[sdiy] VCF caps in modern synths

Roman Sowa modular at go2.pl
Fri Jan 8 17:35:24 CET 2021


My guess is that at low frequencies the currents driving your OTAs or 
transistors or whaetever in filter poles are so small, that it's more 
difficult for them to be so well matched anymore. They spread out 
slightly between poles. So maybe distribution of capacitance among poles 
was different with regards to distribution of pole's gm. If I'm correct, 
you should be able to see the same effect by simply moving the same caps 
between poles.

Roman

W dniu 2021-01-08 o 18:27, ColinMuirDorward pisze:
>  > 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.
> 
>     Roman
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