Bill and Will's Synth
MOTM 485 Construction
"GX-1 Voltage Controlled Filter"


February 2009 -

We turned our attention to the MOTM 485 next for our recording of the "One Mad Track" album this winter.

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 an explanation and Paul Schrieber's initial description of the Module with a photo

Parts - presents a Bill of Materials for "two-dot-oh" builders 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


Paul Writes:

"The MOTM-485 is a recreation of the Yamaha GX-1 LP/HP filter.

"The GX-1 was a massive organ/synth hybrid, costing over $60,000 in the late ‘70s. The best example is the ELP song ‘Pirates’, but was also used by Stevie Wonder and Led Zepplin..

"The actual GX-1 used a pair of these filters to form a bandpass response (the same method used in the MS-20 and the CS-80).

"There is a HP filter set to "half tracking" followed by a LP at "full" tracking. However, you can still use a single MOTM-485 as yet another distinct tone in your setup.

  • Faithfully captures the unique GX-1 timbre
  • 2 audio inputs
  • Voltage-controlled resonance
  • Switchable HP/LP response
  • Switchable half/full tracking of the 1V/Oct input
  • Fully temperature compensated using TEMPCO resistor and matched pairs
  • Not a ‘clean’ filter: has higher distortion than other MOTM filters

the PCB


OK - so - In 2008 (or about that time), Synthesis Technology stopped producing full-blown kits, and moved toward what Paul calls "2.0" (two-dot-oh) DIY. This assumes the builder will buy certain parts from Synthesis Technology - PCB, Panel, and in some cases a Special Parts Kit of the particularly hard to find parts - and will get the rest of the parts from Mouser or Digikey or - well - wherever.

For those who are building this as a "two-dot-oh" project, Will and I, with feedback and review from others, have developed a parts-list / bill-of-materials in the form of an XL spreadsheet (as usual).

Please don't take it as gospel. We've been over and over it and are relatively confident in our specifications - and we hear that several people have used it successfully so you should be good.  The BOM assumes that you get the "extra parts kit" from Synthesis Tech.  Synthesis Technology offers some parts like pots and knobs at particularly good prices... these options are offered in the BOM.

Click here to download our XL spreadsheet Parts List


If you're building this as a "two-dot-oh" project, we also assume you get the panel from Synthesis Technology:

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.


After we've sorted and counted the resistors (part of the first step in building a kit is to account for the parts), we bag them separately to make them easy to find.  To see our handy-dandy Resistor Color Coding Chart, Click here.

resistors are in

Capacitors - ICs - Misc

Via Holes

Misc and ICs are in.  A note - we solder one contact of the ICs first. Then we make sure it's down on the pcb correctly and solder the diagonal contact and finish the other contacts up.

Construction 1 done.

Construction Phase 2

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

Pots, Temco, Wires

the PCB mounted pots

time for the tempco resistor

just like that

in go the caps

the pcb-mounted pots

all the wires are in

Prepare the panel

ready for the jacks


the mounting bracket

fitting the pcb onto the bracket

looks like this

now to fit it onto the panel
for more detail, click here

Wiring the switches

switches mounted in the panel

and wired up

Wiring the jacks

three done - three to go

jacks wired and tied

Construction Done


Set up / Testing

Use Notes


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The fine Print:
Use this site at your own risk.
We are self-proclaimed idiots and any use of this site and any materials presented herein should be taken with a grain of Kosher salt. If the info is useful - more's the better.  Bill and Will

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