DIY Modular Synth: MIDI Distribution

I’m planning to release a PCB and, potentially, a full kit for this project in 2018. It’d be a great help to me if you’d spare the time to tell me whether this would be of interest to you (there’s a form at the bottom of the article).

Let me put this out there at the very beginning of this post, just so that we’re all clear:

I’m not an analog purist. I quite like digital noises, too.

There, I’ve said it. Still with me? Good.

I fully intend for my DIY modular system to contain both analog and digital elements; if it sounds good and/or it’s easy to work with and especially if it’s a cheap build, it’s in. As a result, I have a plan to produce a couple of very quick-and-lo-fi synth voice and drum modules based on Arduino. They take MIDI rather than CV as their main control (although I will be adding CV mod inputs to them). So, I need a sensible way of getting MIDI around my case, preferably without having tons of MIDI sockets all over the place.

To solve this, I turned to a project I’ve already built – a simple MIDI THRU box. I hatched a plan to have this internalised within the case, effectively providing a bus for squirting MIDI to the modules that want it. It then occurred to me that most of these devices run on +5V – so why not combine the two and send MIDI, +5V and ground from a simple 4-pin header?

Now I know there’s already scope for a +5V line in the Eurorack power bus that I’m using. And it also occurred to me that I could break standard and utilise some of the spare capacity in the bus to carry MIDI. But… that wouldn’t be properly managed, properly split MIDI, and I really like the idea of just having one 4-way ribbon do both jobs.

Also: it’s my build, so n’yer. We’re doing it my way.

Anyway, I designed it up and damn if it doesn’t work. MIDI goes in, 5 separate MIDIs go out. +12V goes in, 5 separate +5V lines go out.


Notes on the design & optional stuff

  • J2 is MIDI IN. This could be direct from a MIDI jack (JK1 in the schematic), or from elsewhere in the modular system. In my case, it’ll be from the Master Clock module that I’m in the process of designing so that the same clock gets sent to everything. The Master Clock module will have the main MIDI input for the entire system on its panel.
  • J8 receives +12V and GND from the main power supply. In my system there’ll be a header on the main PSU board (again, in design) specifically for supplying this board. If you want to supply from your main bus just replace this header with the connector of your choice.
  • J9 is a +5V return to my main PSU board – solely so that I can show a “+5V ON” indicator LED on the PSU panel. This header pin is entirely optional.
  • R5, R10, Q1, J6 and LD1 are optional – they’re a “MIDI Activity” LED and driver. Even though the components are on my board, I’ve not yet decided whether I’ll even be using this feature myself (UPDATE: it turned out that I didn’t)
  • The four-pin headers send MIDI, +5V and GND. You could easily adapt this to just send the +5V to your power bus, and have just 2-pin headers to send MIDI to your system instead.

That’s about it. It’s a pretty simple design that doesn’t really expand much on the MIDI THRU circuit that I’ve described previously, except that it’s designed to hide away in your modular and play nicely with your +12V PSU.

Build Documents

Schematic (Board)

MIDI & 5V Breakout Schematic 1

Schematic (Panel, optional)

MIDI & 5V Breakout Schematic 2

Stripboard layout

MIDI & 5V Breakout Board

Flipped stripboard layout

MIDI & 5V Breakout Board Flipped


Interested in building this project?

I’m in the process of designing and procuring a PCB for this project; if you’d be interested in buying a PCB or a full kit of parts please let me know using the form below – it will help me to gauge what level to pitch the initial release at, what format to design for, etc.


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