John Phillips wrote:
"In some amps it's more of an upper-mid EQ control, but in the classic Bassman-derived/Marshall circuit it's a negative feedback loop high frequency control. Turning the presence up takes highs out of the NFB loop.
What this means is that it doesn't so much control the amount of top-end - although it does to some extent - but that it controls the dynamics of the top end. NFB is like a 'regulator' - used to smooth and even out the response - so if you take some frequencies out of the loop, these become less restricted and 'peakier'.
Turning up the presence not only makes the amp brighter, it makes the brightness more spiky and dynamic than the rest of the sound too... which makes it very much more obvious and helps the amp cut better in a mix, and is why 'presence' is such a good name. It doesn't affect any particular frequency, since it's done with a single cap - it simply has more effect the higher the frequency.
The fact that it works on the NFB loop (which is part of the power stage) also explains why it doesn't seem to do all that much at lower volume when the power stage is clean, since the NFB loop is working on an undistorted sound with fairly even frequency balance, but once the power stage goes into clipping this generates a lot more highs, and the presence control become far more effective - on classic Marshalls, it basically becomes the only control that does much when the amp is fully cranked
."
