Bill and Will's Synth
MOTM 480 Construction
(includes Richard Brewster's
Daughter Board and modifications
and some of our own little mods too)
"Dual Cascaded State-Variable Filter"

This is the 480 front panel without modifications.

This is the 480R front panel which accommodates the RB and DFA mods -  additional jacks, and control knobs.

February 2008, February 2010

Although we won't get around to building it for a while, inspired by Adam's success, we also have decided to include Richard Brewster's modifications to the MOTM480 "Dual Cascaded State-Variable Filter" when we build it.  Our purpose in exploring the modifications now, long before we're ready to build it, is to develop a front panel design so we can have it made before we get to building the module.

Per Adam, Richard's modifications provide "...complete, independent access and control to the two filters in the MOTM-480."  Now, in Adam's modification, he only implemented some of the modifications Richard suggests.  We're going to go ahead and implement them all.

Here's how Richard describes his modification:

"In the original SynthTech panel the filters are connected in series, with the high pass output of the first hard-wired to the input of the second. This creates the CS-80 “variable band pass” response from the low pass output of the second VCF. The first VCF has a three-input mixer. Each filter has its own panel pots for initial frequency and initial resonance. The VC inputs are ganged: The 1V/Octave, FM, and Resonance VC jacks control both filters together. The Band Pass output of the first VCF is on a panel jack. The main output is the Variable Band Pass output, i.e. the LP out of the second filter. With a few changes and a bigger panel, this more flexible module can be obtained.

"In conjunction with Larry Hendry I designed a 3U-wide panel with symmetrical features for both filters, so they can be used completely independently. Each filter on the custom panel has:

"5 Pots: Inputs A, B, Frequency, Resonance, FM (reversible)

"8 Jacks: Input A, Input B, FM input, 1V/Oct input, Resonance VC input, High Pass, Band Pass, and Low Pass outputs."

But we're going to include a few of our own little wrinkles too - for one thing, we're going to call the filters A & B:

• Richard completely separates the filters.  To use the filters as originally designed, you could patch the HP output of filter A into one of the inputs of filter B.  Instead, we're going to include a switch (SEPARATE / JOIN) to disconnect the filters or connect them as originally designed (including the 150K resistor which Richard removed).

• He also has completely separate signal inputs for each of the two filters. In our version, the same switch will send the inputs from IN B1 and IN B2 either to the input of filter B (when in the SEPARATE position) or to the input of Filter A (When in the JOIN position).

• Richard also has completely separate CV inputs for each of the two filters.  In our version, we'll wire the 112X Switchcraft Jacks for Filter B's 1V/OCT, FM, and RES inputs such that they'll be normalized to Filter A's CVs until a jack is inserted into them at which point, they'll become discreet inputs.

These extra features should make it possible for us to use the filter nearly as originally designed as well as using the modifications when we want them.  With Switch 3 in the JOIN position, Filter A HP and BP output, and Filter B LP outputs should be the original outputs.

The MOTM 480 has a "difficulty factor" of 5, and with Richard's modifications, It can only be more difficult.  But we'll slog on and see what comes of it.

To make the modifications, we'll need a MUUB4 and a 4-pot "Stooge Mounting Bracket."  We'll also need other requisite parts.

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.

Modifications - presents details of Scott Juskiw's Modification and Daughterboard

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

Background

Paul Writes:

The MOTM-480 VCF is configured as a super bandpass filter with independent corner frequencies and resonant peaks, similar to the legendary CS-80 configuration. This module can create distinctive pad and brass timbres. Built-in audio mixer can filter any audio source! Expansion connector for future configurations!

• Contains Dual Resonant State-Variable VCFs with CS-80 specific characteristics
• Cascaded HP & LP Filters with lowpass (LP) and bandpass (BP) outputs
• Independent Cutoff Frequency
• Independent Resonance
• Three Audio Ins with Level
• Voltage Controlled Resonance
• 5U high x 2U wide

Modifications

1. Dave Brown Max Resonance Limit

Per Dave Brown:

"Replaced R56 and R61 with 1K trimmers. The trimmers fit in the resistor footprint by clipping one lead and slightly spreading the other two leads.

