I got back from my holiday and JOY! the Cynthia lowpass gate arrived from America that I bought on the bay. This is a very special module, and I have been on the lookout for one for quite some time, in the Moog format. I was hoping that it would do what it does and joy upon joy it does. It is a quad vactrol-based circuit based on an original design by west coast synth pioneer Don Buchla in the 1970s, and was a big part of the Buchla sound. Here is his lovely panel, the original Buchla 292
As far as I understand it, vactrols are a special electronic circuit based around a tiny lamp that is enclosed along with a light sensor, and when it recieves a signal the light comes on and the sensor translates it into a control for other circuits. Because there is a time delay when the light goes off it produces a very natural and organic sounding decay which is very useful in electronic music. This module uses the vactrol to control two elements; firstly an audio amp and secondly a low pass filter, which has the effect of decreasing the harmonic content of the input. You can also select to have both being gated at the same time. The net result is a very natural and organic sounding module, and having four in one is really useful, as you can use them in parallel or series. One way of using it is to send it a complex audio signal, such as noise or FM oscillator, and trigger the vactrol with a very short spike and you get wonderfully enveloped percussive sounds
I had to connect up the Dotcom power supply to the Cynthia module which meant I had to 'phone a friend' [Big Al] quite late last night, but he talked me through it, and made sure I had the polarity right. Here is the inside of it. The vactrols are the black squares with 3 legs
The front panel is really well made and printed [by STG] - apart from the fact that each of the 4 sections are labeled 'channel one'!
Here is the patch in the video. Theres a Moog 901 VCO being FM'ed by another 901 which is being FM'ed by the 960 sequencer. The main VCO pitch is being controlled by a random signal via a sample and hold. A COTK VCO is triggering the lowpass gate described above, and the frequency of the trigger is being controlled by a Dotcom 119 sequencer