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

Mike Bryant mbryant at futurehorizons.com
Sat Apr 8 18:35:04 CEST 2023


Probably a FET input amplifier would be best then
________________________________
From: Jonatan Liljedahl <lijon at kymatica.com>
Sent: 08 April 2023 13:48
To: Mike Bryant <mbryant at futurehorizons.com>
Cc: René Schmitz <synth at schmitzbits.de>; synth-diy at synth-diy.org <synth-diy at synth-diy.org>
Subject: Re: [sdiy] what opamp for "warmth"? (I know, I know...)

Actually I do want the distortion! Because it’s nice to fatten up saw and tri waves through the typical OTA curve, or even turn the triangle into sine-like. (I’m adding a parameter for this in my DCO firmware). But I don’t want audible noise (I have a separate noise generator for that..)

/Jonatan

fre 7 apr. 2023 kl. 20:14 skrev Mike Bryant <mbryant at futurehorizons.com<mailto:mbryant at futurehorizons.com>>:
If you just want a simple mixer VCA without the distortion, you could replace the bipolars with MOSFETs.  That would allow a larger signal so a bit less noise
________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org<mailto:synth-diy-bounces at synth-diy.org>> on behalf of Jonatan Liljedahl <lijon at kymatica.com<mailto:lijon at kymatica.com>>
Sent: 07 April 2023 13:46
To: René Schmitz <synth at schmitzbits.de<mailto:synth at schmitzbits.de>>
Cc: synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org> <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>>

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

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<mailto: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

[grab 2023-04-07 at 6.57.54 pm.png]







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