Wednesday, September 30, 2009

CGS Bi-N-Tic Filter

Well, the Super Psycho LFO rebuild / hot rodding is on hold for a couple of weeks. The issue: Each of the six oscillators has a 1M pot in the circuit that controls the osc rate. After thinking of various ways of combining the pot in series or parallel with a Vactrol's light-dependent resistor, I've decided that I'm not happy with any of them. There's just no good way to do it such that there isn't some value of the pot setting that makes the Vactrol ineffective, and vice versa. The right way to do it is to take the pot out of the oscillator circuit and have just the Vactrol in the circuit; then make the pot be part of a voltage divider that will supply an offset voltage, which is added to the external control voltage via an opamp voltage-adder circuit, and the sum of the voltages drives the Vactrol's LED. Problem: Although I can make a 0-5V voltage divider using the existing 1M pots, it's a bit iffy as far as staying well clear of the opamp's specified offset and bias currents. I'd feel more comfortable using a 100K or 50K pot, so it can supply a bit more current. But I don't have any. I'm in on a group buy at Muff's place, but it's going to take a few weeks for that to come together. So until that happens, the Super Psycho is in a wait state again.

In the meantime I have something else to play with:

This is a kit for a Ken Stone / CGS 57 Bi-N-Tic switched-capacitor filter. The kit is from the wonderful folks at Bridechamber and includes all parts. A close-up of the circuit board:

And the panel:

The kit incorporates nearly all of the mods and hot rodding that Ken calls out as options in his notes on this module. (The control that Ken labels "DAMPER" in his notes is called "RESONANCE" on this panel.) Switched-capacitor filters are very unusual in the synth world. I'm not quite sure why that is; they have a reputation for being noisy, but I think that rep might be the result of some early designs of the switched-capacitor concept that used mechanical commutators and crummy caps. I'm curious to build this and see how it sounds. I am going to add one mod: a switch to take the most significant bit of the digital count that goes into the multiplexor which selects one of the eight caps, and move it from the 4-bit to the 8-bit of the counter. That will have the effect of dividing the eight caps into two banks; instead of scanning all eight caps in order, it will scan over a bank of four caps twice, then switch to the other bank. I'm not quite sure what this will actually do; I think it will give the filter two resonance peaks. Anyway, it will be interesting to find out.

No comments: