I'm modifying a guitar effects pedal circuit, and in the circuit the designer has a potentiometer wired as a rheostat in the feedback loop of an op amp for a gain control. There's also a capacitor in the circuit creating a low pass filter. I don't really like this arrangement, as changing the gain of the circuit will affect the circuit's frequency response as well. Is there a better way to use a potentiometer in the feeeback network that would allow the cutoff frequency of the low-pass filter to remain constant, while still having the gain be somewhat adjustable? Some kind of T network, perhaps? Or will having any kind of variable resistance in the feedback network always affect the frequency response in a low pass configuration?

 

The usual way is to put Rs and Cs around the opamp to create the filtering, and then put the gain control as a potentiometer fed from the output of the opamp, with the bottom end grounded.

The wiper position controls the variable level. If you want a lower output impedance than just the pot will provide, add an opamp configured as a voltage follower between the wiper and the output connector.

Another way is to move the pot to the input of the filter stage; that way the filter stage will have a low output impedance, and the pot will not effect the frequency response of the filter stage, just the level.