[sdiy] Single transistor VCA (Was "what opamp for warmth?")

Richie Burnett rburnett at richieburnett.co.uk
Fri Apr 7 15:16:25 CEST 2023


> Curious, how was the second transistor hooked up?

Two NPN's with the collectors connected together (see attached PNG) in the 
case of the VCA used to control resonance in the JX-3P VCFs.  This 
arrangement does result in a very peculiar resonance behaviour in the JX-3P 
filter though! It is very noticeably different when compared to the 
resonance in the Juno-106 which uses a proper OTA to control the feedback 
around an otherwise similar cascaded-OTA type filter.

> How noisy is it...

The single transistor VCA as used all over the place in the TR-909 schematic 
is *very* noisy when the VCA is "open" and passing signal through.  This is 
because the noise gain of the op-amp that follows the transistor becomes 
very large when the transistor is made to conduct.  (As others have said the 
input signal is attenuated right down and then amplified all the way back up 
again.)  It's relatively quiet when the VCA is "closed" though.  You also 
get quite a lot of thump due to control feed-through if the CV changes 
quickly because the transistor's base current ends up going into the audio 
path.  It's also quite non-linear for large signals which results in 
inter-modulation distortion too.

However, all three of these undesirable VCA characteristics are not much of 
a problem in the case of a drum machine where the signal through the VCA is 
either white noise, or some monophonic oscillator tone that will usually get 
some noise mixed in with it at some later point in the voicing circuitry 
anyway.  Control feed-through just results in an attack "click" transient, 
and intermodulation distortion has little effect on noise, and only modifies 
the relative amplitudes of harmonics for monophonic tones.  So it is pretty 
much the perfect cheap VCA for a drum machine! :-)  In fact, in many regards 
it's slightly better than the "swing type VCA" that Roland got away with in 
the TR-808!!!

-Richie,


-----Original Message----- 
From: René Schmitz
Sent: Friday, April 7, 2023 12:51 PM
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] what opamp for "warmth"? (I know, I know...)



You'll find the same VCA in the 909 in a few places. (Bassdrum , Toms, 
Clap..)


Note how far the signal is divided down.


It's noisy, and nonlinear, and has some control feedthrough. Show what you 
can get away with for a single monophonic signal.


The MS10 had the single transistor, the early "Korg35" MS20 had the same.
The later MS20 had a second transistor, which makes things more 
linear/symmetrical and you can run larger amplitudes -> less noise.

According to the schematics the transistors shall be selected. jx3p also 
says "SEL".

Anyone would know what for? Equal gain?


Best,
René

On 07.04.2023 11:04, Adam (synthDIY) wrote:



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.




Interesting.... here's the JX3P version

















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