[sdiy] OTA VCA

Elain Klopke functionofform at gmail.com
Mon Mar 20 16:55:14 CET 2017


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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