[sdiy] VCF caps in modern synths

ColinMuirDorward colindorward at gmail.com
Fri Jan 8 18:27:30 CET 2021


> 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> 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.
>
> Roman
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