Bill and Will's Synth
Buchla 292c "LoPass Gate" Construction


July 2009 -

We turned our attention to the Buchla Lopass Gate.  Why?  'Cause Gino Wong says it's the "real sh*t."  <g>

Thomas White cooked up the PCB - it's here.


Table of Contents

This page has become really long, so here's a table of contents that we hope will make it easier to traverse:

Background - presents Thomas White's list of the Module's features

Parts - presents a Bill of Materials and notes about it

Panel - presents the MOTM format panel

Construction Phase 1 - Resistors, Capacitors, IC Sockets, Power Plugs, MTA headers

Construction Phase 2 - Trimmers, Panel connections

Set up / Testing

Use notes


Thomas lists the features thus:

  • 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 Pot Spacing
  • Compatible with panels
  • Additional Power Connector
  • +/-15v or +/-12v
  • PCB Mounting for Alpha, Bourns, TT, or Vishay/Spectrol pots
  • Ability to use 2 Single or 1 Dual Vactrol (see below)


Will and I used the BOM provided by Thomas White.

Mounting Bracket

This was easy to prepare - just drilled the holes into a short, 3-pot bracket.


We got ours from Bridechamber:

Construction Phase 1

All the stuff in Phase 1 gets soldered using "Organic" Solder.  At every break in the action, we wash the board off to get rid of the flux.


We want resonance and, because the Bridechamber panel has 3 audio inputs and 3 CV inputs, we'll want to use the little on-board summing features; hence the placement of these four jumpers:

Resistors & Capacitors

You can see we left one of the 100K resistors out - the one that's down near the TL074 IC.  We thought we wouldn't need that one because it's for a third CV input and the panel only has CV1 and CV2 jacks and pots.  Later on, we put it in too because we realized we'd need to use it for the "CONTROL" CV.

IC sockets, Diode, Power Stuff


Via Holes

The CONTROL CV resistor (CV3)

we put in that last CV summing resistor

Snack - Italian Chicken and Sausage Stew

the ingredients:

chicken (2 breasts, 2 thighs, 2 drumsticks)
sweet Italian sausage
bell pepper
tomato sauce
chick peas
dried basil

prep onions, garlic, bell pepper - fry bacon

adding a little oil to the bacon grease, fry garlic then add onions, then add carrots and celery

brown the sausage

add sausage to cooking veggies

brown chicken and add to pot

and add bacon

add tomatoes, cut them up a bit, then add basil and oregano

add the jarred sauce and about a cup of wateróready to cook on a very low flameójust enough to simmer slowly

after about 45 min, we add the bell peppers and continue cooking

after another half hour, we add the beans and chick-peas

after another 45 min, we turn off the flame, add the chopped spinach and stir it in.  The hot stew cooks the spinach.  That's  two hours total cooking time.

Done! We served it up with farfale pasta and salad after.

Construction Phase 2

All the stuff in Phase 2 gets soldered using "No Clean" Solder.

The 20K trimmer

we installed this backward (per the schematic) - but, in this case, it didn't really matter; just that the adjustment was  reversed.


This required some consideration - so we set the PCB, bracket, and panel up like this to check things out:

We wanted to mount the PCB on the right side (from the front) of the panel.  So we decided not to solder the FREQUENCY and RESONANCE pots into the PCB, we wired them up instead.  

We figured out the lengths of wire to use:

COAX - we used this for the Audio inputs and the Output. The PCB has just one pad/hole per connection so in all cases, we  connected the shield to ground on the opposite end from the PCB.  We grounded all the jacks with a separate wire:

IN1 (Audio) - PCB to INPUT 1 Pot: 6-1/2 in.; Pot to INPUT 1 Jack: 12 in.
IN2 (Audio) - PCB to INPUT 2 Pot: 7 in.; Pot to INPUT 2 Jack: 10 in.
IN3 (Audio) - PCB to INPUT 3 Pot: 7-1/2 in.; Pot to INPUT 3 Jack: 8 in.
OUT - PCB to OUTPUT jack 10 in.

WIRE - here again, the PCB has just one pad per connection so the grounding came from the jacks. We used the CONTROL jack for that ground connection to the PCB:

-CV - (single) PCB to -CV Jack: 8 in.
IN1 (CV) - (single) PCB to CV 1 Pot: 7 in.; (double) Pot to CV 1 Jack: 6 in.
IN2 (CV) - (single) PCB to CV 2 Pot: 8 in.; (double) Pot to CV 2 Jack: 5 in.
IN3 (CV ) - (double including ground wire) PCB to CONTROL Jack: 8 in.
OFFSET - (triple) PCB to FREQUENCY Pot: 8 in.
RESONANCE - (triple) PCB to RESONANCE Pot: 8 in.
DEEP Switch - (double) to MODE Switch: 5 in.
MODE Switch - (6 wires) to Rotary Switch: 5 in.


OK - so we started with the switches - the rotary switch and the SPDT "deep" switch.

Rotary Switch - we needed six wires - we have five colors - so we marked one of the white ones with a bit of heat shrink. We later removed the heat-shrink from the switch-end 'cause we realized it would be in the way - but we immediately soldered that wire to the switch pad A. Here's how the wires worked out:

S1 - red - switch pad 3
S2 - marked white - switch pad A
S3 - green - switch pad 12
S4 - blue - switch pad D
S5 - white - switch pad 7
S6 - black - switch pad C

SPDT - We used two red wires - polarity doesn't matter here:

Heat Shrunk and Soldered:

Control Pots

Next, we dealt with the FREQUENCY and RESONANCE pots. We used "chicklets," from Bridechamber, to make the connections.

CV inputs


OK - so on the PCB side of the output and input, there's only the one pad for the signal.  The ground connection needs to come separately.  But we wanted to use coax for the audio ins and the output so we prepared those bits of coax to solder into the PCB like this:



The OUTPUT coax soldered in like this:

The ground (shield) is fed fron the jack-side of the coax.

Audio inputs

The audio inputs required two sections of coax; one from the PCB to the panel-mounted potentiometer, and one from that potentiometer to the jack. So the grounded coax shield connection needs to come through the potentiometer from the input jacks. Here's the neat way we dealt with this issue:

this is the version 2 chicklet from Bridechamber - check it out - extra holes!

so we prepared the coax like this, with bit of resistor lead soldered to the shield

they soldered into the chicklet like this.  The coax from the PCB has its other end prepared with the heat-shrink (like we showed for the OUTPUT connection above) and is soldered into the V2 chicklet; shield in pad 1 and signal in pad 2.  The coax to the jack is soldered into the V2 chicklet with shield in pad 1 and signal in pad 3.  The extra holes are perfect for this - piece of pie - easy as cake.

there are little moments of brilliance in this ocean of idiocy <sigh>

Mounting Bracket

We stuck the ICs into their sockets and mounted the PCB:


We started with the input pots - they hold the mounting bracket onto the panel. 

the MODE switch


we snipped the locating pin off the rotary switch

and fitted it into the panel

the CV pots


The ground will come from the CONTROL connection - so every jack's ground is linked together:

Connections Done

Set up / Testing

Use Notes


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