Resonant Lopass Gate

Resonant Low Pass GateQuantity: 2 (of course)

This module is derivative of the Buchla 292c Lopass Gate.  The PC board and assembly instructions were made by Thomas White, described on his site on the Lopass Gate page.  The circuit is similar to the Cyndustries Quad Low Pass Gates, that I also built, and which is also based on the Buchla 292c.  I have always loved the sound of the LPG.  Although I had four of these in the Cyndustries module, I wanted to build Thomas White’s design for several reasons.  The QLPG has but one pot for each gate, which corresponds to the Offset pot on the Resonant LPG.  It sets the initial frequency.  The RLPG adds an attenuator pot for the control voltage input, plus a second CV input, and features a resonance booster with pot.  With two RLPG modules I have two lopass gates in a 4U space with more panel features.  I also decided to use VTL5C4 Vactrols, which have a slower response time than the VTL5C3 used in the QLPG.

Construction Notes

The Alpha pot part numbers in Thomas White’s part list are for the PCB-mount versions.  Unfortunately those are the wrong part numbers, because they are audio taper, and the circuit calls for linear taper pots.  I was building with the 1U BrideChamber panel and bracket, so I ordered linear Alpha pots, mounted them on the panel (using #5 O-rings on the bushings) and wired over to the board.

I found an interesting knob for the rotary switch, Mouser #506-PKGP-70B-1/4, which though a bit pricey at $3.75 USD each is really well suited.  It is a smaller diameter than the standard MOTM know, but has a pointer indicator that fits the fingers nicely.

Since I built the 1U panel with one signal input, I used the direct audio input option on the PC board.  I made use of the control voltage mixer to mix the CV and Control inputs.


How does it sound?  Very much like the QLPG, naturally, since it is based on the same Don Buchla design.  Of course the resonance (filter mode only) can be turned way up!  Do take the time to view Thomas White’s video demonstrations.


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