Looking at the LM13600 data sheet, down through all the example circuits, this use of the LM13600 does NOT match the circuit shape of a VCA. The negative feedback and the capacitor between the OTA and the darlignton pair actually make this look like a voltage-controlled low-pass filter.
The app notes say that each stage of the LM13600 configured as a VCF generates a -6 dB/octave filter. So, in the Polysix, the two stages cascaded together would give a -12 dB/octave filter.
Looking at the values for the resistors and capacitors, and knowing the control currect (Iabc), one can calculate the cutoff frequency of the filter. The circuit values are clear from the schematic. It's Iabc that's trickier.
For Iabc, there's a tiny barely-legible note in the schematic shows that the IE is 1.2 mA, which is then knocked down to 1/3∗IE (which is 0.4 mA) which is then divided between the two halves of the LM13600. As a result, the maximum (?) control current to each 13600 is 0.2 mA. Running this through the F_cutoff equation in the LM13600 datasheet, you get:
Fc = (Ra∗gm)/((R+Ra)∗2∗pi∗C) where gm = 19.2∗Iabc
Fc = (220∗19.2∗0.2e-3)/((10E3+200)∗2∗pi∗470e-12)
Fc = 28 kHz
This suggests that if the Polysix really drives this LM13600 to the 0.2 mA level suggested by the note in the schematic, that this element will open up to just beyond the range of human hearing (20 kHz). That's pretty reasonable.
With less current drive, the 13600-based VCF will close down, cutting out the high frequencies (often noisy and hissy) first. Since the current drive appears to be based on the magnitude of the input audio signal, it suggests that the filter opens when the input is strong (loud) and closes when weak (quiet).
Is this the behavior that the designers intended? Is this the right interpretation for this portion of the "Compressor/Expander" circuit? If so, I'm still troubled by the fact that the current drive circuit (the level detection circuits) seem to be railed at +15V. How are they doing their job?
Thoughts?
Chip
--- In PolySix@yahoogroups.com, "chipaudette" <chipaudette@...> wrote:
>
> Hi Folks,
>
> Based on a previous post regarding frequency response of the synth's output, Johannes suggested that I check out the "compressor/expander" circuit on the KLM-368 effects board. I totally agree that the circuit has an interesting effect on the sound of the synth. I was hoping to figure out how it worked so I could better understand its purpose.
>
> Below I show a schematic of this circuit along with some annotation and discussion of the individual elements:
>
> http://synthhacker.blogspot.com/2013/02/mystery-circuit-polysix-post-effects-vca.html
>
> Am I getting it right? What is this thing actually doing? Is it just a noise gate?
>
> I'd love your thoughts.
>
> Chip
>