I know that audio signals are AC signals (negative/positive voltage).
Assume a typical Fuzz (guitar pedal) effect circuit below. It seems signals stay between 0 and 9 V (9V is the power/battery voltage).
Also, when I look at some through an oscilloscope however, the GND is considered 0 V, and the voltage goes from 0 V to some mV/V, only positive. I used for this a line level signal (not guitar audio output, since I don't have a guitar myself).
So I'm a bit confused about the 'negative/AC' voltage. Can someone clear that up?