I designed this module mainly for the purpose of increasing the resonance of the Cynthia/Buchla Quad Low Pass Gates.  There are quite a few low pass gates circuits around the internet.  I was looking over Scott Stites’s Mutant Vactrol Filter project.

One variant of Scott’s filter is organized as two Buchla Low Pass Gate circuits in series, combined with an input mixer and a feedback circuit for resonance control.  It occurred to me to build just the input mixer and feedback circuit as a separate module that could be patched to the low pass gates. Here’s the schematic.  As you can see from the photos, I built two of these circuits on a MOTM prototype board on a Stooge 4-pot bracket behind a custom Front Panel Express panel.

To patch a couple of LPG circuits in series with the mixer in the loop:  route the signal inputs to the mixer; the output of the mixer to LPG input one.  Then LPG out one patches to LPG input two and LPG output two is patched to RES IN on the mixer, using a multiple to take this as the output signal.

I tried using the new module not just with the LPG, but with all sorts of filters and effects.  It adds increased resonance effects to the MOTM-410, which is also a vactrol-based filter with a fixed resonance.  I also found it useful with the Encore Frequency Shifter.  It worked with the MOTM-440, but didn’t do much that the internal resonance control already did.  When not in service as a feedback mixer, this module makes a nice two-input audio mixer for any purpose.  It’s AC-coupled and features audio taper pots.

Here’s a demonstration of resonance enhancement on the Low Pass Gates.  In the following recording you hear the LPG first in the both mode, sort of bland, then in the low pass mode, and finally in low pass mode with the resonance turned up to about six.  This cycle is repeated two times.  The clip switch is turned on, which decreases the strength of the feedback for more control.