[sdiy] Transistor VCA - was Re: what opamp for "warmth"? (I know, I know...)

brianw brianw at audiobanshee.com
Fri Apr 7 20:22:38 CEST 2023


Since the rest of the circuit is powered with unipolar +5V, I assume that means all signals already have a positive bias. So there's no need to inject a bias into the signal.

If, instead, you wanted to add an input jack for external signals, you'd need a stage to shift the audio into the positive range before the VCA, otherwise there'd be some severe distortion.


On Apr 7, 2023, at 1:44 AM, Roman Sowa <modular at go2.pl> wrote:
> A picture is worth a thousand words - see attached
> It's from a circuit powered with single 5V
> The output needs a coupling cap too.
> Roman
> W dniu 2023-04-07 o 04:20, brianw pisze:
>> How would you use a single-transistor VCA in a circuit?
>> Would you need to AC-couple the input, bias with DC of about half the positive supply, feed through the transistor VCA, and then AC-couple to the rest of the circuit?
>> I'm assuming most of the analog signals will be bipolar, but some synths are all positive voltages so a 1TX VCA would fit nicely (like the TR-808).
>> Brian
>> On Apr 6, 2023, at 12:31 AM, Roman Sowa <modular at go2.pl> wrote:
>>> That's a single transistor VCA and indeed it sounds amazing, although a bit noisy as there is a lot of gain involved. I used it in my simple VCO and it makes fat sound out of otherwise boring clean oscillator.
>>> Here's a short video showing waveshapes on a scope. The input to VCA was variable trapezoid to add even more fattness:
>>> https://youtu.be/algsxOo6lns
>>> And when you add 2nd transistor in reverse, that VCA becomes clean as baby butt wipes
>>> Roman
>>> W dniu 2023-04-05 o 20:32, David G Dixon via Synth-diy pisze:
>>>> Whatever the VCA is in the Korg MS-20 really warms up the sound of that synthesizer.  You can find the schematic on the internet.  I have something redrawn by a fellow named Marjan Urekar that I looked at years ago.  It involves two transistors.  It is evidently also the VCA used in the Roland JX-3P.
> <1transistorVCA.png>

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