"I had very weird distortion and strange modes at high resonance levels. ... Decreasing the input level to '8' eliminated both effects but I wanted to avoid them at both maximum input level and resonance. I adjusted the maximum resonance to limit the maximum output voltage to +/-13 volts."

picture from Dave Brown

2. Disconnection of the Filters

The MOTM schematics refer to one filter as "HP."  Richard Brewster calls this "Filter 1" and we're going to call it "Filter A." The other filter is referred to as "LP." Richard calls this one "Filer 2" and we'll call it "Filter B." As Richard explained, the two filters are connected in series, with the high pass output of the first (A) hard-wired to the input of the second (B).  This is done through resistor "R68" (150K).  By removing this resistor, the two filters are separated.

3. Switch to Re-Connect the Filters

Our idea is to have a switch to re-route the input to filter B in the original way, through the 150K resistor.

4. Input Connections

A. Disconnect Inputs from Filter 1

Per Richard's mod, we'll remove R25 from the motherboard.

B. Make Connections at switch

We'll re-mount resistor R25 - and an additional 150K resistor for the fourth input - on bits of wire connected to the switch.  With these resistors and also with R68 on the switch, we can easily make the various input connections we are aiming at.  Here's a schematic of our idea:

Here's how we'll mount the three 150K resistors on the switch and make the connections:

With everything hooked up this way, when the switch is in the "up" (JOIN) position, the inputs to the filter and relationship between the filters should act much like Paul Schreiber's original design only with four inputs instead of three. With the switch in the down, (not printed on the panel) SEPARATE position, the filter should be set up much like Richard Brewster's design.

5. Board-mounted Pots

VR1, VR2, VR3, and VR4 will not be soldered into the PCB - they will mount on the panel instead.

6. Disconnect Voltage Control Inputs

Per Richard, four traces need to be cut to separate the voltage control inputs for each filter:

• to the right of R18, on the top of the board. The right end of R18 will connect the 1V/OCT input of Filter A

• to the right of R19, on the top of the board. The right end of R19 will connect the +FM input of Filter A

• to the right of R21, on the bottom of the board. The right end of R19 will connect the -FM input of Filter A

• between R66 and J3, on the bottom of the board. The left end of R66 will connect the RES IN of Filter B

7. Build Buffers for Extra Outputs Using MUUB4

Here is how the components and connections lay out:

8. Other connections

Here's how Richard describes the other jack connections:

• The right end of R18 will connect the 1V/OCT input of Filter A.
• The left end of R66 will connect the RES IN for Filter B.
• The right end of R19 will connect the +FM input of Filter A.
• The right end of R21 will connect the –FM input of Filter A.
• Wire the IN1A pot at the VR1 location (center and ground only).
• Wire the IN1B pot where VR2 location (center and ground only).
• The left hole where R68 was will connect the HP Filter A output.
• The lower end of R55 (33K) will connect the LP Filter A output.
• The left end of R40 (10K) will connect the HP Filter B output.
• The right end of R50 (4M7) will connect the BP Filter B output.

9. Normalization Wiring for Filter B CV Input jacks

We'll wire the jacks for filter B CV inputs so that if no plug is inserted, Filter A's CVs will connect to Filter B's. Looking at the panel back, those three connections would be here:

Parts

Will and I have developed two a parts-lists / bill-of-materials-es in the form of XL spreadsheets - one for the stock 480 per MOTM 2.0 - and another for the 480R mods based on the parts list on Richard Brewster's Site and our own figuring.

Click here to download the 480 XL spreadsheet (apx. 250K).

Click here to download the 480R XL spreadsheet (apx. 250K).

Click here to go to our Bill of Materials Page.

Panel

For our FPD panel design, click here.

But Scott Deyo made a panel - and we got ours from him.

480DB - MUUB 4 Construction

Now that we've essentially figured out what's required to do the project, we'll see about ordering the Front Panel and get beck to this in a couple months. 

MOTM 480 Construction with Modifications

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.

Construction Phase 2

All the stuff in Phase 2 gets soldered using "No-Clean" Solder and the PCB doesn't get washed off from here on.

Set up / Testing

Use Notes