CGS Dual CMOS Filters

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Ken Stone’s Dual CMOS filters, a.k.a. “Twin Wasp” VCF.

Here’s a link to Ken’s description of the board.

Continuing the squeeze-it-into-1U-with-small-knobs approach, I designed this Front Panel Express panel for the Twin Wasp.  Key to the design is a miniature rotary switch, Mouser part #633-MRK-112A.  It’s single-pole, twelve-position, configurable to limit the number of positions to your needs.  At over twelve bucks a pop, it’s pricey.  But at least the knob is included.  The switch mounts in a quarter-inch diameter hole and even could fit in a slightly smaller hole, the bushing is that tiny.

Compared with the Bridechamber 3U Panel, the only features I had to give up were the inverted CV inputs and the spread CV, which is an external modification.  And mine is 1/3 the size!  I have created similar 1U panel designs for a dual CGS Steiner VCF and a dual CGS Serge 1973 VCF.  But I probably will not make those, because I have so many filters as it is.

The construction pic follows.  You need nimble, steady fingers to lay-solder the five wires to the itty bitty PC-mount legs of the rotary switch.  This filter uses four of the obsolete CA3080 OTA chips, the heart of so many 1970’s synthesizer circuits.  I was happy to finally use some of my stash of 50 that I’ve been hoarding for a few years.

I made one circuit change after taking the photo here. The FREQ pot sets the initial frequency. Ken Stone’s wiring diagram showed the CCW lug going to ground. But it really needs to go to -15V. And the summing resistor should be 270K or 300K, instead of the 100K on the schematic. I just added a 200K resistor in series with the wire going to the center lug on the pot.

I have to learn how to use this filter now!

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One Response to CGS Dual CMOS Filters

  1. slovo says:

    It’s a tricky one, isn’t it? I have the cynthia dual wasp and it seems like about 75% of the combinations of dials produce basically no sound. Even when the two filters are set in similar states, you need to be very careful fine tuning your cutoffs. The Cynthia one is a bit trickier I guess, with the serial/parallel blend and “divergence” pots.

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