Korg FK-1 filter schem

[Paul Perry (Frostwave)]

http://www.korganalogue.net/korgother/Synthepedalmanual/synthpedalschematic.pdf

Thanks to Ben at the Korg korgucopia site.
Appears to be an early keyboard filter.
I haven't had a close look at the schem yet, but all Korg stuff from this era is extremely oddball, so here's hoping!!

http://reviews.harmony-central.com/reviews/Keyboard+And+MIDI/product/Korg/FK-1+VCF/10/1

[Isaiah]

Ace, thanks!

It seems like a band-pass filter, with an envelope follower to control it or the option of a manual control.
Or something, that's just a guess from the schematic.

I'd like to build just part of the filter by itself (JFET and first three transistors) with a low-pass response.
I love discrete circuits.

The opto coupler looks weird, almost like the resistive element is a pot.
Is that right? Or is it really just 2 LDRs in series with the output taken from in between them?

[DDD]

LDR seems to be the "differential LDR", i.e two matched (on a single chip maybe) LDRs in single case. These LDRs are more stable as voltage dividers under temperature changes. And usually 3 terminals instead of 4 :-)