[sdiy] OTA VCA

[Tom Wiltshire]

Seconded.

With a linear VCA response, typically you'd want an exponential envelope to give you a natural sounding decay. As it happens, that's exactly what you get when you stick an RC combination after a Gate pulse, and that's what you'll get from Ray's design.

That AR generator of Ray Wilson's needs serious pruning if you're going to build one-per-note. The bit that really matters is just the Attack/Release pots, their diodes, and that 2.2uF cap C3. I'd be trying to get rid of pretty much everything else. You can get it down to 2 resistors, 2 diodes, and a cap.
The VCA on the other hand is pretty minimal already, so seems like a good choice.

I'm afraid I can't help with (3). I've no experience with the NJM chips.
There is a difference between the 13700 and the 13600 (of whichever version). As Wikipedia puts it: "The darlington output buffers on the LM13700 are different from those on the LM13600 in that their bias currents (and hence their output DC levels) are independent of IABC pin. This usually results in performance superior to that of the LM13600 in audio applications."

HTH,
Tom

On 20 Mar 2017, at 17:30, cheater00 cheater00 <cheater00 at gmail.com> wrote:

> Hi Elain,
> It just depends on whether your AR has exponential or linear output. Linear into linear might not sound the way you'd like it.
> 
> 
> 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
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