ABOUT THE CIRCUITS
I know very little about how these circuits actually work– 99% of the circuits come from the internet, especially the following places:
– electro-music.com DIY forum
– electro-music.com Lunetta forum
– Nicholas Collins’ “Handmade Electronic Music” (2006)
– Fun with Seamoss website (no longer up)
– Hackaday Logic Noise series
– Ken Stone’s Modular Synth site
– Yu Synth DIY

Thanks to all those folks for sharing their designs! Please don’t look at my circuits, go see the originals. Those folks know what they are doing. My circuits are only here for reference, just to show you can build a great-sounding synthesizer with extremely primitive circuits.

BUILDING THESE CIRCUITS
Don’t just build these circuits– breadboard them first! I made many small modifications that may not be reflected in the schematics; also, I barely understand anything about electricity so take everything with a grain of salt. If you know your way around electrical engineering you will probably see about a million things I’ve done incorrectly. That said, my synthesizer works and sounds awesome, and I haven’t blown anything up (except for a handful of ICs, resistors, LEDs, pots, transistors, and other components).

MY SYNTH “FORMAT” AND CONSTRUCTION
Originally I intended to follow the Eurorack format (which I think is ±12V with +5V for digital modules) but I’m not skilled enough (nor did I have the right wall warts) for that. Instead, my synth uses ±9V (from a +20V wall wart) and +5V (from a +6V wall wart). The size is mostly Eurorack (3U tall) and I’ve built my cases standard rack size widths (19″). My panels are constructed out of 1/8″ plywood, mostly oak, and decorated with sharpie markers. For connectors I use #6 nuts and bolts and connect them with alligator clip patch cables I make using clips and boat trailer wire I get at the grocery store (comes in cool colors). My general focus has been on building basic modules that can be patched to make cool sounds. I also use a lot of LEDs because blinkenlights are awesome.

I currently (Feb 2016) have about 45 modules that include around 15 oscillators (mostly square waves) and lots of ways to manipulate them (filters, logic modules, shift register, sequencer, comparator, dividers, envelopes, LFOs, amps, etc). I also have a sort of “skiff” that includes a very primitive keyboard and a breadboard mounted into a panel for easy circuit building. My synth is built into 4 19″ x 6U cases (24U of modules) constructed out of wood from reclaimed pallets.

In addition to the DIY modules, there is also a broken-down Korg Monotron Duo I use primarily for the excellent low pass filter and a Korg Volca Keys I separated from its case and shoved into my rack.

MORE INFO
Circuits, code, panel design, PureData patches, Arduino sketches, etc: https://github.com/chrisbeckstrom/diy_modular_synthesizer/
Hackaday IO project: https://hackaday.io/project/5826-diy-mostly-analog-modular-synthesizer

LIST OF MODULES

updated February 20, 2016

OSCILLATORS/GENERATORS

--- AUDIO RATE

VCO (saw, square, triangle, short pulse)

VCO (saw, pulse)

voltage controlled digital sound source (arduino) with MIDI input

-- LFOs

Dual LFO (square, triangle)

square LFO ("pulse")

LFO (not working)

LFO x3 (square, "triple clock")

LFO x5 (square LFOs)

-- OTHER

CMOS noise

White noise (HP, LP, regular)


MODULATION

--- VCAs

VCA (op amp + transistor, diode)

VCA (transistor)

low pass gate VCA/filter

dual VCA (vactrols)

VCA (vactrol)

--- SEQUENCING

10 step sequencer (CV and gate)

dual 4-stage shift register

--- ENVELOPES

envelope generator (A/D)

slope generator (rise/fall)

--- CONTROLLERS

shotgun shell photoresistor

expression pedal input

arbitrary voltage controller (keyboard-type thing with gate and CV outputs)

ribbon controller

--- OTHER

sample and hold

HARMONICS/WAVESHAPING

low pass resonant filter, voltage controlled

voltage controlled filters low pass + high pass (can combine for band pass)

twin T drum/filter (not working)

voltage controlled diode clipper (positive and negative)

EXTERNAL INPUT/DISTORTION

Preamp (741)

preamp x4 (set gain x5)

preamp/distortion (with lots of gain)

stereo preamp (for iphone, etc)

iPhone holder + power with audio outputs

LOGIC

quad NAND gate (only 1 works)

logic inverter x3

sequential switch

quad AND gate

r/2r ladder

MIXING

4 input inverting mixer

diode mixer / ring modulator

passive attenuator x3

3 input non-inverting mixer

passive attenuator / i/o (headphones)

EFFECTS

delay (pt2399)

reverb (send and recovery)

UTILITY

dual control voltage processor

simple slew (capacitors)

gate to trigger converter (x1 active, x2 passive)

clock divider (powers of 2)

clock divider (/2 /3 /4 /5 /6 /7 /8 /9 /10 /16)

dual comparator

physical switch (bolts)

breadboard

MISC

trunk line (4 busses via cat5) x2

jack converter (banana, 3.5mm, bolt, 1/4")

jack converter (bolt, 3.5mm, 1/4")

misc (button, switch, power connections)

oscilloscope connections

power supply (+-9V)

monotron duo

external controller power (for skiff)

power (+5V)

3.5mm jack to bolt converter