[sdiy] VCF caps in modern synths

Mike Bryant mbryant at futurehorizons.com
Fri Jan 8 17:36:26 CET 2021

I’ve just looked at the 3320 circuit diagram and it shows 300pF capacitors.  With through hole I wonder if you can keep the stray capacitances  low enough to be worrying that much about matching capacitors ?   You may be better off trying to add small parallel capacitors on each 300pF in turn in situ to see where you can improve the Q.

From: ColinMuirDorward [mailto:colindorward at gmail.com]
Sent: 08 January 2021 17:28
To: Roman Sowa
Cc: Mike Bryant; synth-diy at synth-diy.org
Subject: Re: [sdiy] VCF caps in modern synths

> Close matching of capacitors gives close matching of the real pole frequencies, and this requires less feedback gain to reach self-oscillation.

To reach self-oscillation, is it to be expected that more feedback gain is required at lower frequencies? I'm trying to imagine how less-well-matched capacitors  would contribute to the frequency-dependancy factor.

On Fri, Jan 8, 2021 at 8:20 AM Roman Sowa <modular at go2.pl<mailto:modular at go2.pl>> wrote:
>> do not show microphonic effect
> This was fixed in X7Rs over 20 years ago.  I keep seeing this coming up as a reason and ask for people to show me a modern quality X7R (from Samsung, Yageo, etc) that has microphony.  I'm sure there are some cheap and nasty ones that still do it but the quality brands fixed this for smartphone use ages ago.

Maybe I'm biased, and you are probably correct. Not so much nowadays as
it used to be. But still I was able to see microphonic effect on new
1210 10uF cap. It was just 1mV bump but still. So I withdraw my claim
about "piezoelectric transducers"

>> dielectric absorption
> Again barely an effect with modern units.

my very recent experience proved it's still a problem. By mistake I put
X7R diped multilayer THT cap instead of C0G in oscillator timing. They
look the same, little yellow thingies, only differ with one letter I
can't see without glasses. And it caused huge spike that could be
explained only by dielectric absorption. By "huge" I mean nearly 10% of
full voltage span. Replacing with C0G made the spike disappear.
And it was in only tiny 1n cap. Might get worse in bigger capacitances.

>> and all those things end up as distortion in audio path.
> < 0.001% on an AP in my latest mic amp/ADC combo.   Do you need any better ?

no, I don't, you win.

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