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.
Crow
/∗∗/
On 2/25/2013 11:10 AM, 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
>