CMOS Modular Synthesizer

Low-cost, high-effectiveness DIY modular synthesizer system aimed at beginners

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From the day I picked up a soldering iron, the big goal was always to have some sort of never-really-finished, always-being-expanded modular synthesizer system. In my dreams it occupies an entire room of my luxury beach-side mansion where I sequence and patch squelchy noises that I record and sell to continue my exuberant lifestyle. Only one part of that dream is likely to happen but it's coming one step at a time.

On my journey to build this enormous analog monstrosity that my future children will probably have to sleep amongst after it takes up what would have been their bedroom, I found that online resources tended to either assume you knew nothing or assume you already are an engineer and don't need to be told anything.

There was nothing to bridge the gap between these two states... so that's what this project is. Learn about electronics through mechanical squelching sounds, blinking lights and chin scratching.

This DIY modular synth system is very much in the "Lunneta" style. Steve Lunetta was a man I never met but who seems cool who was really interested in abusing CMOS logic chips to make sounds from them. It turns out this is a really great way to have a lot of fun making a horrible racket without spending much money. If you want 1V/Octave tracking, temprature compensation and other things that make tonal music possible, you are going to be dissapointed. If you want to make sounds like a dying cyborg trapped in a meat grinder then you are in the right place!

Panels are for eurorack size blanks - 3U tall and some multiple of HP wide.

The banana jacks are color-coded for easy patching – consult article #000 for more in-depth explanations:

  • Red - output
  • Black - primary (data/audio) input
  • Yellow - CV input
  • Blue - Clock input
  • Green - Trigger input
  • White - Gate input

Every article contains a schematic, a perfboard layout, a drill template for a eurorack sized panel, as well as an explanation and analysis of the circuit. Everything runs off a simple 9-12V DC wall-wart type adaptor, and outputs signals that range from 0-5V.

Though I have designed these to be beginner friendly, I reccomend you first know how to read a schematic and how to lay out and build something on perfboard as well as on a breadboard. Neither of these are tough though, so don't worry! You can also definitely use these projects to learn about all of those things, but ultra-simple projects to get your feet wet might make things go smoother. Also, don't expect everything to go right the first time. Hunting down your mistakes and fixing them till everything gets right is part of the fun! Have patience!

New modules come as fast as I can make them (there is a long to-do list!) but the modular generally takes a back seat to other, bigger projects so progress can be sporadic

  • MODULE 005 – R-2R

    Castle Rocktronics02/09/2015 at 04:44 0 comments

    R-2R is two "R-2R ladders" in a tight 6HP package. The four inputs act as a 4-bit analog to digital converter that is perfect for generating stepped waveforms to drive oscillators as a CV source or to create more nuanced sounds than the raw square-waves everything has been putting out till now.

    The article gets really tucked into DC analysis as we pick apart how and why the R-2R ladder works. You will be sick of Ohm's law by the end of it, but you should also never have to read up about it again!

    – 6HP


  • MODULE 004 – Dual Shift Register

    Castle Rocktronics02/09/2015 at 04:37 1 comment

    Two deceptively powerful and useful little tools tucked in nice and tight on a small panel. Chain them together for a larger 8-bit register!

    The article covers what shift registers actually are, a tiny sampling of their many uses and a look into a circuit building block that we'll be getting very familiar with – the leading edge detector. There is also what I hope will be the last of my woes from working with the acrylic for the front panel.

    – 8HP


  • MODULE 003 – 4xLFO

    Castle Rocktronics02/09/2015 at 04:13 0 comments

    Four square wave LFOs with little blinky lights to show you how fast they are going. Endless fun. Just don't stare into them too long or you will hurt your eyes.

    This article is very short as the insides of this module are just the same as 001 – 4xSQUARE, only with bigger capacitors. However, I ran into a few problems while making it, which I write about briefly, mainly to do with drilling and measuring out panels – both of which I ballsed up terribly...

    – 12HP

  • MODULE 002 - Output Mixer

    Castle Rocktronics02/09/2015 at 04:04 0 comments

    The other thing you can't not have is some way to get the sounds together on one cable and out to an amp or your computer. That's exactly what this module is for, with a little added extra: built in overdrive! 4 channels that are summed together and stepped down to line-level so you don't blow anything up & smooth, but gritty overdrive for when you want to get aggressive. Very nice when you have pulsing ambient drones going on.

