This is an audio effect circuit that give extra oscillation to your guitar, percussive, or semi percussive instruments, you can call it a funk box. This  guitar effect circuit is basically a damped oscillator that is set slightly below its critical oscillation point.  The oscillator part is constructed by feeding back the output signal through a bandpass circuit, with the center frequency is adjusted by potentiometer R7 to add an extra “twang” , adjustable from low to high pitched timbre. Potentiometer R4 is used to adjust the gain of the oscillator, should be set to damp the oscillation at the desired decay time.  To adjust it, turn the knob slowly until a steady oscillation occurs, and turn back slightly just before it happens (the oscillation should ceases slowly). Here is the schematic diagram of the circuit:

Funk Box circuit schematic

[Circuit’s schematic diagram source: seekic.com]