My Modulus analog synthesizer is the combination of the the other equipment described on these pages along with three large banks of analog synthesizer modules.  The Paia Keyboard is the centerpiece, with the MIDI/Joystick interface, Chameleon, and Paia Fatman sitting on it.  The three cabinets are constructed from 3/4″ oak plywood with iron-on wood edges in the front.  The back is 1/4″ luan plywood.   Each cabinet measures 40-1/4″ wide x 8-1/2″ tall x 5-1/2″ deep outside dimensions.  Each cabinet has a module front panel space measuring a total of 38-3/4″ wide x 7″ tall.  That’s enough space to hold fourteen of my modules in each cabinet.  Modules are standardized to a size of 7” tall by 2-3/4” wide. This puts the knobs at a horizontal spacing of 1-3/8” and the 1/4” jacks are spaced 11/16” horizontally. Vertical spacing is 1″ overall. This gives a nice density of components, not too big but still plenty of room to turn the knobs and insert patch cords.  The cabinets have 1/4″ captive nuts in the sides.  The side brackets are made from 3/4″ oak plywood with walnut edging.  The threaded knobs that hold everything together are from Woodcraft.

Power Supply

The synth is powered from this unit.  Inside is a commercial +/-12V switching power supply and a homemade +5V supply that uses a LM340-5 regulator.  There’s also an AC transformer that outputs 12VAC for the Paia Fatman.  The meters confirm that everything is working.  And there’s a temperature-controlled fan that helps keep things cool.  The fan controller uses a 10K ohm thermistor (TH1 and TH2 on the diagram) to sense the temperature and if it gets warm enough the comparator (IC1) trips and turns on the transistor which turns on the fan.  The trimpot (P1) allows adjustment of the trip point where the fan turns on.

MIDI & Joystick Panel

The MIDI and joystick control unit sits on top of my Paia Keyboard but provides control to the modular synth above.  It’s connected through two DB25 ribbon cables to the patch panel which is the leftmost module in the bottom cabinet.  There are two physical MIDI inputs, selectable by a switch.  There is a three-way select switch to choose the MIDI channel to “listen” to. The thumbwheel is the selector for the interface’s operating mode:

  1. mono mode – when the thumbwheel is in this position, the last key pressed generates the same control voltage at all four jacks on the modular panel interface.  The gate and trigger signal is the same for all four corresponding jacks.
  2. duo mode – in this position, the last two keys pressed generate control voltages and corresponding gate and trigger signals in the jack sets 1 and 2.  This is a polyphonic mode that is normally used by connecting the panel jacks to independent VCO-VCF-VCA chains to generate two different pitches/tones at the same time.
  3. trio mode (arp up) – like the #2 duo mode, but the last three keys pressed generate independent CV/G/T outputs.  Can be used to generate three pitches/tones at once.  The “arp up” refers to the arpeggiator function – there’s a speed control on the panel that sets the arp rate – the gate and trigger outputs “strobe up” at the set rate, or not at all if the arpeggiator is off.
  4. trio mode (arp down) – just like the #3 mode, but the arp direction is down.
  5. trio mode (arp up/down) – just like the #3 and #4 modes, but the arp direction alternates between up down.
  6. quad mode (arp up) – just like the #3 mode, but four CV/G/T sets are output corresponding to the last four keys pressed.
  7. quad mode (arp down) – just like the #6 mode, but the arpeggiator goes down.
  8. quad mode (arp up/down) – just like the #6 and #7 modes, but the arp direction alternates between up down.

The interface is capable of receiving MIDI velocity and setting a velocity control voltage output to the patch panel. My Paia keyboard doesn’t output velocity, so there are also selectable random velocity modes to keep things interesting.  The interface also can decode and send control voltages for pitch bend, aftertouch, mod wheel, and sustain for equipped MIDI keyboards.  The joystick portion of the interface connects to the patch panel and takes a control voltage in from the panel, passes it through the joystick, and sends a scaled output back to the patch panel based on the position of the stick.  There’s also an internal mode where the control voltage is not brought in from the front panel but is just generated internally.  Portions of the schematics are based on the Polydac hardware, but the software running on the PIC16F877 is all my own original design.


The sequencer is a MFOS 16 step sequencer.  It’s a stock build with no significant customizations or modifications.

Drum Machine

A Yamaha DD5 Drum Machine was hacked to make my drum machine module.  The front panel has buttons for all the stock functions of the DD5 – changing volume and tempo, selecting patterns, start/stop, and so on.  It also has modifications to add pattern select LED indicators, a tempo indicator, and multiple outputs that can be used to trigger analog synth modules.  The DD5 tone board is mounted in the top cabinet behind the panel interface module.  My add-on circuit is based on a Microchip PIC16F877 and has digital interface circuitry used to drive the module’s trigger outputs, the display LEDs, and interface with the DD5 board.  Interface buffering circuitry lets the modular synth trigger the start/stop function, intro/fill-in function, and four selectable drum sounds.  The four drum sounds are those that were triggered from the pads in the original unmodified DD5.  The DD5 case and pads were discarded so these inputs mimic the signal generated when a pad was hit in the original unit.  Each input buffer circuit has two inputs – one is connected to a jack and the other is connected to a pushbutton on the front panel to manually trigger the function or drum sound.  The DD5 board has connection points for the 4 pads, the 6 selector switches, the intro/fill-in and start/stop buttons, the tempo and volume controls, and the pad assign button.  All the points are wired to the header connector which plugs into the front panel’s cable harness.

