Modular Audio Synthesizer

Cabinet A
Cabinet B

This DIY modular audio synthesizer system was constructed over a period of 7 years starting in 2006.

It is built to the 5U format and uses 4mm banana jacks. Yellow jacks represent outputs, while blue jacks are inputs. The oscillators follow the 1 volt per octave standard. All panels are made from aluminum plate, with paper decal labeling covered with contact film. Modules use a mix of circuit boards provided by others in the Synth DIY community and scratch-built perfboard designs. The cabinets were also handmade from pine and plywood. Water-based stain and urethane was then applied as a finish. Each cabinet is powered by a 1.5A Power One bipolar 15V power supply.

Cabinet A

This cabinet contains a complete synthesizer voice. It also houses the MIDI to CV converter and output mixer.


An eight channel MIDI to CV converter based on a kit from PAIA. The midi mode switches have been brought to the front panel in lieu of using DIP switches.

CGS58 Utility LFO

A dual LFO from Ken Stone which outputs square, pulse and triangle waves. A control is available which continuously varies the slope of the triangle from saw to ramp. It also controls the pulsewidth of the pulse waveform.

Thomas Henry Quadrature Generator

Four outputs are provided for this LFO which are 90 degrees out of phase with each other. Voltage control of the rate is also possible. PCB comes from Thomas Henry's design.

Triple Envelope

Three envelopes are available from this module. An ADSR is provided which has been built according to Thomas Henry's Synthesizer for the 21st Century book. Two simpler AR envelopes are provided which are based on Hoshuyama's EG 3.1. Switches are fitted to turn each into an AD envelope. These behave very quirky and exhibit lag when many triggered repeatedly.

Digital noise

This module is built around Ken Stone's CGS31 Digital Noise PCB. The VC high-speed clock section has been broken out so it can be patched elsewhere. The noise itself is based on a linear feedback shift register circuit using the 4006 CMOS IC. The module also includes a Thomas Henry Sample & Hold circuit with a lag control.

Control Voltage Manipulator

This module contains many submodules. It was built to deal exclusively with control voltages. A Porteniometer based on Aaron Cram's circuit is provided. This acts as a voltage controlled lag circuit. A switch has been fitted which also turns it into a VC LFO. A comparator based on a op-amp design is provided, with voltage control over the threshold. Two op-amp based inverters are also provided. A counter based on the 4029 CMOS IC provides clock divisions of 2,4 and 8. Finally, the last submodule provides crude diode-based logic. A switch is provided to move between OR and AND logic. It is a very leaky circuit which can also be used for audio effects. All circuits in this module have been built on perfboard.

Mixer Interface

This module also includes some submodules. A four channel mixer is provided which can also be split into two 2 channel mixers. Output and input jacks are provided to move between banana and 1/4" jacks. A driver for a spring reverb has been constructed. It drives a very strange reverb module pulled from a dead Kimball organ.


This module contains an echo circuit based on a Princeton Technology 2399 Echo IC application note. Voltage control of the speed has been added using the FET approach. This module gets very gritty as the speed of the delay is lowered. After a certain point, the sound drops out entirely with a satisfying zipper noise effect. This circuit has been built on perfboard.


The PCB for this voltage controlled oscillator comes from Ray Wilson at Music From Outer Space. The module is temperature compensated and extends down to LFO rates.

CGS35 Synthacon Filter with Drive

This module uses a PCB from Ken Stone. It works in reverse: three inputs can be provided at different frequency bands which then appear at the output. In this way, it can be used as a signal scanner. It is a very aggressive diode filter. A switch has been fitted to overdrive the output amplifier in an open loop.

4069 Dual VCO

This module was built on perfboard and based on Rene Schmitz 4069 CMOS oscillator. Two VCOs are provided which output floppy saw waves. A saw to triangle waveshaper has been fitted to each output to provide floppy triangles. As the rate gets lower, the triangles lose more symmetry.

MFOS State Variable Filter 2006

The PCB for this polite filter also comes from MFOS. It provides voltage control over the Resonance. It is very clean and tracks 1V per octave over a decent range.


This module also comes from MFOS via PCB. The slope of the VCA can be switched from linear to logarithmic.

MFOS ADSR with Inverted Output

This ADSR PCB comes from MFOS. It can be switched between two ranges. An inverter was added to provide an anti-ADSR voltage on a separate output.

CGS29 Rev 2 Wave Multiplier

This module consists of three different wavefolder circuits. It is based on Ken Stone's CGS29 PCB. The first circuit folds the wave over on itself up to four times. This circuit works best with a triangle or sine wave. The second circuit uses a Lockhart folder circuit two combines two waves into a new waveshape. The last circuit adds extra feedback at sharp inflection points of the input wave.

Analog Chorus

This module is essentially a panel mounted Washburn A-SC7 chorus pedal. This pedal uses an analog Bucket Bridge Delay circuit to provided the chorus effect. Voltage control over the speed has been added with an LM13700-based voltage controlled resistor circuit.

Cabinet B

This cabinet was mainly used for experimental modules. It contains two reverb tanks, as well as a drum machine and panel mounted MFOS Soundlab mini synth. It remains in an unfinished state.

Thunder Bay Foundry

This circuit contains three submodules. A CGS30 Bandpass Filter PCB provides a summing bandpass filter with three inputs. Two spring reverb driver circuits based on the Hot Springs design are present. One drives a Hammond module, which the other drives a module pulled from a Biamp mixing desk.

SF Nonlinear Filter

This original design builds a non-linear shaper circuit out of a chaos circuit. Switches are provided to move between different range, activate a smoothing circuit and literally change the color of the sound by the LED currently used. It can also act as a generator when pushed into feedback. Voltage control of the feedback is also provided.

CGS49 Dual Wasp Filter with Mixer

Two Wasp CMOS filter circuits are combined on this module thanks to Ken Stone's CGS49 PCB. A mixer system has been added to combine the output of the two filters. Mode switching uses On-Off-On switches in lieu of rotary switches. This allows for some strange filter node combinations.

Hexagram Cloud Generator

This original design is based on the 40106 CMOS IC. Six oscillator circuits are linked together to produce a noise cloud. Divergence of two of the oscillators is also controllable. The Modulate input moves all six oscillators at once.

Power Converter Voltage Scaler

Voltage scaling, offset and rectification functions are provided by this custom design built on perfboard.

Ring Modulator

A classic passive germanium diode and transformer based is provided in this module. A switch is provided to move between AC and DC coupling on the modulator input.

Univox SR-55

A Univox SR-55 drum machine has been rehoused in this section of the cabinet. Trigger inputs have been added to each drum sound.

MFOS Soundlab 2006

The Soundlab is a complete mini-synth project provided by MFOS. Two oscillators, a filter, an envelope, an LFO and a VCA are provided to make a complete synth voice. Each circuit has its own quirks due to the simple, low component count of each design. This module is a rehousing of a 2006 revision of the Soundlab. It has been modified to run on 15 volt bipolar power and to work with banana plugs. Note that this module does not have any knobs attached.

Looping Logic Sequencer

This original design implements a step sequencer with a shift register circuit which feedback on itself. Data inputs are provided to inject steps into the sequence. The output is a combined control voltage which sums all the steps. Thus an additional bit in the shift register completely alters the output sequence.