[sdiy] OTA VCA

Elain Klopke functionofform at gmail.com
Mon Mar 20 21:13:50 CET 2017


Hey guys,

Thanks!

Cheater: I have no idea what linear vs. exponential looks like unless the
description tells me so, lol. I'm such a hack and constantly grabbing a
piece of this or that circuit to suit my needs....

Tom: I had initially trimmed it down to the schmidt trigger, diodes, pots,
capacitor, and the buffering op-amp. I'm guessing that the buffer isn't
really needed since one AR generator is going to one VCA, but shouldn't I
keep the schmidt trigger there to give the envelope a nice 0-5 volt gate
pulse when the key is pressed? Or is just a five volt rail getting
connected through the key to the diodes enough?

Dave: Thank you, I'm glad there's no real distinction between them. I
should probably get a few extras for some VCFs or something.

-Ian

On Mon, Mar 20, 2017 at 2:20 PM, Dave Magnuson <abide at dmdrafting.com> wrote:

>
>
>
>
> On Mon, 20 Mar 2017 16:58 Elain Klopke, <functionofform at gmail.com> wrote:
>
> 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
>
> _______________________________________________
>
>
>
>
>
> Hi Ian
>
>
>
> I have used the NJM13600 and NJM13700 almost exclusively and have had no
> problems in any of the circuits I’ve tried.   I have little experience with
> “real” LM13xxx however….
>
>
>
> Dave M
>
>
>
> _______________________________________________
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>
>
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