[sdiy] OTA VCA

Dan Snazelle subjectivity at hotmail.com
Tue Mar 21 15:50:33 CET 2017

I was going to suggest he also look into "keyer" circuits which usually are a few diodes , Passives and maybe a BJT 

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> On Mar 21, 2017, at 6:46 AM, Roman Sowa <modular at go2.pl> wrote:
> As I sometimes do, will not try to answer your question, but put you on another track maybe.
> Since there's a bucket full of 300 AR+VCAs planned, why don't you try the simplest VCA there can be like 2-transistors and opamp from MS20. Or take a look into CS80 or PS3000 series. There's a lot of VCAs there and obviously they are trimmed down to very few absolutely necessary parts by clever Japanese engineers.
> I reckon you'll be summing a bunch o VCAs on several buses so that may go down to 2 or maybe even one transistor per key and one opamp per bus.
> As for AR envelope - do you need the times to be voltage controlled or fixed? If fixed, then 2 resistors, 2 diodes and one cap from gating signal. If voltage controlled, there's a lot to choose from, and I wolud start by looking into mentioned above monster polysynth schematics.
> Also, AFAIR one of Korg Lambda or Sigma or the 3rd one in the series I can't remember the name, had also a lot of AR+VCAs for keying - look there.
> Roman
> W dniu 2017-03-20 o 16:55, Elain Klopke pisze:
>> So I'm going to put this can of worms on the table and back off slowly
>> before opening it....
>> Due to various reasons (not the least of which was getting shafted by
>> the person  I purchased a large quantity of LDRs from) I'm switching
>> over to an AR envelope generator and VCA combo for keying my notes on
>> the paraphonic synth.
>> I decided to go with some circuits that I know and have assembled a few
>> times, the AR generator and VCA from Ray Wilson's original Sound Lab
>> mini synth. I'm going to try to trim away the bits of the AR generator
>> that have to do with the triggered mode since these are going to be
>> gated by the key presses only and see if they still work.
>> Question time: Ray's VCA is a simple half-a-LM13700 with the CV going
>> into the Bias input and the op amp bit doing its amplify-y thing.
>> 1) In his notes, Ray talks about the response to the CV being linear.
>> Since I'm just using this to switch the notes to the rest of the synth
>> with a little preset attack and release that shouldn't be a problem,
>> right? It would only do strange things if I tried feeding it an
>> exponential envelope from an external source?
>> 2) If I hard-wire the initial gain "knob" to no initial gain, it's going
>> to be silent until the envelope hits it?
>> 3) Is there any noticeable difference between an LM13700, NJM13700, and
>> NJM13600 besides the price? Because at 148 chips for a 3-octave
>> keyboard, price is kinda a thing.
>> -Ian
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