This is the Resonant Gate designed by Craig Lee, based on the original low pass gate Don Buchla circuit. The complete discussion is on Muff’s message board.
There have been many variations on the low pass gate. All share the central concept of using a Vactrol in the signal path as an amplitude controller. Various options provide for different levels of feedback, or resonance. Most low pass gates make us of a manual switch to select the mode of operation. This one is different in that it features a voltage-controlled feedback, which is labelled Mode. The mode is variable from no feedback at all to feedback large enough to make the circuit self-oscillate. The Mode pot sets the initial level of resonance, which can be added to or subtracted from by an external Mode CV through a reversible attenuator. The initial frequency is set by a pot I labelled ‘FR’ for frequency. (This is labelled ‘Offset’ on the schematic diagram.) External CV of frequency also uses a reversible attenuator.
My panel uses the ‘small knob’ MOTM grid format, which allows me to fit two identical resonant gates into a single 1U wide panel. This choice required me to omit the second CV input that the PC board provides.
A problem was reported about the resonance circuit. It turned out that due to a characteristic of the Vactrol response, the resonance level could fall off over time. (See the full discussion, linked above.) I solved this for myself with a simple modification that set the minimum resonance to greater than zero. This means that the plain VCA mode is no longer available, and there is still some resonance at the lowest setting of the Mode pot. That’s OK with me. I like some resonance in the module at all times. Here’s the backside of the completed module:
My modification consisted of determining a parallel resistor for the resonance Vactrol. I described the procedure for finding the value in the post listed above. The end result is a resistor soldered across the Vactrol leads on the top of the board. Detail:
Finally, here’s a short demo of the resonant gate. It is filtering a triangle wave. A MOTM-800 envelope generator is hitting the CV. The demo starts out with the lowest resonance with the gate all the way open. Then I bring up the CV and play around with the controls a little.