[sdiy] VCF caps in modern synths

ben gebhardt benjamin_gebhardt at yahoo.com
Fri Jan 8 16:14:36 CET 2021


 My rules of thumb for capacitors is always evolving.  Currently:
NP0 when economically viable.  Always when 10n or less.  NP0's above 10n are getting more affordable now, I would definitely have this as a first choice for any time constant or filter.
I've used SMD film caps for the 10n to 1u range.  A former colleague has assembly issues with them, but I believe that was maybe CM related as they've never cause me grief.
I've also alternatively used THRU-box film caps as they are cheaper usually (even with an assembly cost up).
I've stayed away from X7R for the past few years, the more I learn about them, more more they seem problematic for anything except bulk capacitance.  I'll have to investigate this notion that they are no longer microphonic.
I would like to believe DC bias doesn't affect capacitance as Mike has indicated, and yet Murata's website has an FAQ page about this topic and that graph doesn't look pretty:
Does the capacitance change when a DC voltage is applied to ceramic capacitors? Are there any points to be aware of regarding changes in the capacitance? | Murata Manufacturing Co., Ltd.

Seems they've incorporated this into their web-sim engine.  Doesn't seem like dated material to me.  (I'd like to be proven wrong! it would make my designs much cheaper)
Also, in my understanding capacitor related distortion doesn't show up with traditional steady-state sine based THD measurements.  
Off topic but regarding SMD resistor distortion, I disagree that "In fact it is SMD resistors that have become the main source of distortion in surface mount products."  I think this is an over-generalization.    On Friday, January 8, 2021, 09:42:15 AM EST, Mike Bryant <mbryant at futurehorizons.com> wrote:  
 
 Hehe.  I remember I built a Moog ladder filter using 47nF X7Rs in the early 1980s and the capacitors were so bad back then it actually killed the Q so you couldn't get the thing to self-oscillate no matter how hard you tried.  I expect Behringer use newer versions in their Model D and get the right sound nowadays.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of mark verbos
Sent: 08 January 2021 14:15
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] VCF caps in modern synths

why? If you like the sound of a filter that uses a certain (in this case “whatever” ) cap, why would you hope that manufacturers use something else? If you don’t like the sound, that is something different. Oddly, when I think of an SH-101 I think of something that is almost too stable. 


Mark





> On Jan 7, 2021, at 9:19 PM, ColinMuirDorward <colindorward at gmail.com> wrote:
> 
>  I would hope that today we at minimum have a predominance of C0G types since they don't cost much more than a cent each.

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