I very quickly drew the negative feedback filter controlled by the presence control when using Vintage mode on a 2001 3 Channel Dual Rectifier. The red line represents the -3 dB half boost mark from the point of the most filtering. I tried drawing the entire phase inverter, but it didn’t make a difference on the plots, so I removed it and kept it simple.
R2 is being instructed to act as a variable resistor with 11 points to represent each 30 degrees of travel from 0 -10 on the control. Anytime a part of the plot goes below the red line, the feedback is lessened by quite a bit and those frequencies pass the phase inverter more easily. The maroon line on each picture is representing the control being at maximum and a massive amount of the frequency band is passing easily.
Aside from the stock values of 25k and 100k, I tried 10k and 50k as replacement values. 10k actually gives a lot of flexibility, but doesn’t end up being able to make a flat response when the control is turned all the way down. That may, or may not, be a problem for a person, but it might make getting more treble a little easier.
I haven’t yet plotted the Modern/Raw attenuator circuit to see what the differences are, or what changes would do; I’ll try to get to that soon.
I hope this helps.
- Presence and Voicing of the Mesa Boogie Dual Rectifier 3 Channel Amplifier
- Mesa Rectifier Presence Controls