Audio equalizer circuit using multiple feedback bandpass filter for each frequency band
Just a note: When designing these filters, one can set the center frequency, Q, and maximum gain at the center frequency. By choosing the capacitors to be equal, you have given up the ability to independently choose the gain at the center frequency.
These filters have to be driven by a low impedance source for accuracy, so you can’t put a pot in FRONT of them, unless you want to add a buffer after each pot.
It makes the most sense, then, to use a 4th op amp as an inverting summer after these stages to provide independent gain controls for the three channels. The 3 pots would be the input resistors for this summing stage, connected as variable resistors. Fixed resistors in series with the pots determine the maximum gain. Choose the pot values and feedback resistor value appropriately to meet your gain specs.