Excuse my ignorance on this. I want to do boost the audio output of the Pisound micro with a TL072 opamp. What voltage should I use to power up the opamp, 3V3 or 5V?
What are you trying to do in particular? What is the differential output driving? The Pisound Micro’s output should be at ‘line level’ which is good enough to go to other audio devices or into an audio amplifier that then goes to headphones or speakers.
For TL072 you have to use +5V and -5V at least, but up to +12V and -12V seems to be recommended, depending on the gain you need.
For audio purposes, you should derive your own clean +5V and -5V using voltage regulators and some DC/DC converter like a charge pump inverter.
Alternatively, there should be some so called ‘rail-to-rail’ opamps that can amplify signals even if it’s working close the provided supply voltages, that could be usable with 0V and +5V range.
The differential outputs of Pisound Micro are AC coupled, so each + or - pin goes ± ~1V RMS or ~1.41V peak to peak, around you bias point. When differential signal is converted to single ended, it’s already ~2.82V peak to peak, so it’s close to +3.3V limit, that’s why to amplify it further, better to opt for +5V rail-to-rail op-amp.
You’d have to bias the AC coupled signals at 2.5V, for this, another op-amp and some resistors or a dedicated Virtual ground generator IC would be needed.