Adding external filter input to polysynth, running 6 wires off one jack to input of 6 vcfs?

Hi to the knowledgeable or, if you’re like me, enthusiastic folks

I recently picked up a korg polysix (kiwisix modded) that I’ve been tweaking to extend its functionality or make subjective improvements in sound. So far I’ve removed the post-effects VCF/noise gate for less noise and a clearer sound (made the ensemble usable IMO), shorted a capacitor for increased bass response, removed a diode to allow simultaneous saw and pulse waveforms. I think I’ll be done for the time being after trying one more adjustment: I would like to be able to run other signals through the creamy VCF and effects section. To do this, I was planning on replacing the chord memory input with a jack running 6 wires to each of the VCFs on the 6 voices, but I’m not sure if this is sound electronic design. Would there be any buffering circuits or other considerations that I should take into account to provide a well-functioning filter input?

> 6 wires to each of the VCFs

I don’t have the schematics, but the SSM2044 has positive and negative current inputs, so it’s fine to just feed a signal to one of these through a resistor of adequate value. No risk of damaging anything - clipping will occur in the worst case.

You’ll be connecting the same signal input to 6 inputs through 6 resistors. The device sending the signal to the input will “see” a rather low impedance, but as long as it’s above 5k (that is to say, as long as each resistor to the SSM2044 input will be above 30k), you’ll be fine. Worst thing that can happen is a drop of signal. Of course, the device you connect is expected to have a low impedance output. Synth, modular, pro audio gear good ; guitar or mic bad.

The only real problem I see with that is that each of the VCF is gated by the VCA ; so depending on how many keys are pressed you’ll hear 0, 1, 2, 3… 6 times the input signal going through possibly detuned filters. That’s why true polysynths very rarely have external inputs (and when they have, it’s usually through a dedicated filter).

Does the Kiwi Six upgrade allow for multitimbrality on separate MIDI channels? If it does, a mod to install all six inputs makes sense. If you can’t, it may make more sense to only add one input and use it in mono/unison mode. If you make a small external mixing board, you can hook up 4 external monosynths and run them through one filter and the FX to act like a Mono/Poly. If you get a Midi Pal or something similar you could use the function that allows several mono synths to be played like a poly synth.

Olivier, you are awesome! Thanks for the comprehensive reply. If I wanted a high impedance input, it would have to be something like 6M ohm resistors on each line,right?

Audiohoarder, thanks for your input. Yea, it seems like implementation could be a bit messy, but this could allow extra oscillators or noise to be added from an external source and mixed with the oscillators from each voice, so it seems worthwhile to connect to all 6 voices warts and all. If just filtering or effecting a signal, I’d put the synth in unison and hold modes (use PW to turn oscillators off). Only thing I’m really worried about is potential for noise b/c I’d have to run wires past the power supply, effects, and digital control section.

The kiwisix mod does not make the polysix multitimbral. Plus I think to specifically route distinct input signal to each voice, I’d have to drill separate input jacks for each voice, which scares me!

Noise shouldn’t be too much of an issue if you use well shielded cable and don’t tangle it. The less distance your wire travels, the less outside noise can influence the signal.
Keep us posted, and let us know how it goes!

> If I wanted a high impedance input, it would have to be something like 6M ohm resistors on each line,right?

Yes, but then you won’t hear anything at all because the higher the resistor, the lower the current that goes into the filter ; and thus the overall level. And adding some gain after that to compensate will also boost up the delicious analog noise floor. You have to find a compromise between a good signal level and a not too low input impedance.

I was just thinking about that! Interesting. Not to mention I’d have to actually make a circuit to add gain after the input which would make things many times more complicated. Guess it’s time to put that breadboard to use.

For testing values of resistors, that is. I’m not keen on stuffing another gain stage into the polysix.

Hey, so I wound up using 19.5 kohm resistors for 3.25 k input impedance. It’s pretty cool, but signals have to be really loud to compete with the internal oscillators. Pretty much have to red line into a preamp and then take the output from there. Still capable of some nice processing on external sounds. Running a DW8000 patch with filters wide open into the polysix can give some interesting textures. Running a drumbeat from the sonic potions LXR in can yield cool filtering opportunities or an alternate ‘side chaining’ effect where the LXR beat crunches the polysix oscillator sound as the combination periodically overdrives the VCA

Oh man… vocals through the chorus and ensemble effects sound amazing

Any audio demos? :slight_smile:

I’ll put some up sometime soon. Need to get my studio set up again…

So I think I’m going to do this to my DW-8000 and also try to add CV ins for filter cutoff, resonance, and delay time. For these connections, is there any need to add resistors or capacitors if taking the output of, say, an MS-20 and routing it to the global CV input for cutoff etc? Or what factors would I need to consider and research?

@audiohoarder

Well I haven’t gotten around to recording any demos with the polysix, but I just did an external input mod to my DW8000 and recorded a mess about with it on guitar. Really cool! I forgot to put a resistor between the input and the filters, though…, It’s a bit hot haha

https://dl.dropboxusercontent.com/u/8335661/DW8000%20guitar%20multi%20fx%20fafffff.mp3

Once the delay is added, it makes a very full sound! This sounds like a very useful mod.
Now if there were an easy way to trigger the envelope when the input reaches a certain threshold you could have an “auto wah” and delay. :slight_smile: This may be an easier mod on the PolySix.