Op amp feedback network question

Discussion in 'The Projects Forum' started by bitrex, Dec 17, 2009.

  1. bitrex

    Thread Starter Active Member

    Dec 13, 2009
    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?
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    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.