The circuit is a dBx-style noise shaper: the more amplitude the dry signal provides, the wider the response of the filter. This squelches MN3005 noise at low levels and allows more high frequency content at higher levels. The 2SA798 is a standard V/I converter that is "locked on" (filter wide open) if the "OFF" signal voltage appears. The timbre change is of course due to the VCF changing vs. amplitude
This reply was similarly timed to my own discovery, through the example circuits in the datasheet of the LM13600, that the circuit in the polysix is wired as a VCF not as a VCA. The datasheet also gives an equation for the cutoff frequency as a function of the circuit element values (resistors and capacitor) and as a function of the control current ("I_ABC") provided to the LM13600.
|The LM13600 at the Center is the Mysterious Post-Effects VCF in the Korg Polysix. A simple modification (the brown clip leads) can dramatically alter the high-frequency response of the Polysix.|
|Measuring the Control Current to the LM13600 VCF (click to enlarge)|
The important result to see in this result is that, even with the effects "off", the filter's cutoff frequency for the low notes is surprisingly low (4-5 kHz). Wow. A second result is that, on my synth at least, having the effects "off" does seem to open the filter somewhat, but certainly not all the way. Hmm.
It is possible that these calculations are wrong. It would be good to confirm these values by measuring the cutoff frequency directly. I did this by, first, measuring the frequency content of a sawtooth wave at TP4 on KLM-368. This point on the circuit is pretty much the signal that is input to the first stage of this LM13600 when no effects are active. Second, I measured the sawtooth wave at the input to R168 on KLM-368, which is the output of the second stage of the LM13600. By comparing the output to the input, we can see the frequency response of the LM13600 for whatever control signal the LM13600 is receiving. An example graph comparing the frequency response (relative to an ideal sawtooth) is shown in the figure below when pressing the C1 note.
|Measured Frequency Content at TP4 (before LM13600 VCF) and R168 (after LM13600 VCF) . The Effects Are Off.|
The rest of my measured cutoff values are shown in the table below. As you can see, they continue to support my conclusion that this VCF stage is definitely controlling how much high frequency energy is coming out of this synth. If you like a really buzzy, sizzling sawtooth, you might want to consider modifying this portion of the synth (though at the expense of additional hissing noise).
As an example of a modification to improve the high-frequency response, I took a single clip-lead and connected +15V from R125 to the base of Q14. This is shown in the picture at the top of this post. By applying 15V to Q14, it causes the transistor to pass a lot of current, which then induces more control current to flow into the LM13600, which causes the filter to open all the way. When I measured the frequency response (graph below), I see that the frequency response is the same coming out of the filter as going in...the cutoff frequency is somewhere above 20 kHz. This happened no matter what note I pressed.
|Applying +15V to Q14 Forces the LM13600 Filter Wide Open. The output signal (R168) has no high-frequency roll-off relative to the input signal (TP4).|
Update: The bypassing / removal of this post-effects VCF is explored in much more detail (including an audio demo) is this follow-up post.