I recently bought an USB sound card to do some measurements/experiments with. What I want to do first is to build a simple amplifier for the headphone output.
Therefore I took the sound card apart and traced out the in- and outputs. Interestingly the ground of the female audio jack is not connected to the USB ground, it is DC biased with about 1.67 volts with respect to USB GND (half the AVDD supply voltage).
The problem I now face is that I don't know what to do with the audio jack's ground on my amplifier board. The amplifier board uses the USB power supply and obviously I can't connect the audio jack's ground directly to the USB ground...
Can I just pull the audio jack's ground to USB ground via a resistor?
Here is a simple schematic of the sound card (the male audio jack should be female actually):