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. (more…)