[sdiy] what opamp for "warmth"? (I know, I know...)

Jonatan Liljedahl lijon at kymatica.com
Fri Apr 7 14:46:47 CEST 2023


How noisy is it, compared to an LM13700 for example?

I'm looking for a simple 2x4 channel waveform mixer so this two-transistor
vca looks interresting. My waveforms are already unipolar (0 to -3.3v). I
don't mind some distortion unless it turns my triangle into a sine or
square :)

By the way, regarding VCA noise, there's a lot about minimizing distortion
in LM13700 designs, but what about noise? I understand higher input level
gives less noise for more distortion, but what about Iabc? Does lowering
Iabc from say 500uA to 250uA (and doubling the gain in the I-V opamp
buffer) give better or worse noise performance? And how about other pros
and cons? According to my sims, a lower Iabc at least gives less cv
bleed-through.

Cheers
/Jonatan


On Fri, 7 Apr 2023 at 13:57, René Schmitz <synth at schmitzbits.de> wrote:

> 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
>
> [image: grab 2023-04-07 at 6.57.54 pm.png]
>
>
>
>
>
>
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>
> --
> --synth at schmitzbits.dehttp://schmitzbits.de
>
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-- 
/Jonatan
http://kymatica.com
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