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Modified Korg Poly-61 (New!!!)

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I wasn’t very familiar with Korg Poly-61.  I’d had a DW-6000 and, having only seen the occasional picture of the P61 on the web, I thought it was another largely digital hybrid, encased in a plastic shell.  One appeared for super cheap on my local Craigslist ($25, I’ve paid more for Casios!) – the seller said on the phone that it had several issues, so I figured I’d collect it for parts which I’ve done with other synths (harvest the CEM or SSM chips, some keys, a chorus board, etc). 


But when I showed up to buy it, I was surprised that it was actually quite weighty, made of metal and wood, like a Juno or a Polysix.  Then I went back on the internet and downloaded the service manual and found out that my initial assumptions were _very_ wrong.  The P61 is nearly all analog, but for the DCO’s and the horrible, horrible digital parameter access/control.  What’s good:  the main DCO is of the Juno type (well-described elsewhere as a sawtooth core under digital timer control), not a sampled waveform; EG’s are analog from the SSM2056; the two LFO’s are simple but analog; VCFs are interesting 12dB/oct OTA based; VCA is serviceable Korg transistor+op amp.


The digital control of all that analog is awful though.  Not only is it a ‘one-at-a-time, punch in the parameter number’ design, but there’s not even a slider (Rhodes Chroma, DX7), encoder (Moog source), or rotary pot (AX-80) – you get to use only incremental up and down buttons.  In a word, it sucks.  I can only guess that Korg was trying to dramatically cut the price of the unit.


So, if you don’t absolutely need presets (the Juno-6 comes to mind), the P61 is a perfect mod candidate.  There’s a number of parameters that the microprocessor controls in a global (all voices) way, with a single CV.  The list of mods that I made is:



Now, the work here isn’t particularly beautiful, because I didn’t want the project to take more than a few days.  But it all seems to work pretty nicely.  Some of the controls could be made a little more linear or logarithmic, depending upon the function, but this P61 is now _fun_ to play.  Having voices instantly adjustable is really great for this synth because of the chord hold and arpeggiator functions.  You can tweak a drone, or filter sweep an arpeggio. 


How does the P61 sound?  It’s not as warm as the Juno-60, but the filter is more aggressive and the 2nd oscillator can add weirdness or give you some detuning for a phasing effect, like the Juno chorus.  One nice thing is, with the mods being purely analog, there is no stepping at all, as you can sometimes hear on the Juno.

Here's a Flickr link to some photos of the modified synth:
And here's the Youtube link to a video demo:
Want the schematics?  Please email me (see address on Home Page here)