[sdiy] OTA VCA

[Andre Majorel]

On 2017-03-20 10:55 -0500, Elain Klopke wrote:

> 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.

In case you didn't know, you can do a single-transistor VCA. See
for example the Korg MS-20, PS-3200 and probably MS-10, PS-3100,
PS-3300 etc.

The Korg PS-3200 has full polyphony over 49 notes and for this
reason is a good source of cheap techniques. See

  http://jhaible.com/legacy/polykorg/jh_polykorg_clone.html

-- 
André Majorel http://www.teaser.fr/~amajorel/