VCF stompbox schematic, is it ok

[diyvenom]

HI! I have this (the schematic below) built and working, but I think it is adding in way too much noise. At first there was a lot of AC hum too, but not as much if the buffer at the bottom goes to an opamp and then to the out. I just don't know much about these issues, what causes noise in this case? I don't know much about that so I could use any tips you have.

This is a voltage controlled filter with res and cutoff, meant to be a stompbox so it's on 9v, no negative supply, thats why it has offsets, but I don't much understand that part.




*1 chip is 13600, same as lm 13700 I think, opamp is 741 quad type

[Transmogrifox]

The Korg Delta DL-50 has a really nice VCF using 13600 chips. 
http://www.korganalogue.net/korgother/DL50Delta/KorgDeltaService.pdf
Page 7/21 has the schematic for this section of the Synth circuit.

It also has the classic exponential generator for the CV.

To adapt the Korg Delta filter to single supply 9V, you would set up your power supply so you have +9V, +4.5V, and 0V.  The +4.5V is your "virtual ground" composed of a pair of 10k resistors in series between 0V and 9V with a really large capacitor (like 470uF).  See any number of 9V stompbox circuits for this virtual ground.  The Tube Screamer is a good example circuit;  look for "Vb" or "V/2" reference.

Virtually speaking, your power supply becomes +4.5V, ground, and -4.5V.  In cases where the virtual ground reference is sinking or sourcing DC current this virtual ground reference may need to be buffered by an op amp.

The circuit you have shown above has a number of DC bias problems (for example the input bias changes as you change the volume pot).  I would guess your DC conditions are guilty for the excessive noise.

The 1k resistors from power supply to ground are a bit small.  This pedal will eat up a lot of power for a battery operated stompbox.

Another good reference to work from is the DOD FX25 envelope filter.  This one is a second order filter similar to what you have shown (note it is different).  The use of the DOD FX25 schematic will be a good reference for how to deal with LM13600 (and LM13700) OTA's in a 9V single supply configuration.

I hope that helps.