[sdiy] Elka Synthex VCA/VCF is extremely interesting

[Aaron Lanterman]

Check out the 410CST5800 schematic, right column - that's where the  
VCA and VCF live:

http://manuals.fdiskc.com/flat/Elka%20Synthex%20Schematic.pdf

The schematic takes a bit of practice to read.

Each VCA consists of two OTAs - the output of one drives the input of  
the other! It looks like one of the OTAs is a VCA whose job is to  
modulate the CV modulating the audio, at least if I'm reading it  
right. The OTAs are 13600s, 4Q and 8Q on the schematic.

The filters consist of two CEM3320s, 2Q and 6Q. Find them is tricky  
since their bits and pieces are spread all over the schematic. It's  
hooked up with electronic switches all over the place.

The CEM3320 consists of OTA-buffer pairs, with a tap point between the  
OTA and the buffer for you to hang a cap (or whatever) off of, so in  
theory you could recreate this with LM13700s or something, if you  
added your own expo converters.

The last two OTAs in the chain appear set up as straight single-pole  
LPFs.

The first two look tike a typical single-pole OTA-C LPF, except each  
has a connection from the input to where the cap normally hits ground,  
and there's a switch that you can use to switch that ground connection  
in and out. Similarly, where you normally see the input of a LPF, you  
see a switch. So I'm guessing that these are used alternately - i.e.  
one is on, and one is off, and you can use this to switch between a  
highpass and lowpass LPFs.

It appears that the output of a four-pole block can be tapped from the  
output of the last pole, the output of the last pole with different  
amounts of gain  (based on resistors I see), or from output of the  
second pole. Whatever the output is is fed back to the resonance input  
pin of the 3320 (pin 8), which internally in the chip is fed back to  
the negative input of the first OTA.

How interesting - I haven't seen anything quite exactly like this!

I'm procrastinating... I'm supposed to be writing up solutions to a  
homework assignment. ;)

- Aaron, blessing his parents who are here to help take care of the  
new little one (and his mother-in-law who has helped the past two  
week, and who is now heading home)