[sdiy] OTA VCA

cheater00 cheater00 cheater00 at gmail.com
Tue Mar 21 17:04:14 CET 2017


If you're looking to make your own action you could make the envelope
part of the keyboard action so that's less electronics still. I think
that hammond divide-down from the early 1900s might have done that.

On Tue, Mar 21, 2017 at 3:50 PM, Dan Snazelle <subjectivity at hotmail.com> wrote:
> I was going to suggest he also look into "keyer" circuits which usually are a few diodes , Passives and maybe a BJT
>
>
>
> Sent from my iPhone
>
>> On Mar 21, 2017, at 6:46 AM, Roman Sowa <modular at go2.pl> wrote:
>>
>> As I sometimes do, will not try to answer your question, but put you on another track maybe.
>> Since there's a bucket full of 300 AR+VCAs planned, why don't you try the simplest VCA there can be like 2-transistors and opamp from MS20. Or take a look into CS80 or PS3000 series. There's a lot of VCAs there and obviously they are trimmed down to very few absolutely necessary parts by clever Japanese engineers.
>> I reckon you'll be summing a bunch o VCAs on several buses so that may go down to 2 or maybe even one transistor per key and one opamp per bus.
>>
>> As for AR envelope - do you need the times to be voltage controlled or fixed? If fixed, then 2 resistors, 2 diodes and one cap from gating signal. If voltage controlled, there's a lot to choose from, and I wolud start by looking into mentioned above monster polysynth schematics.
>>
>> Also, AFAIR one of Korg Lambda or Sigma or the 3rd one in the series I can't remember the name, had also a lot of AR+VCAs for keying - look there.
>>
>> Roman
>>
>> W dniu 2017-03-20 o 16:55, Elain Klopke pisze:
>>> 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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