This is a very basic circuit diagram of 3 band tone control. This basic tone control circuit was originally intended for home audio use, but should be able to be hacked into an effect circuit with very minor modification.
Resistance value of resistor R1 is left up to the user depending on gain needs, R2 is unknown – for experiment purpose. The op-amps are not critical, any standard ones could be used (eq: 741, LM324, etc).
Power supply used for this circuit depends the op-amp IC used. Use the IC datasheet to know the recommended voltage. Some op-amp IC needs symmetrical power supply to work, just find the op-amp which require general power supply (asymmetrical). A regulated power supply will be better for this circuit.
For better sound performance, use high quality component such MKM capacitor (non polar), tantalum capacitor (bipolar), metalfilm resistor, etc.
Download basic tone control circuit in PDF document:
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what is the value of the gain resistor r1