One of the reasons I wanted to test this was I was getting some shrill high frequency dominating the sound with the presence control at max settings.
Starting with the GFB engaged, I tested the output with the presence turned all the way down and here is the shot:
Here is the presence at full:
Here is the output with no global feedback:
As an aside, I'm becoming really underwhelmed with the resolution on my scope and wonder if I should be doing this on an analog scope.
But my questions are; first, since this is a guitar amp, what kind of realistic phase compensation do I need to concern myself with? I was thinking of adding the 8k2/200pF circuit shown in parenthesis on my schematic.
Second, what kind of linearity should I be looking for? What can be inferred from the wave forms above?
Last, any thoughts on taming the HF in the presence circuit? I think more useful would be some kind of actual small bandpass filter to shut actual mid frequencies for an active mid boost.