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This is part 1 of my Polysix mod series. Here is a demo video

Polysix mod by Tubbutec from Tubbutec on Vimeo.

Todays mod: Add pseudo portamento and voice detune/spread to your Korg Polysix

These are actually two different modifications of the Polysix I developed recently. I always missed the possibility of portamento in my Polysix, unfortunately it is quite difficult to add real portamento, because of the polyphonic nature of the Polysix. But it is possible to add some envelope-to-pitch functionality and this is basically what I did.

The second modification relates to the Polysix’s unison mode. In unison mode all the six voices are played simultaneously. But because this would not sound much different from playing one voice, the voices are slightly detuned in this mode. this is where my mod cuts in. It is now not only possible to adjust the amount of  detune but it is also envelope and LFO controllable. The results range from fat phasing basslines to bell like sounds to weired huge strings.  It is also possible to detune the voices when not in unison mode, the result is similar to a detuned polyphonic synthesizer which is also quite a nice effect.

Although the following description might sound a bit difficult to understand to beginners the ciruit itself is fairly easy to build. All you need is some soldering skill.

Portamento mod

The Polysix only uses one exponential converter for all six voices. The CV-voltages are multiplexed, fed into the expo-converter and ten demultiplexed and distributed to the six voiced. We want to alter the pitch exponential rather then linear, otherwise the lower notes would “glide”  faster then the higher notes. This means we have to cut in before the exponential converter. We also would like to have only one potentiometer to control the amount and maybe one switch to control the polarity of the effect.  This means we have to demultiplex the envelopes of the voices, control the amount and polarity and then feed it into the expo-converter.  And this is basically what I have done here.

Picture 1 shows the portamento part of the circuit.  IC1 is an analogue switch with 8 inputs and one output. Inputs x1-x6 of IC1 are fed by the outputs of the envelopes, A, B, C is the address buss of the Polysix multiplexing circuit. IC1 multiplexes the envelope voltages and they get into IC2A where some offset compensation and amplification is done.  I wanted some kind of behavior that the note is detuned in the beginning and gets less detunded during the attack phase of the envelope until it reaches its normal pitch after the attack phase.

The picture above illustrates what is done to achive this. First the maximum amplitude of the envelope is substracted by R2 and TR2 to get 0V at the maximum amplitude. Then the Voltage is inverted be IC2A. TR1 adjusts the maximum envelope amplitude and therefore the maximum amount of detune. You don’t have to use a trimmer here, I used one to set ecxactly 1 Octave as maximum. If you dont care, just use a 100k resistor instead od TR1.

P1 is a potentiometer to adjust the amount of portamento, SW1 changes the polarity of  the signal to get either positive or negative envelope amount. The output of IC2B is connected to the input of the Polisix’s expo-converter. (See later article for a connection diagram)

Next: Voice spread modification