I've been thinking about this for a while, and thanks to some pointers from the Synth DIY mailing list, I've managed to get a nice workable mod for realtime filter cutoff and resonance on my Poly 61.
The digital signal for the filter cutoff can be found coming out IC 42, where a simple DAC is created with 6 signals and some resistors. These lead to IC 36, which buffers the signal and sends it to pin 12 of connector 10A (cpu to voice board).
Similarly, filter resonance comes from IC 36, a buffer that is connected to IC 28. Pin 13 is the location on the connector.
Take the signals from the connector cable by carefully cutting wires 12 and 13 and stripping them. Solder wires and insulate propers (this can be tricky, and messy).
For the cutoff pot, you can create a voltage divider between +5 volts and the signal coming from the CPU board (max cutoff is -5volts). There are many places to get 5 volts, I used Pin 26 on IC28.
For the resonance, use a ground point for one side of the pot, and the cpu board side for the other, the signal is 0-10 volts.
Probably not the cleanest mod in the world, but simple to do, and it really adds a new level of expression to the cheap and easy to find Korg Poly 61. One note, when you program a patch now, the cutoff level will determine the max cutoff amount the potentiometer will affect, and resonance amount will determine the minimum resonance setting for that potentiometer. In other words, for full range, set the cutoff to 63, and the resonance to 0.
Enjoy non stepped filter sweeps!
8 comments:
Great Job. I understand not everything. What do you mean with CPU board. Where is it located. I see for the resonance 2 black wires going to 12 and 13 from the ribbon and one blue wire, but this one i don't understand were to connect. The two blue (or green ) wires from the ribbon goes to the cutoff. I think that i understand this. but can you please expain me where i have to connect the pins of pots going to CPU ? Thank you in advance. And again , really nice job you did.
Hello here phger again.
If i understand the cutoff i think is like this.
Blue wire goes to PIN26 and the two green to 12 and 13.
I understand the resonance : the two blac wires goes to the same 12 and 13 of the ribbon. And the blue goes to ground ( this i can choose, where did you put it ). You think i understand. Thank You very much.
Last question that will me understand a lot :
Is 12 the CPU side and 13 the rihgt ribbon ?
first thing to note, my battery died on my p61 after I did this mod. I don't think its related, but i need to investigate further.
the cpu board is the one on the left side, connected with the ribbon cable. should actually be labeled, as you can see in the 2nd pic.
the last picture should explain how to wire the pots. the resonance pot: pin 1 goes to the right side of the ribbon cables' pin 13 wire, pin 2 of the pot goes to the cpu side of the ribbon cables pin 13 wire. The 3rd pin on the pot goes to ground (any ground point on the chassis).
for cutoff, you need a wire from pin 1 of the pot going to +5v. I used pin 26 on ic 28...that might be why the battery drained, i still need to investigate. for pin 2 on the pot, use the right side of the ribbons pin 12 wire, and for pin3 use the left, or cpu side.
hope that helps.
Thank a lot for quick respons !
Nice work. I found a similar mod in German, but it was more complicated and I couldn't quite make it all out.
I will try this ASAFP. I have two P-61s(one broken)and have to swap out a midi board which will require a little soldering. Now I have motivation to start and a test machine.
Any findings about the battery drain?
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