Bill and Will's Synth
MOTM 410 Construction
"Triple Resonant Filter with Dual VCLFO"

       

February 2009 -

We turned our attention to the MOTM 410 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

Modifications - presents details of our implementation of Scott Juskiw's "Calibrate Bandpass Filters" modification and our own idea for switching R68 in and out from behind the panel.

Parts - presents a Bill of Materials for "two-dot-oh" builders and notes about it

Panel - presents the MOTM format panel

Daughterboard Construction - here's the daughterboard we're building to elegantly accomodate the modifications

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

Construction Phase 2 - Trimmers, Panel connections

Set up / Testing

Use notes

Background

Paul Writes:

The MOTM-410 contains three voltage-controlled bandpass filters, and a dual voltage-controlled sinewave LFO. The module is based on the Korg PS-3100 filter structure. It uses the same "vintage" components: Vactrol resistive elements for a smoother, 'liquid' phasing sound, and JRC4558 bipolar op amps. However, the control section has been updated to include:

  • Voltage-controlled LFO Rate (0.02Hz to 100Hz)
  • Voltage-controlled LFO Depth (0-100%)
  • Individual filter outputs
  • Master SWEEP control voltage

The MOTM-410 can be used as a "fixed" filter bank: just set the DEPTH to zero. Because narrow filters are used, vocal formants can be created using the independent center frequency controls.

The on-board LFO structure contains 2 "detuned" voltage-controlled LFOs (one LFO is running 20% slower than the other one). The LFOs have true sinewave outputs for best sweeping effects. You can use the panel controls, or plug in up to 3 seperate control voltages (SWEEP, RATE and DEPTH)!

Another unique LFO feature is the MODE switch. It controls how the 3 filters are swept as:

  • Single - All 3 filters are swept together by LFO #1.
  • Dual - Filter #1 sweeps with LFO #1. Filters #2 & #3 sweep with LFO #2.
  • Dual Reverse - Filter #1 sweeps UP with LFO #1, Filter #2 sweeps UP with LFO #2 while filter #3 sweeps DOWN.

The MOTM-410 has 3 separate audio outputs, which can be used to create frequency-dependent stereo panning. Using 3 inputs on your mixer, you manually pan Filter #1 LEFT, #2 to CENTER, and #3 to RIGHT. The LFOs then sweep the stereo field, based on the frequency content of what you are playing. Or, turn off the LFOs and use the fixed filter to "direct" the sound to a specific mixer channel, again based on input frequency.

The MOTM-410 can be used with any line-level audio source (synth outputs, mixer effect sends, etc) or integrated in a modular system. The kit version is moderately difficult (4 -5 hours construction time).

Modifications

Calibrate Bandpass Filters

Scott Juskiw writes about his "Calibrate Bandpass Filters" modification:

"Changed R53, R56, and R59 to get the bandpass filters to track each other more closely. I have two MOTM-410s and when all six filters were tuned to generate maximum output from a 1 KHz sine wave, the settings for the FREQ knobs varied between 2.0 and 4.5 (a knob variance of 2.5). After making this mod, the knob variance at 1 KHz is zero and is less than 0.5 over a 10 KHz range.:

We've decided to permanently replace the three resistors, R53, R56, and R59, with 500K trimpots.  We'll do so by installing a separate PCB - specifically, an MUUB3 - to hold the trimpots neatly.  Admittedly this is an extravagance because the trimmers could be soldered directly into the 410 PCB - but not quite as elegantly. 

Switch input attenuation - R68 in and out

While we're at it, we're going to put R68 on that MUUB3 too - so we can switch it in or out.

Parts

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.

This BOM doesn't account for our little daughterboard modification stuff... if you're interested email us, OK?

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:

Daughterboard Construction

Build Daughterboard

Right - so let's make the little daughterboard for those resistors... we'll use an MUUB3 (from Tellun) and put some headers on there so we can plug wires from the 410 PCB in there (we'll get around to specifying this in more detail later on.)  For now, we're just going to put three headers with the default resistors on there - 180K for R53, R56, and R59.  When we get around to it, we'll replace those resistors with the 500K trim pots.

We'll also put two headers on the daughterboard for the R68 resistor.  One header will go to the default R68 resistor.  The other will go to nothing (but it will hold the wires in place)

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.

Resistors

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

We've left out R53, R56, R59, and R68. Those will be installed in the MUUB3 daughterboard we're going to include.

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.

Capacitors


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

ICs - Misc


Misc & ICs


Vactrols


the transistors


Misc, ICs, and poly caps 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.

OK - now for soldering the "via holes."

Done with the "organic" solder


Construction 1 done - northern and southern views <G>

Construction Phase 2

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

Wires


the standard wires


wires for the mods


wires soldered in for R53, R56, and R59 - blue and white


and now, as well, the wires for R68  - green and white


OK - some the wires are in - but...


now the "Sweep" pot


so all the wires are in

Prepare the panel


ready for the jacks

Bracket


the mounting bracket - now we're going to use stand-offs to mount the MUUB3, but for a detailed description of classic bracket installation, click here


the 3/4" stand-offs plus a little 1/8" spacer to avoid the bracket screw


the daughter board attached

Wiring the switch


switch

Wiring the jacks


all wired up

Construction Done

 

Set up / Testing

Use Notes

 

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