Friday, August 16, 2013

Polysix - Disabling the Built-In Detuning

As many of you know, I replaced the Key Assigner in my Korg Polysix with an Arduino and a DAC chip. The flexibility of this setup allows me to do all sorts of arbitrary pitch manipulations such as pitch bending, aftertouch-driven vibrato, and pitch detuning. My implementation of detuning was discussed in this post and I really enjoy having that capability.  One incomplete aspect of my detuning modification is that I did not properly address the interaction of my detuning with the fixed amount of detuning that is already added by the Polysix when it is set to Unison mode.  This fixed amount of detuning confounds my ability to properly add just the right amount of own amount of detuning.  So, as described below, I've decided to simply disable the Polysix's built-in detuning.

The schematic above is an excerpt from the KLM-366 PCB.   It shows the elements that create the fixed amount of detuning when the Polysix is in Unison mode.  When in Unison mode, the Polysix enables this circuit  by applying a LOW voltage to Pin 6.  This allows IC30 to do its job -- which is to take the "detune" voltages created on the right side of the chip (along with R116) and multiplex them onto the "X" pin.  The multiplexed output then goes off to get added with the main Pitch CV signal so that the combined Pitch CV signal creates the pitch that you desire plus the small amount of detuning generated by this circuit.  When the Polysix is in any mode other than Unison, the Polysix applies a HIGH voltage to Pin 6, which disables this chip and prevents the detune voltages from being output.

To disable this built-in detuning feature, I am going to apply 5V to Pin 6 so that the chip is always disabled.

To implement this modification, I need an easy source of 5V and I need an easy place to inject it into Pin 6.  Looking at the full schematic, and looking at the real-life PCB, I see that I can grab 5V off R121 and I can inject the 5V via R118.  The trick is to grab the correct side of R121 and of R118.  After a little probing using my multimeter in "continuity test" mode, I found the side of R121 that was connected to 5V (the left side) and I found the side of R118 that was connected to Pin 6 of IC30 (the right side).  As shown below, I soldered a short jumper wire and gave it a try.

I added the short red wire in the middle to jumper R121 to R118
The result is that the detuning has been successfully disabled.  Now, when I switch to Unison mode, it sounds just like if you load up the Chord Memory function with 6 of the same notes.  If your Polysix is in really good tune, you get a single tone with a very slow phasing sound.  Then, when I activate my own detuning, the 6 voices spread apart by a controlled amount, which is what I want.

It's a nice and easy modification that achieves my goals.  I do think that detuning is an important part of the Unison sound, so I do not recommend this modification if you have no other way of creating the detune effect.

In fact, if you're relying upon the built-in detuning, my taste would have preferred more detuning.  If you, too, like a good amount of detuning, you can easily increase the built-in amount of detuning by changing R116.  A larger resistor will result in more detuning -- a smaller resistor will result in less.  Try a couple values and you'll find what you like!

Thanks for reading...

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