I'm modifying an audio circuit to add more tone controls, and found a method I want to try.

It involves separating the wanted frequencies into a boost circuit and the less wanted frequencies around the boost circuit.

Instead of filtering them to ground, I've figured out how to do this using a inverting op amp to boost, but the circuit I'm trying to modify uses a non inverting op amp setup and I couldn't find out how to do this.

My idea was to do it using a filter like R4 and C1 in the picture, but I am concerned that this path might create a second feedback loop to the non-inverting input and I dont know what that would cause; So I thought I might ask here.

  • Is this the way to achieve what I'm looking for?
  • Am I completely wrong?

The schematic is just for proof of concept, in reality I will be using three parallel filters to separately control low, mid, and high frequencies.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ What exactly is a "tone control"? Also, "boost circuit" has a pretty well-defined meaning in electronics that is very different from what this is, which appears to be a filtered amplifier of some sort. (n.b. I work pretty exclusively in power electronics. It's possible that within the context of audio the words mean something very different.) \$\endgroup\$ – Hearth May 25 at 13:31
  • \$\begingroup\$ A tone control is a circuit that either cuts or boosts/amplifies certain frequencies of an audio signal, for example if you want the 'tone' to be more 'bassy', you either cut the higher frequencies or boost the lower ones, most eq's use a combination of bost cutting and boosting, cutting being filtering to ground, but since i am not looking for that extreme of a difference and cutting generally means that you also lose a bit of the wanted frequencies, so thats why i wanted to try tone controll using just boost \$\endgroup\$ – Nook May 25 at 13:45
  • \$\begingroup\$ OA2 is a inverting filtering op amp, it lets frequencies of more than 884hz around the op amp, and frequencies below that get amplified. The idea came from this video: youtu.be/4izZ-bwaTMY he calls it a 'treble bypass' which allows high frequencies to pass around the clipping/amplifying stage, but he uses transistors and the circuit I'm trying to modify has a non inverting op amp 😅 \$\endgroup\$ – Nook May 25 at 13:47
  • \$\begingroup\$ Adjusting R4 will have no effect. OA1 will just act as a unity buffer for all frequencies. Since the voltage at the non-inverting input and the output are the same no current will flow through the R4 C1 combination regardless of frequency. Because of the finite voltage gain of OA1 this will not be quite true as the frequency is increased but still to no useful effect. \$\endgroup\$ – Kevin White May 25 at 23:56
  • \$\begingroup\$ And if i where to replace R2 with a highter value, to raise the gain above unity? Then if i get it right, the voltage at the output would be highter than on the non inverting input 🤔, but then wouldnt the current tend to flow through c1 and r4 back to the non inverting input? Because thats the opposite direction of how i want it to go \$\endgroup\$ – Nook May 26 at 7:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.