Bill and Will's Synth
MOTM 440 Construction
"Discrete OTA Voltage-Controlled Lowpass Filter with VC Resonance"

September 2008 -

We need the MOTM 440 for our recording of the "One Mad Track" album this winter.  We've moved our building shop to the basement and off the dining-room table so here we go!

Whereas we're going to try to provide good photos here our most complete set of pictures so far is the MOTM-120 photos - we won't get that detailed here - many of the steps are similar and you might take a look at those if you feel the need.

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

Background

Paul Writes:

You have probably heard of the SSM2040 filter chip, used in the Prophet 5 Rev. 2 synth, the Voyetra 8, and others. Although short-lived, the SSM2040 filter had a unique sound different from a Moog, ARP or other 4-pole lowpass. Why? The design used a clever discrete OTA gain cell.

The MOTM-440 offers an updated SSM2040 architecture using matched NPN/PNP pairs and features a switch that adds a second audio feedback path to boost bass response at higher Q levels. This makes the filter "growl and rumble" even more! In addition, voltage-controlled Q allows for more sweeping effects. Three audio inputs and three CV inputs make the MOTM-440 the killer lowpass filter in your system. The internal gain structure is such that over-driving the filter is now possible (unlike the Prophet 5) to get even more nasty sounds. Did we mention it self-oscillates at high Q?

So order a MOTM-440 filter for a fraction of the price of a P5 Rev. 2, but without the worry of obsolete parts!

Parts

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

Panel

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.

Click here to see the handy-dandy Resistor Color Coding Chart I found at "Electrical & Electronic Knowledge Share" on line.

resistors are in

Whereas we are vigilant about orienting all the resistors, caps, etc. consistently so their values can be read easily (in case we need to trouble-shoot them later), we oriented the resistors with the "tolerance" stripe on the left (relative to the text on the pcb).  Why did we do it this way?  'Cause when we started out doing these builds, we thought the gold stripe is so pretty and easy to see... and we put it on the left - well - just because.  But now, we do it so all our modules are consistent with each other <shrug>.  You might want to do it the opposite way - with the "tolerance" stripe on the right.

here all but the Poly caps are in - per Paul's instructions, the polys wait 'till after the ICs

ready for Misc & ICs

Misc & ICs are in.  To see a bigger, high-rez picture, click here. 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.

OK - now for soldering the "via holes."  On this photo of the back of the pcb, we've marked them in red.  Again, to see a bigger, high-rez picture, click here.

Construction Phase 2

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

trimmer, poly caps, pots, wires - for a high rez pic, click here. For details about soldering in the Coax, click here.

Getting ready for the big, bad Tempco... the "Silicon Grease" (Mouser part# 567-120-2) ready to go -

Cognizant of Paul's instructions and of Gino Wong's warnings to be careful, we used a twisty tie as a little scoop to put this dollup of the "Silicon Grease" on the transistor array IC. For a high rez pic, click here.

Making a little bit of a mess, we scooped a tiny bit of the silicon grease back off. But it was probably a good thing because when we'd squished the Tempco down onto the IC, it looked perfect. For a high rez pic, click here. For some other notes and details about putting in a Tepmco resistor, click here.

ready for the jacks

the jacks are in - we're mounting the PCB on the bracket

filing out the POT holes; sometimes the paint makes the fit too snug.

the PCB bracket mounted
for more detail, click here

panel mounted POTs and switch are in

all wired up - for a high rez picture click here.

ready for knobs

knobs are on

construction is done.

Set up / Testing

Use Notes