I know the description says this a filter, but how would this circuit be used?
Basically a low pass filter (it cuts high frequencies) that takes some of the output signal and feeds that back into the buffer of the first stage - if you feed back enough signal the circuit will oscillate. I have no idea how it would sound with a guitar, so I supose we are waiting for somebody to build one and post some clips.
Today's circuit is another low pass voltage control filter:
I took the liberty of a small mod on the above schematic.
The part of the circuit that is supplying the reference (or bias) voltage is connected through the 1MΩ resistor to the non-inverting input of the op amp; if you measure the DC voltage at the non-inverting input, it will be half the supply voltage.
If the reference voltage were use anywhere else in the circuit, I would need the capacitor and two resistors (I won't go into the techie explanation), as this is not the case I can change the circuit slightly for the same result (reference voltage at the non-inverting input) and save the cost of the capacitor and one resistor, plus a bit of board space:
I've laid the board out as per the top left diagram:
Ten cuts and three jumpers.
To change the board to the bottom right diagram:
Change R2 to 2.2MΩ
Change R3 to a jumper
To invert the control voltage (CV) response as suggest in the small box, connect the anode of D1 to b12 and the cathode to d12.
It's also possible to make the LED switchable at the cost of an extra LED and a DPST on/on switch: