Sunday, 20 September 2015

Overcycler 2, the return

Overcycler, my homemade polyphonic hybrid synthesizer, while being a working synth that I made music with, had quite a few caveats in its design, here are a few of them:

  • Being a 5 PCB design, I tought it was a good idea to have voice cards but it just complicated the design and made the enclosure a lot harder to build.
  • The enclosure was too small, not very practical, and overall badly built. The fact that it was too small also forced me to make a lot of hacks to fit everything inside.
  • The display was too small, which made parameters hard to read fast.
  • VCAs were noisy, bleeding audio even when fully closed and gain staging was wrong, which made the sound too tiny past the filter.
  • Using the VCA to compensate for resonance volume loss, it seemed like a good idea, saving the cost of one more VCA per voice but it made the synth sound small with resonance, as the SSM2044 lose its overdrive the more you added resonance.
  • DAC / sample & hold mechanism for CV control. Not a bad thing in itself but very sensitive to noise.
  • Link capacitors in some parts of the voice design had too small values, leading to poor bass response.
  • Overall just a prototype that was left unfinished.

So, as the title says, I'm making Overcycler 2, a full rebuild / redesign of Overcycler. Besides having a more enjoyable synth to make music with, I hope this will make it a properer DIY project that others can build. As you can guess all the previous issues were addressed:

  • Single PCB design, roughly A5 size (153mm x 209mm).
  • Bigger, better built, nicer enclosure, with wooden sides for that vintage vibe :)
  • Bigger display, 40x4 characters instead of 20x4.
  • Lots of work on gain staging, to try to make the synth always sound good.
  • Using the pre-filter mixer to compensate for resonance volume loss, requiring no additional parts and nicely doing the job. That was kind of an eureka moment when I thought about it :)
  • 8 channel DAC chip to handle the CVs. They are quite expensive so I chose to ditch multitimbrality (which never worked on overcycler) to keep the costs low.
  • Bigger and better quality link capacitors.
  • This time I really want to make it proper. 

By the way, I added a voltage controlled noise generator of my design and also removed the VC master volume to replace it by a dedicated potentiometer.

Right now I have a fully working voice on breadboard, a prototype PCB that is sent for production and I just finished drilling / preparing the enclosure, here are a few pics of it:

Now compare it to this and you will see what I mean by better built :)

To be continued when my PCB arrives!

Project page:

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