    In the article we cover op-amps as inverting amplifiers (and summing amplifiers), using op-amps on a single-rail power supply as well as decibels and how diodes in an op-amp's feedback loop create clipping.

    – 18HP


  • MODULE 001 – 4xSQUARE

    Castle Rocktronics02/09/2015 at 03:59 0 comments

    You can't have a synth, modular or otherwise, without oscillators! But why have one when you can have four? This is a slight twist on the popular 40106 oscilator bank, with very simple CV control of the oscillators added with diodes. Very low parts count, a nice easy build.

    In the article we cover why it oscillates, as well as why the diode allows for pitch control and gating. On top of that, we figure out how to calculate the pitch of the oscillators and how to figure out where the gating effect of the diode kicks in.

    – 18HP


  • MODULE 000 – Power & System Overview

    Castle Rocktronics02/09/2015 at 03:29 0 comments

    The only module you will use 100% of the time. It's slightly dishonest to call it a power supply though, as really you use a DC wall-wart as the power supply, this is just a switch, an LED and a bus board. It uses generic connectors DIP spaced connectors (XH-2.54 type) to allow for easy attachment and detachment of modules for rearranging. There is a fuse so you can ignore current draw till it blows and then be thankful you had a fuse since you weren't tracking current draw (it's very low though). Also includes a daisy-chain output for powering guitar stompboxes off the same power supply and an external ground connection for interfacing with other synths and gear.

    There is also a general overview of the system, including the colour scheme for the banana jacks, some generic circuit building blocks like input and output protection, and what the general ethos of this DIY project is.

    – 4HP


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ebrahim0687 wrote 09/20/2016 at 09:33 point

Hi there,

Module 04 on shift registers PDF link doesnt seem to be working. It redirects me to an image of the panel.

Any chance of fixing this?


  Are you sure? yes | no

davedarko wrote 02/09/2015 at 09:21 point

Nice project! This is exactly what I had in mind the other day for my workbench next to my PSU - I thought about having a pulse generator and more stuff that allows me to make music at some point and this is somewhat the same and more! Is there a standard size for those panels? I have a 30cm * 70cm box and use A4(210*297) / A3(297*420) sheets of plywood for the covers anyway, but always wondered.

Is there a video where you show it in action? Would be cool to see that!

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Castle Rocktronics wrote 02/10/2015 at 01:51 point

There are a few modular standards which define the power-supply, output swing and physical size of modules. I am using mounting rails intended for the "eurorack" standard - the most widespread today and also the most compact. However, this is *not* a eurorack modular. Eurorack uses a +/-12V supply and tends to send and accept +/-10V or so signals. This system is much lower power, I just use the same size because it was easiest to get rails for that I could put into a rack mounted unit. Other common systems have similarly silly names: Frac Rack, 5U, MOTM, dotcom, Serge. It is definitely possible to get all of these to talk to each other if you really want though, but you tend to have to make a "breakout" box or module.

More specifically, all of these modules are 3 rack units high, or 3U. This is 5.25" or ~134mm tall. 1U is 1.75"/44.45mm. The width is measured in horizontal pitch or HP. 1HP is 0.2"/5.08mm. This is the same height and width standard used by the eurorack standard.

There are drill templates for each of the articles so you can definitely just drill holes in whatever you want! I just have two big acrylic blanks that are the same size as one row on my rack when it is empty. I went for acrylic because you can just score it deeply with a boxcutter and then snap off what you need. It's also easy to drill... when you do it right. The first three or four articles have a little bit at the end dedicated to mistakes I made when drilling and cutting.

Video is not possible for now, unfortunately. I live in China so no youtube here. I am hoping to finally get some audio snippets up on the webpage this week though.

  Are you sure? yes | no

davedarko wrote 02/10/2015 at 08:56 point

Wow, thank you for those insights and informations! You've done an awesome job with this project page btw., since it's not a link drop but a very detailed write up (obsessive tagging ;) ). I'm looking forward to the audio snippets.

  Are you sure? yes | no

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