powerful (up to 613mW into 33 ohms on battery).
9) Battery can be changed out when it runs out of juice.
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) nearly inaudible hiss. Hissing sounds may come from sources instead.
Cons:Warning: Spoiler! (Click to show)
1) Short battery life - ~8h.
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 disgustingly bad
5) Certain songs 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:
Warning: Spoiler! (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!
O2 can be used as an pre amplifier as it does not change the sound signature much, but it improves the sound stage. Edit: This only applies to the stock O2. Modified O2 has too much coloration.
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 PM me for questions regarding op amp rolling for the O2.