12V Stereo Tone Control
The following diagram is the circuit diagram of 12V stereo tone control which also available in kit, you may find the kit at electronic part store around your place.
The circuit build based on ordinary tone control circuit, using 2 transistors FCS9014 in each channel, so there are will be 4 transistors in this 12v stereo tone control. 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. Any small signal NPN transistor such as BC547B, 2N3904, C828, or C945 can be used to replace FCS9014.