Is this powerful enough to comfortably drive the 6XX, or should I go for standard gain O2 amp only? Also does it make a difference to plug in DC power adapter when powered by usb?

100% upvoted

Jul 11, 2021
Yes you will want this or another amp for the 6XX headphones. A normal device (PC, phone, whatever) will have to "max out" the volume to get it to a barely-acceptable volume. The amp in the o2 dac/amp has more than enough power for whatever volume you want. No you cannot use the device with just DC. It's super annoying, and I just had to buy a new plug after moving to a 240v country, but the open source patent doesn't allow anyone to modify the design to something more modern and reasonable.
Jul 16, 2021
What is the actual input voltage required?
Jul 28, 2021
(Going from memory so numbers might be a tad off - and I'm assuming the DAC gets its power from the usb cable) It semi depends on the max output current of a adapter as well, but 14 to 15 VAC is a good amount. Probably ideal adapter specs: 16VAC is also fine but the higher you go risks over heating the psu circuit. Too high and the circuit gets fried. The adapter's output current can safely be 1000's of amps and it wont' break anything but your wallet. The minimum current I'd say is around 320mA (140mA for each channel plus a little more for essentially just energizing the circuit). You can actually go lower than 320 but the O2 won't reach the rated power specs. 12VAC might work if it has 1 or 2 A output current but I wouldn't do that unless that's all you can find.