Re: Presence knob
The presence control is part of the negative feedback circuit. The negative feedback circuit reintroduces the audio signal 180 degrees out of phase back into the amp and this tightens up the overall sound of the amp, limits distortion, etc. When you turn up the presence control you actually send the high frequencies of the negative feedback circuit to ground so they do not get reintroduced back into the main audio signal and so they do not cancel out the in phase high frequencies in the main signal...the result is that the amp gets brighter. Lew