[sdiy] OTA VCA

Rutger Vlek rutgervlek at gmail.com
Tue Mar 21 12:04:23 CET 2017


Hi Elain,

Fun project! From my experience the lm13xxx sound the same due to an
identical ota topology. Only if you use the darlingtons on the extra pins
you may hear a difference.

I second the suggestion for the simple vca plus ar. Preferably with few
transistors. Even a simple minimoog vca would be cheaper. I just restored a
Hohner string performer that has exactly the full polyphonic vca design
that you're looking for. Have a look at its schematics. It uses cmos logic
after the keys to trigger the envelopes and has a release control.

Rutger

Op 20 mrt. 2017 16:58 schreef "Elain Klopke" <functionofform at gmail.com>:

> 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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