I'm surprised at the low number of posts here for a such commonly recommended amp. If anyone is interested in some cheap but effective mods to improve the sound of the o2 this post can help but keep in mind this was done with the desktop version of the amp with mains powersupply not batteries. It would be recommended that you have some basic knowledge of electronics.
The o2 amp never sounded good to me and I do not believe that this is what 'transparency' sounds like, if you like the sound then more power to you but If you are disappointed and want an amp that sounds more vivid, more detailed and natural then I recommend these mods. Testing was done by ear only, objective blasphemy, I know, but I can guarantee that some of the greatest with the he560 which had perhaps some of the worst synergy with the o2 out of the headphones I used with it (hd600, hd650, he400i and ath-m50), If the he560 could sound good with o2 then I knew I was doing something right
Surprisingly the biggest problem with O2 lies in the power supply capacitors, it is lacking in some capacitance. I tried many different variations of caps and eventually found the perfect balance, beyond a certain point more caps either made no difference or actually made the amp sound too laidback and smooth in the bass and mids, all 'slam' and no 'punch'. very high capacitance can also put a strain on your power supply.
The caps I recommend are:
2x 100uF Electrolytic
2x 47uF Electrolytic
2x 10uf Electrolytic
4x 0.1uF (100nF)
polypropylene Film capacitor
optional 2x 4.7uF Electrolytic
optional 2x 0.47uF (470nF) polypropylene film capacitor
Any caps will do as long as they are rated for at least 35V to match the stock 470uF caps.
Be sure to keep the dimensions of the caps you buy in mind, even with the same voltage rating and capacitance they can vary greatly in size
To connect caps you must solder 3 wires onto the existing caps on the underside of the PCB for positive, negative and ground. there are 2 solder pads for the positive side and negative side and 4 solder pads for ground marked by the black dots, you can solder each wire on any of these pads. the new caps are paired in series and wired in parallel with the current caps, here is a diagram for clarity:
Unfortunately there isnt enough room for the new caps inside the standard O2+ODAC enclosure.
I have the new caps on a small breadboard, the wires feed out from the back and are connected to the breadboard, not elegant but totally reversible and easy, well worth the cost of a few capacitors.
if you like the results and want something more robust my suggestion would be to solder the caps to some perfboard, drill 3 small holes on the top of enclosure, 2 for mounting the perfboard and 1 for the wires.
if you are using the standalone O2 then there is room for a piece of perfboard in place of the ODAC inside the enclosure, keep the height restriction in mind when choosing caps. secure the perfboard with a nut and bolt on the outer mounting hole and feed the wires up from the bottom of the pcb through the inner mouting hole:
https://www.jdslabs.com/images/products/35/full/O2-pcb-assembled.png.
Op Amps
Changing the op amps is also a good idea regardless of what people say. The 4556 and 2068 might be technically good chips but they really are some of the worst sounding ones I listened to, any well regarded audio op amp will be a big upgrade. if you are skeptical you can pick up a couple of NE5532s for a few dollars and swap out the 4556s, it will sound smoother and fuller.
The LM4562 is the best price to performance chip Ive tried, one of the most clean and detailed chips and only around $5 each. It works very well in place of either the 4556 or 2068. if you prefer a less analytical sound the OPA2132 could be a better choice but I havent tried it, only the expensive OPA2107 which is a very good chip but not to my taste and shouldnt cost more than $10
most, if not all bipolar audio op amps such as the LM4562 and NE5532 will work perfectly fine in the o2, they are very stable and versatile. I tried many other chips including non-audio op amps and JFET op amps with zero problems, some sounded very good aswell.
After modding you will be left with an amp that begs for you turn it up, the exact opposite of the stock o2 for me.