[sdiy] CGS30 BPF modded for guitar use

[lanterma at ece.gatech.edu]

On Nov 13, 2010, at 3:43 PM, Dave Kendall wrote:

> Mad idea I know, but it's a request from a guitarist mate. I want to find a way to mod this circuit  http://www.cgs.synth.net/modules/cgs30_bpf.html
> for use in a guitar stompbox. FWIW, I believe the circuit to be the same as, or a near clone of the roland SH5 BPF.

....

> I don't understand the circuit , 

This is a fairly standard "state variable" filter design, although it's drawn in a way that makes it hard to see at first. Numbering the op amps 1 to 4 from left to right, OA1 is just an input mixer/buffer, OA2 mixes the input signal with two feedback paths. OA3 and OA4 are integrators. It's drawn so that the bandpass output is on the right, but that makes it hard to recognize at first, and requires the drawing to be kind of folded in and twisted strangely; most SVF schematics show OA4 to the left of OA3. You could tap a lowpass output at the output of OA3 if you wanted.

Anyway... the feedback path from OA3 to OA2 is a second-order feedback path that allows the filter to resonante. (I'm handwaving a bit here - the 100K pots control the gain of both integrators, which sets the resonant frequency.) The feedback path from OA4 to OA2 is a first-order feedback path that dampens the response; more feedback with this path means less Q, and less feedback equals more Q (take it out altogether and you'll get an oscillator.) That's how the 50K pot is controlling the resonance. The diodes in the feedback path prevent things from getting too crazy; you'll see similar diodes, IIRC, in things like the Oberheim SEM filter and the Roland Jupiter-6 filter (although I think only one of its SVF's has the diodes); in both those filters, OTAs are used instead of the 100K pots to get a  voltage-controllable critical frequency.

- Aaron