This is 12V stereo tone control circuit electronic. Take a note that you must connect the circuit before the amplifier module. The output of tone control must be connected to the input of the amplifier.
There are will be four potensiometers used for this tone control circuit: volume level, bass level, treble level and balance adjustment. For better result, use better components with small tolerance value for example metalfilm resistors, MKM capacitor etc.