AMZ presence control frequency question

[nepalnt21]

http://www.muzique.com/lab/tone3.htm

quick question regarding this page, why is it that as the values of the components change from version to version, the center frequency of the mid boost/ cut stays pretty much in the same arena? was it deliberate? how can i manipulate that center frequency?

[GGBB]

Simplifying the BMP tone circuit, what you basically have is two RC filters - one high-pass and one low-pass - with a passive blend control mixing the results. They are the first cap 4000p followed by the resistor 22k to ground, and the first resistor 39k followed by the cap .01u to ground. The knee of any RC filter is determined by the product of R and C: 1/(2*Pi*R*C). Math says that you can get to the same product using any number of multipliers (mathematically an infinite number) - for example 100 = 4x25 = 10x10. So in an RC filter, you can increase (double/triple/quadruple/...) the resistance (or capacitance) and decrease (halve/third/quarter/...) the capacitance (or resistance) by the same factor and end up with the same knee frequency.

This roughly consistent center frequency is intentional - this type of tone control generally works "best" with the center within a somewhat narrow range. Too high and the bass side starts affecting the mids too much; too low and the treble side starts affecting the mids and lows too much. The "flat" sound is also affected since flat is really a mid scoop, and shifting that scoop too low or high doesn't sound "right".

You can manipulate that center frequency by adjusting both RC filters in the same direction and by the same amount. For example double both caps and the center frequency is halved or moves one octave down. You can manipulate the resistances to achieve the same effect, but note that resistances impact attenuation and the tone pot value has an effect, so the tone pot value should be changed by the same factor as the other resistors. It's generally easier to play with cap values to adjust frequency than resistance values since we have very few steps available for pot values.

[nepalnt21]

that is great! thanks so much for the response