[sdiy] VCF caps in modern synths

Tom Wiltshire tom at electricdruid.net
Fri Jan 8 19:03:28 CET 2021


Or stick 470pF caps on it and turn up the cutoff frequency CV.

Honestly, why not? More juice through the OTAs should help with the very-low-current effects Roman mentioned, and bigger caps would mean that a few pF of stray capacitance isn’t such a big deal. Although I agree that 470pF isn’t a huge jump up.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================



> On 8 Jan 2021, at 16:36, Mike Bryant <mbryant at futurehorizons.com> wrote:
> 
> 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.
> 
> Roman
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