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I am trying to understand the differences in operation and frequency response of the following tone control circuits. This question only applies to the sections circled in red. The rest of the schematic was included for context.

Circuit A:

A

Source: General Guitar Gadgets - Design Your Own Distortion

Circuit B:

B

Source: Wampler Pedals - How to design a basic overdrive pedal circuit

I know that these tone circuits are RC filters but I do not understand why they are different and how the differences affect the sound. This is my first question here. I have just begun tinkering with effects pedals. I wish to build my own custom effects one day.

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The first schematic is horrible and may well lead to unwanted op-amp oscillations due to it creating instability when the pot (R6) is wound down to low ohms. Also, adding those diodes across the op-amp output is a perfect no-no for anyone who has studied op-amps. It's asking for trouble because an op-amp output should not be loaded this way. It looks like the circuit was created by someone who didn't know what they were doing and settled on trying various things out without care and attention.

The 2nd circuit is marginally better in that it doesn't provide the facility to put diodes on the output but, might still cause instability as the pot (Tone 10 kΩ) wiper is wound to the left terminal. Look in virtually any op-amp data sheet and they will tell you that putting a capacitor to ground on the output is asking for trouble.

My advice is stay away from both these circuits but, in the 2nd circuit, you could add a 470 ohm resistor in series between the left pot terminal and where it joins C3.

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  • \$\begingroup\$ Very informative but my question was about the tone controls (circled in red). I probably should have been more specific in my question. I know that these circuits are RC filters but I am not understanding the way they operate or how they affect the frequencies coming out of the op amp. \$\endgroup\$ Sep 4, 2023 at 14:01
  • \$\begingroup\$ @Dragonfruit1966 I have covered those "so-called" tone controls in my answer. I have been quite clear about this when I refer to R6 and Tone 10 kohm. Try reading a little more carefully. The "so-called" tone controls as shown are plain and simply bad engineering and that's the end of it. \$\endgroup\$
    – Andy aka
    Sep 4, 2023 at 14:11

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