[sdiy] LM13700 high frequency oscillation issue

[drheqx]

You fixed it already. Now just add some bypass caps on those pins to
ground.  Very small values in the pf range like 22pf.

Thank you for mentioning this circuit because i need to build this also.
Mike

On Fri, Mar 21, 2025 at 9:47 PM ackolonges fds via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Hi Synth-DIYers!
>
> I've been a bit perplexed by an issue I'm having with an LM13700 VCA, and
> I'm sure a lot of you are intimately familiar with the chip, so you may
> have encountered this and have some advice for me.
>
> I've been building the Ray Wilson Dual Log/Linear VCA (
> https://musicfromouterspace.com/index.php?MAINTAB=SYNTHDIY&VPW=1910&VPH=829)
> in many forms throughout the years, and it's always worked great for me. I
> even did a very small one with 0402s and TSSOPs with no issues. However,
> I've recently made a version with 0603s and SOICs to introduce my students
> to soldering SMT, and I'm finding the Darlington buffer on the output
> causing high frequency (500kHz to 1MHz) oscillation at high amplitudes at
> higher gain settings.
>
> It's the exact same circuit that I've made with other sized components,
> and I've built it multiple times with the same result, as well as using a
> couple of different varieties of 13700. When probing the circuit at pins
> 7-8-9-10 with 4 channels of an oscilloscope, I've noticed that the
> oscillation goes away, and returns as soon as one of the probes are
> removed.
>
> Has anybody on here experienced a similar issue? Perhaps I should be
> adding an extra capacitor somewhere to keep the circuit stable? Any
> suggestions on position and values to try?
>
> Thanks a lot for any advice you may have.
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