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Thanks for the feedback guys - I'm perfectly happy with 'it works well'. We have enough amps that 'make the magic smoke come out of your headphones' on Head-Fi .....
Well, here's another vote that it works well.
I bought mine from aerohoff at diyAudio, since I've never soldered before, but it was everything I was expecting so far. I don't have any high-impedance or low-sensitivity headphones to test yet, so I have nothing to say regarding the improvement over unamped listening. I'll be able to informally test that once my FA-011's arrive, since they require an amp for maximum performance.
For anyone needing more, I think the O2 is just difficult to subjectively review in detail, because it's easier to go into detail about coloration than lack thereof and neutrality:
What I can say is that my low-impedance HTF600's and Re-ZERO's seem to sound very similar to the way they do without the amp. The O2 actually might be making them sound a bit clearer, but if that observation is "real," it's probably because the O2 lets me use full volume output from my computer instead of using digital volume controls to drop 40 dB (which loses tons of precision). I definitely can't detect any coloration, noise, or distortion though.
Still, that's pretty much to be expected from any O2 that's built properly: The O2 measures well by design, and any amplifier that measures well across the board will necessarily sound the same as another that measures well across the board. (If the load doesn't require amplification, well-measuring amps should also sound the same as an unamplified signal, once volume-matched). A well-measuring amp like the O2 is inherently neutral, which makes it more of a reference point that colored amps can be contrasted with. That's different from saying "all amplifiers sound the same," because the vast majority don't, but that's a direct result of coloring the signal in measurable ways. To say otherwise would imply that electrical signals have esoteric (or magical) properties that can only be detected by headphone or speaker drivers in combination with the human ear, and which cannot be detected by any other objective tests or instruments. (That doesn't mean we can directly measure a complex phenomenon like soundstage, but an amp's effect on soundstage still results from the interaction of simpler, measurable characteristics like distortion and crossfeed.)
Some people actually do believe in "magical" signal properties, but this viewpoint essentially renounces the possibility of a true high-fidelity amplified signal or a pure "wire with gain amp." By extension this viewpoint would imply that headphones cannot be tested or measured on their own terms, since their sound changes from amp to amp, and no amp is objectively "correct." At the same time, no amp can be tested or measured on its own terms either, because the sound of an amp definitely changes depending on the load (headphones). In short, if you believe that an objectively correct amplifier cannot exist, the O2 shouldn't hold any special interest for you. It will just be another amp to test in combination with your particular headphones, so you can see if you personally like the combination of the two.
I fall into the camp that believes that an objectively correct "wire with gain" amplifier is possible, and that the O2 has accomplished that goal. Aside from curiosity, there are two major reasons to specifically want such an amp:
- You may want an amp that delivers a high-fidelity amplified signal, regardless of whether genuine high-fidelity is actually your favorite cup of tea. In this case, subjective impressions will be irrelevant to you, because objective measurements have the final say on whether an amp is high-fidelity or not.
- You may be looking for sheer audio enjoyment like most, but you'd rather limit your variables to just testing different headphones instead of testing different combinations of amps and headphones. (It's definitely cheaper. ) In this case, you necessarily believe that listening to your headphones through the O2 means listening to your headphones as they really are...so it would be a bit silly and redundant to subjectively comment on the amp's sound by itself. Unless/until you become convinced that the amp is coloring the sound, there's not much to say beyond, "It works, and it sounds like my headphones."
Finally, a third camp seems to concede that a correct "wire with gain" amplifier is possible and that well-measuring amps will sound the same, but they've argued that the O2 designer has only published the results from a few handpicked objective tests. These criticisms seem to be vague and speculative to me though, rather than genuine calls for specific additional testing. (I could be mistaken, but they seem to come from a persistent emotional disbelief that an inexpensive amp could actually do the job correctly.)
Most O2 owners will ultimately fall into the second camp, so that should help explain the relative shortage of subjective impressions. It sounds as neutral as it measures (unless you overload the input stage), so there's not much more to say. It doesn't add extra warmth or fullness to my music, but it isn't supposed to. It also presents the soundstage "as is" rather than augmenting it. At the same time, my expectation bias will make me less sensitive to any kind of coloration anyway, so that just gives me even less to say from the perspective of subjective sound quality.
Most O2 owners probably fall into the same category.
Short of the expensive Violectric amps or the Benchmark DAC1's built-in amp, I'd say the O2 is basically the definition of a neutral reference amplifier. (It has to be, unless the objective measurements are lies, omitting something, or unable to capture mysterious or magical properties of an electrical signal that somehow influence only drivers and ears.) My ears agree with the objective measurements based on the only test I can perform, which is comparing easily driven headphones with and without the amp. You're unlikely to get reliable subjective impressions with more detail: If you read impressions that say it's too cold, it probably means the reviewer prefers warm amps to neutral amps. If you read impressions that say it's too thin, it probably means the reviewer prefers full amps to neutral amps. (If you read impressions that say it isn't neutral, it probably means the reviewer is using the wrong reference point for neutral, unless something just went wrong with the build process.) The problem is, these impressions basically say more about the reviewer's preferences than the amp itself, and they'll be meaningless to you unless you know how your preferences compare with the reviewer's.
Bottom line: If you want a stress-free neutral amplifier, you'll like the O2, and you won't have to worry about how it might be altering your music. If you believe in the existence of a neutral amplifier but don't know whether you'd like it or not, the O2 is a great reference point, and it's the only one that won't cost you an arm and a leg. If you already know that you want the fullest and most "euphonic" sound possible, the O2 definitely isn't for you, and you should look for amps with harmonic distortion and crossfeed instead.
