[sdiy] LM13700 high frequency oscillation issue

[ackolonges fds]

Hi Roman,

Thanks a lot for your reply. I'll try some 10pF or 100pF capacitors. Just to clarify, do you mean adding them in parallel to pins 7/8 and 9/10? Or do you mean adding them to those points, with the other side connected to ground? I could try both I suppose.

Apologies for getting your hopes up, but unfortunately when I mentioned the TSSOP, I was referring to the other chip in the design (TL084) - I haven't yet seen a TSSOP 13700, but I'd certainly like one as well!

Thanks again
________________________________
From: Roman Sowa <modular at go2.pl>
Sent: Sunday, March 23, 2025 2:07 AM
To: ackolonges fds <ackolonges at hotmail.com>; Synth-Diy mailing list <synth-diy at synth-diy.org>
Subject: Re: [sdiy] LM13700 high frequency oscillation issue

The answer seems to be hidden in your question itself.
If oscillation stops with 4 probes hooked, then maybe try to add 10pF
capacitors to 7-8-9-10 pins and it should go away. Or even 100pF at the
emiters.

But most of all - where did you get LM13700 in TSSOP??
I'd love to have that.
Or maybe you mean quad version - AS13704?

Roman

W dniu 2025-03-22 o 02:43, ackolonges fds via Synth-diy pisze:
> 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 <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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