MIDI for the Korg Polysix

Polysix-M top view

Note: links to Johannes' retrofit pages fixed 2010-01-18

The Polysix is a six-voice analog programmable polyphonic synthesiser that Korg produced in the early eighties. It was produced just before the era of digital audio technology in synthesisers; it even has analog oscillators (VCO's) instead of the digital variety (DCO's) that had started appearing at the time. With two LFO's per voice (one dedicated for PWM), SSM 2044 4-pole filters (the closest thing you'll ever get to a Moog filter on a chip), knobs and easy to use controls it's definitely one of the better among the smaller analog polyphonic synthesisers.

Several years ago, during the summer of 1989 I decided to try and retrofit a Polysix with MIDI. At the time, Korg USA had made a retrofit, and Miditec in Canada had one I believe, but I hadn't seen either of them. I don't know if Kenton (UK) had their interface out yet; at any rate, I wasn't aware of it. So I decided to go it alone and see how it went. Well, someone had told me that it had a single Z80 microprocessor controlling it; adding MIDI would have been a question of adding some hardware and rewriting the software. As it turned out, the Polysix had two single-chip processors, one 8048 and one 8049. Ugh. Just reading the original software from the chips was awkward, not to mention rewriting the complete code in Z80 assembler while adding the MIDI functionality.

Anyway, to cut a long story short, a few months later I had a design and a prototype for the retrofit. It is built on two boards, one CPU board which replaces the original two microcontrollers, and a small analog board which provides control voltages for the bend and mod wheels functions. Based on a Z80 CPU, the CPU board features RAM, ROM, lots of parallel I/O and a serial interface for MIDI. Apart from installing the new boards, the Polysix must be modified by adding MIDI connectors, and cutting a few tracks and patching in some jumpers on the internal boards.

Over the years, people have asked me if I had a kit available, would I be able to copy the diagrams etc. While I made some copies for a few people, hardly anyone attempted to build it. One of the reasons for this was that I never made any PCB foil patterns; the two synthesisers that I've retrofitted have hand built boards based on copperless veroboard (an excellent prototyping method, by the way). After having put up my design notes on the web, a few people starting building the retrofit, and I also got contacted by by various people offering to make PCB layouts for the design.

Just in time to celebrate the 10 year birthday of the retrofit design, a very nice guy called Martin Wise in the UK put a lot of work into designing and testing PCB layouts with great success. He's also very kindly made them avilable to me for putting them up on the web.

Another guy called Johannes Haustensteiner took it a step further and redesigned the electronics to fit on one board, and added a number of features, like a noise generator, individual parameter control via MIDI and programmable destinations for the bend and mod wheels. He's made it all available as a complete kit, which not only includes the PCB, but also a PC-based editor/librarian and printed manual! Johannes is also planning future software upgrades to the retrofit.

So, if you want to retrofit your Polysix, there are a couple of ways to go about it:

  • Johannes' kit is available in two versions, either as a complete kit which includes ready-assembled PCB, additional components for modification of the Polysix, printed user's and installation manual, new panel labels, and editor/librarian software on 3.5" PC disk, or as a cheaper DIY kit with just the bare PCB, a programmed EPROM and the software disk.
  • You can have a go at my original retrofit, using either Martin's PCB artwork or the real prototype way, using veroboard.

    So, which way to go? I'd recommend Johannes' kit, since it contains more features than my original design, including a noise generator. Johannes is also planning future software updates which will add even more features. However, my original design continues to be available in case you want to build it all from scratch.

    I'm afraid I can't be of much practical assistance to anyone attempting to build an interface, since I no longer have a Polysix myself, however, I'd be happy to answer any questions. I'd also be happy to hear from anybody who's built a retrofit of mine.

    Comments? Ideas? Questions? Mail me!

    This page is (c) Copyright 2000 by Ricard Wolf

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