Waveform Visualizer

This is my original design to display synth waveforms oscilloscope-style on an old monochrome Toshiba TLC-271A 128 by 480 pixel LCD screen.  I say “oscilloscope-style” because it’s not calibrated for voltage or frequency, although the signal input level is adjustable.  It is a handy way to see if a module is outputting a square wave, sine, triangle, or whatever.  The processor is a PIC16F873.  The input signal goes to the PIC’s A-to-D converter.  This PIC doesn’t have a lot of RAM available, certainly not enough to capture and store a 128 by 480 pixel image.  So the real secret of this circuit is the dual port RAM design which allows the PIC to store samples in an 8K byte 6264 static RAM chip at the same time the video output circuit is reading the same samples and sending them to the LCD display.  The display-output side of the circuit generates the data, dot clock, and horizontal and vertical sync signals to feed the LCD.  The little schematic at the end is an add-on PWM brightness control for the LEDs which surround the display. 

Divider & Mult

This is my own design.  It takes any signal and buffers it into a logic level square wave which is then run through a divide by 2, 4, and 8 chain (IC5/IC6) or a divide by 3, 6, and 12 chain (IC7/IC8).  The outputs of each chain are fed to IC9/IC10 as the “gate” versions along with a “trigger” version that’s generated from the 0.001uF/100K capacitor and 40106 buffer chip.  A switch  on the front panel selects the IC9/IC10 input to allow each divider section operate in either gate or trigger mode.  The outputs of everything are converted back to bipolar voltages suitable for use as a control for any other module in the synth.  There are also two “multiple” patch points that allow a single control voltage from another module to be split into up to 3 output copies.  The bottom part of the panel provides left and right audio and two control voltage patch points that are connected from behind the cabinet down to the Chameleon.  This provides a path to route audio to the Chameleon’s aux input and control voltages to the Chameleon’s “External 1” and “External 2” inputs.

Dual Quantizer

My Dual Quantizer is essentially two MFOS voltage quantizers built in one module.  One is a stock MFOS circuit board and the other is a clone that I made myself.  There’s also a modifications board that buffers an optional internal clock input as well as providing trigger outputs and inverted main outputs.  The first schematic shown is a copy of the MFOS schematic, but with the clock generator (U1A) isolated from the main circuit.  There’s also an added trigger output connected to U1C.  The second schematic is the modification board that adds new functionality to both quantizers:

  • Buffering an external input clock
  • The trigger output buffer
  • A inverted main output

The output of U1A in the main circuit connects to the normally-closed switch contact of the panel jack.  When a plug is inserted into the jack, this U1A output is disconnected and instead the signal coming in on the plug is used.  The tip connection of the jack connects to the modification board CLK_INx point.  The output of the modification board connects to the CLK_IN point on the main circuit.


Modulus has four MFOS Voltage Controlled Oscillators (VCOs).  There are two in the middle cabinet and two more in the bottom cabinet.


Modulus has two MFOS Low Frequency Oscillators (LFOs).  


Modulus has two MFOS Low Pass Voltage Controlled Filters (VCFs).


Modulus has two MFOS State Variable Voltage Controlled Filters (VCFs).


There are two MFOS Attack / Decay / Sustain / Release (ADSR) units.  These were modified to add a simple oscillator that provides an auto-repeat function, an inverted ADSR output, and LEDs to display the state of the ADSR cycle.  The schematics show the interconnection diagram for the MFOS board and the modifications board and the MFOS schematic with connection points annotated for where the modifications connect.  R10/C2/IC1A is the repeat oscillator.  IC3 and the associated parts invert the ADSR output.  Everything else is essentially diode logic to provide LED indicators for the ADSR states.


This is a dual Attack / Decay / Sustain / Release (ADSR) generator based on a design by yusynth.  Each ADSR is based on a CMOS 555 timer (ICM7555).  Both normal and inverted outputs are provided.  And of course there’s an LED to display the state of the ADSR cycle.

Dual AR

This is a MFOS Attack / Release (AR) generator.  The little add-on circuit board combines attack and release internal logic states to drive the yellow state LEDs on the front panel.

Dual VCA

Modulus has two MFOS Dual Voltage Controlled Amplifiers (VCAs). 

Noise & S/H

Modulus has a MFOS noise cornucopia and a MFOS Sample & Hold (S/H) in one module.  The noise modification board adds a low pass graininess output and buffers the random gates to a bipolar output suitable for connecting to any synth control voltage input.

Ring Modulator

This is a stock MFOS Ring Modulator. 

Reverb & Panner

This module is a combination element consisting of an original reverb drive and amplifier circuit along with a MFOS stereo panner board.  The reverb drive board connects to an Accutronics spring reverb unit which is mounted to the back of the lower cabinet.  The schematic’s transistor drive arrangement is based on the recommended drive circuit for the spring reverb unit.  The module input mixer allows several sources to be combined before sending them through the reverb spring.  There’s an LED that indicates when the unit is getting near clipping levels.


Every modular synth needs a mixer to combine the various audio elements. This is my design. It has 4 main inputs that each can be sent to the FX1 or FX2 loops or to the main output. The FX loops return to the main output along with 2 auxiliary inputs. There’s also a headphone amplifier built in.


Nothing exciting here – these speakers are just a set of amplified computer speakers gutted and fit behind my standard size panels.  They really don’t have much frequency response, but they help give a nice audio presence to the user sitting in front of the synth.  These are not the only speakers I have connected to the synth, there is a large speaker/amp unit under the Modulus table that puts out most of the sound.

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