Resonant Lopass Gate Circuit Board Project

Here's a little project that have offered the PCB for, inspired by legendary synthesizer designer Don Buchla. To buy a PCB, visit Synthcube in the USA. Boards are ROHS approved "lead free" materials and have a silver finish to them instead of lead. The board size is 3.8 x 2.5 inches and they should fit most cabinets. RLPG's are compatible with nearly all modular formats, but work particularly well with all the popular 3U and 5U high modules like those from Synthtech, Blacet, Doepfer, and all of the modern Eurorack format makers out there.


  • Up to 3 Audio Inputs Up to 3 Control Voltage Inputs
  • Offset (Frequency) Control
  • Resonance Control
  • VCF/VCA/Both Modes (Rotary or 3PDT Option)
  • "Deep" Switch to set lower freq range
  • Inverted CV Out (From CV Mixer)
  • Jumper points to bypass mixer or remove resonance circuit
  • MOTM 5U Pot Spacing
  • MOTM (MTA-156), (MTA-100), and Euro/- - Doepfer Power Connector options
  • +/-15v or +/-12v
  • PCB Mounting for Alpha, Bourns, TT, or Spectrol pots
  • Ability to use 2 Single or 1 Dual Vactrol (see below)
  • "Cool" people use Vactrols


This is a distant cousin PCB project that is a derivative version of the famous Buchla 292c Lopass Gate. Don Buchla himself gave approval to this endeavor for us all to learn a bit more about analog synthesizer module design, and for this we owe him a debt of gratitude as a true scholar and gentleman (RIP Don. This is not the first, nor will it be the last Lopass Gate module to come out to the world. People have become transfixed on the magic of Vactrol's (Light Dependent Resistors or LDR's) which are a combination of a resistor and LED in a light-tight package that seals out the light of the world yet somehow glows with an internal magic, that for the Synth_DIY crowd becomes hard to. Many have contributed to this PCB project and to the research supporting Vactrol integration to modular synthesizer design through various designs and ideas made public through the wonders of the Internet. These people include Don Buchla, Peter Grenader, Chris MacDonald, Mark Verbos, Grant Richter, members and others. We would like to thank them for their valuable contributions to our Synth_DIY community and for their brilliant projects which have helped to spawn a crowd of Vactrol addicts.

Building this PCB project requires the same fundamental practices which to build other pcb-based projects or modules that you have built. If this is your first module, or your 100'th I hope you find the documentation and support offered within to be satisfactory. However, if this "is" your first module I highly recommend you to find and frequent the dirty Internet watering holes for us synth junkies otherwise known as and

Project document download

Notes: Nearly all project documents in this zip file are in Abobe PDF format. Part numbers for parts on the Mouser part list are very outdated and will likely need to be updated. Some users have mentioned using a "Log" pot for the Resonance control provides better control over the total resonance amount.

Single or Dual Vactrols?

You can use a variety of LDR/Vactrol parts for this including 2 single vactrols, 1 dual-vactrol or homemade units if you are really up for a challenge! Here is a great diagram showing how the recommended VTL5C3/2 (dual LDR) fits to the board instead of 2 single parts.

User Community:

Dave Brown has a great build report, mods, and updated parts list at:

Board Versions:

This board has gone through several revisions from 3.0 to 3.4 now. The basic build guide in the docs works for every board. The changes in each revision can be attributed to adding the Euro power connector and optimizing circuit traces more than any other factor. However, revision 3.2 of the board requires one *tiny* jumper due to a circuit trace that got separated from the ground plane when I beefed up the thickness of the traces. The fix is to jumper a pin 5 of the mixer IC to ground (Which can be found on pin 10 of the same IC). This error was fixed for revisions 3.3 forward.