[sdiy] Elka Synthex VCA/VCF is extremely interesting

[Osamu HOSHUSHUYAMA]

Hello Aaron and List,

I trimed the VCF part from the schematic.
This will be much easier to see.

  http://houshu.at.webry.info/200706/article_3.html

(I'm sorry that the analysis is in Japanese as usual.)

Sam HOSHUYAMA
Saitama, JAPAN

----- Original Message ----- 
From: "Aaron Lanterman" <lanterma at ece.gatech.edu>
To: "SDIY" <synth-diy at dropmix.xs4all.nl>
Sent: Monday, April 21, 2008 12:31 PM
Subject: [sdiy] Elka Synthex VCA/VCF is extremely interesting


> 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 VCFs.
> 
> 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)
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