Chord Organ

Version 0.1 from August 2021

Digital polyphonic synth lead on breadboard. I build it while having some slack time, waiting for new DIY kits for my techno rack project. The idea is taken from the Chord Organ from Music Thing Modular.


  • ✅ 4 simultaneous voices (polyphony)
  • ✅ 5 different waveforms: sine, triangle, square, saw and “tuned” noise
  • ✅ 13 different chord shapes (major, minor, 7th, …)
  • ✅ 48 semitones (from C-2 to C+2)
  • ✅ CV IN for the root note (1 volt per octave)


It is based on an Arduino Nano. Luckily I found the library the_synth. That made the implementation with some additional circuitry a relatively easy task.


The logic and design of the chord shapes are taken from the Music Thing Modular’s approach. They can be edited in the code.

Improvement Potential

  • Remove noise and crackling:
    • This is the elephant in the room. Especially in lower tone regions you can here it. I guess this is related to the software. So far I opened an issue in the repository of the fundamental library, let’s see. Fixing it by myself would be beyond my programming skills and I would rather go for building the original Chord Organ.
    • A portion of the noise can be filtered by additional circuitry or following filter modules. But of course this comes with costs for the dynamic range as well.
    • The noise is most noticeable with the sine and triangle waves. With the rest of the waves, it almost gets lost in the overtones.
  • Add a CV IN for the chord shape, just like the original module has one.
  • Add an opamp at the end for amplification
  • Implement it as an Eurorack module on stripboard or PCB.

Also see the comments on Instagram, Reddit and Mod Wiggler.

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