Pros: Good sound to price ratio, Very versatile for modding, Delivers the sound without coloring, enhances soundstage, decently powerful.
Cons: short battery life (8-9 hours), long charge time (12 hrs using 18VAC, 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 really depends on the audio 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, and there goes the poison .
**DX50 is running Firmware1.2.8, DX100 is running FW 1.2.7. Both DAPs are using the same songs, FLAC quality converted from CD. Using silver plated copper interconnects. Headphones used is Custom One Pro.
Comparing O2 with other amplifiers:
I have the Aune T1, so i decided to compare the T1's amp section vs the O2.
The O2 and T1's on board amplifier are very close sounding to the point that you would not hear much difference unless you really listen in great detail. The O2 has a tiny bit less bass, less 3D vocals and sounds slightly less engaging. The highs have not much noticeable difference, probably due to the headphones i'm using. In this case, The former is clearly the winner, but hey this is a portable amp, and it performs almost as well as a small desktop amplifier .
**Source used in this test is in FLAC format, digital from laptop, using the Aune T1 as a DAC for both amplifiers. The tube on T1 is a NOS Philips JAN 6DJ8. LO cable is a 5N silver RCA-3.5mm cable for the O2. Headphones used is Custom One Pro.
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 overall, including build quality, sound and price to performance:
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) 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 modders)
7) Current limiting OPamps used at output to protect expensive IEMs.
8) Very powerful (up to 613mW into 33 ohms on battery), can also drive nearly any headphones out there to a decent listening volume.
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.
1) Short battery life
2) Source dependent. = Need to get a good DAP or DAC
3) Big and bulky, not pocket friendly
4) On high gain, it sound's 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:
Suitable OPamp replacements for O2: JRC(NJM) 2068DD can be replaced with NE5532 or LM4562 according to the O2 blog site.
Plugging in Fitear TG334, at completely 0 volume, you can still hear the music playing through the O2.
O2 can be used as an pre amplifier as it does not change the sound signature much, but it improves the soundstage.
O2 tends to clip easily when a low quality source (eg. a phone) is used.
If you do not like to use the equaliser, then change the interconnect cables to change the sound signature.
I hope the above review can help!