Ok, before I start, and because two other threads that discuss the O2 have been locked, I'd like to say let's keep this fun, it's audio, let's enjoy it! Let's keep arguing about whether or not to do something has merit or not, and if someone is as dumb as tree moss for doing such a thing, to a dull roar. (eg. "Why did you replace partA with partB, do you even know how electricity works, doofus?")
Alright, well I've built two separate O2's, one that follows the letter of the BOM, and one that has some very slight modifications (ceramic capacitors in feedback loops replaced with polystyrene caps) which will also become my guinea pig for as long as my interest holds. I also built myself a Beta22 almost a year ago, three channel, two cases.
Now I've spent a lot of time with my Beta22, and completely love it, loved it from the first time I played DSOTM through it. The O2 on the other hand, I was not blown away by, I found it to be cluttered around 80Hz (open E speed picking), and a few other issues, but I don't hate the thing, it is my portable headphone amp of choice at the moment, and I've been spending a decent amount of time listening to it.
What I said in the previous paragraph has caused a fair amount of bickering in other threads, something to the effect of me being biased, imagining things, or preferring the sound of a colored amp (I'm fine with that). Not the worst insults ever thrown at someone, but I did take issue with the implication that I don't know how or what instruments should sound like. With the Beta22 snare drums sound like standing next to someone hitting a snare drum, not a recording of a snare drum, speed picking on a guitar's open e string has dynamics and is not just turned into a buzzing guitar. To me that's the difference between the O2 and Beta22.
Ok ok, enough about me...
Usually my minor gripes were responded to with some iteration of, "Do some measurements!" Well now that I've built my second O2 I've decided, Hey WHY NOT!?!?!?
First test I've done is Total Harmonic Distortion + Noise (THD+N)
Test was conducted with an NTI Minilyzer and Minirator.
Minirator was producing a a -10dBu sine wave, all amps volume knobs turned up to their maximum.
Numbers in a grid like fashion (color separations are octaves):
Nothing too crazy yet, the O2 fares better than the Beta22 up till 160Hz, then they're pretty much even all the way till 20kHz. And the polystyrene caps seem to have no effect on THD+N.
Now I don't have a pile of test gear laying around my house, so I have to figure out what I have at work (I work for an AV company) and how to use it. The more equipment I find, the more tests I'll do. And as I am admitting right here and now, I may misuse a piece of test gear, if you feel that I am, and that it's corrupting my results, please let me know, I'll re-test!
Got suggestions for tests? I'll do my best, bare in mind I don't have deep pockets for purchasing things, but I am pretty handy with a soldering iron, so if I can build something to use to test, let me know, I always like making new things!