Just found these in an empty shoebox i just opened. Getting the urge to build them again!
Most of the components for the O2 are still available and/or can easily be substituted with a part of the same characteristics the only thing that is a problem to source for the original layout is the
DC power socket AFAIK. One thing to watch out for IIRC is that the LED is used as a voltage reference by the amp so you can't just change it out for any random LED unless you have the knowledge to refactor the circuit (I believe it also uses an AC adapter rather than a DC one which is worth nothing as you can kill the op amps plugging a DC one into it).
Thought I'd link mine here as well though this thread seems a bit abandoned
always interesting to see what people are upto.
My own take on a buffered cmoy (with quite a lot of researching other people's designs):
(Really hard enclosure to take a good photograph of that looks as good as it does in the flesh)
(Work in progress shot of the circuit inside)
OPA2228 gain stage into a NJM4556 buffer, power supply/rail splitter aside its loosely similar to the O2 in concept but with a fully regulated/filtered dc virtual ground instead of the ac-ac setup on the O2. All power supply capacitors are Panasonic FM series, anything near the audio path Nichicon FG (no electrolytics in the signal path) - dc blocking caps are Cornell Dubilier 0.47uf Polypropylene (not cheap for a reasonable quality 0.47uf in that physical size - miss my Nichicon Polypropylenes but they took up half the circuit board and then some).
Drives my HD600s very nicely - the 2228 is good for longer listening sessions but if I want higher quality audio its swapped out for the AD8066.
6.3mm and 3.5mm outputs on the front (the 3.5mm is disabled if something is plugged into the 6.3mm jack) and RCA inputs on the back.
EDIT: I am NOT an electronics engineer or audio engineer so be nice if you spot any glaring errors in the pics, etc.
