Tonestack Values and Impedance

[Marshall Arts]

I am experimenting with tonestacks and found something interesting in that process, but unfortunately, I don't fully understand it. I will read more about passive filters and the theory behind them, but maybe someone could give me an easy explanation for this (questions below).

On the Lab L5 Amp, I found the following tonestack:



Compared to a fender twin tonestack (below), the values for caps are higher and the values for resistors/pots are much lower:



I found an explanation an Aion build doc for the values in the Lab L5:
"Since this portion of the circuit operates at very low impedances only achievable with op-amps (and certainly much lower than a tube amp), the resistors and pot values are around 20% what you'd see in a F/M/V tonestack while the capacitors are multiplied by five to compensate. The tone controls are identical to the classic setup in frequency and behavior."

So, the values are different, because the output imepedance of the first op am stage in the L5 is obviously lower than the output from a first tube stage in a Twin Reverb (which in my understanding is a standard triode gain stage with rather high impedance).

But even in a Marshall-Schematic, where the tonestack is behind a cathode-follower (low impedance), the values are similar to the high-impedance fender schematic. Here is the Marshall tonestack I am talking about:



Also, looking at the ROG approach (and many other circuits with tonestacks following op-amps), I am not sure, what values to start breadboarding with (I dont fear the work, but would like to understand first).



So, how do I calculate the correct values for caps, resistors and potentiometers? Which values would you choose for a classic fender tone stack (http://www.guitarscience.net/tsc/fender.htm) following an op-amp buffer?

Thanks!

[thomasha]

Quite interesting,

I had a similar problem with the dr. boogey pedal, the initial versions had weird values at the tonestack, and this was also to compensate for the changes in the circuit, where the original uses tubes and the pedal uses fets. I think there was a simulation, or at least some comparison using the tonestack calculator, where you can  try to match the curves.

The supro tone control and the big muff have the same problem, values are quite different too.

I assume that with the first circuit the loss should be higher due to the low pot values, but at the same time the caps are larger, which would bypass a lot of signal through the top.
In the end, if you analyze the filters separately you will see that the cutoff frequency is a function of the resistance and capacitance, when increasing one you decrease the other to compensate. This is usefull if you want to reduce the loss caused by the tonestack.

You also need to think of the circuit after the tonestack, if it's an opamp the high impedance helps, if it's another triode, as in the marshall amp, the input impedance is lower, and more signal returns to ground, than enters the next stage.

Check this one: https://elektrotanya.com/PREVIEWS/63463243/23432455/fender/fender_frontman_212r_sch.pdf_1.png
it's a fender solidstate amp, it has higher pot values, but just in front of the next stage there is a 22k and 100k voltage divider dumping part of the signal to ground.