[sdiy] OTA VCA

[Roman Sowa]

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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