[sdiy] VCF caps in modern synths

Mike Bryant mbryant at futurehorizons.com
Mon Jan 11 12:32:05 CET 2021


Not sure I'd take Wikipedia as a source of definitive information.  

Meeting class 1 involves meeting a whole series of parameter limits.    X7Rs are never going to meet the dielectric absorption limits nor the capacitance stability under bias for class 1 so will always be in class 2 where microphony is allowed.  But that doesn't mean the manufacturers are going to deliberately make the parts so.  The people who have pushed for X7R to get better are the phone manufacturers - originally Nokia and Ericsson, and later Apple and Samsung.  This is why these parts are so cheap nowadays.   An engineer designing a commercial product won't be thinking about classes, they will be thinking what is the minimum spec needed and hence minimum price needed to do the role of each component in the design.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Brian Willoughby
Sent: 11 January 2021 07:28
To: synth-diy at synth-diy.org
Subject: Re: [sdiy] VCF caps in modern synths

One thing that I hope someone can clear up is the definition of C0G, NP0, X7R, etc.

When it was first explained to me that I need to select C0G or NP0 to avoid unwanted microphonic effects, I got the impression that was part of the definition of those designations.

I mean, sure, you can get lucky and find a batch or a brand of X7R that isn't quite detectably microphonic in a particular application, but if it's not part of the guarantee that comes with a particular designation, then what's the point of convincing people to gamble?

Who are you going to trust? The testing facilities of a major capacitor manufacturer, or a small batch of a few boards that have gone through ad-hoc testing with amateur equipment?

We're talking about the difference between Class 1 (NP0, C0G) and Class 2 (X7R). According to Wikipedia, all Class 2 capacitors exhibit piezoelectricity.

If you need Class 1 performance, then that's what you should buy. Just because Class 2 is getting better and better doesn't mean you should start specifying X7R when you need Class 1.

I'm sure that if the companies making these capacitors were able to get Class 1 performance from their improved Class 2 manufacturing process, then they'd just change the designation when they announced the new part numbers.

Brian


On Jan 10, 2021, at 19:41, Eric Schlappi <eric.schlappi at gmail.com> wrote:
> my two cents on filter caps:
> 
> I have experienced piezoelectric effects with x7r caps recently, I now only use them for decoupling.
> 
> One of the bigger upgrades for my filters recently was upgrading from 5% C0G/NP0 to 1% C0G/NP0. I had previously considered 1% too expensive but they really aren't anymore and it can really make a difference as far as getting your filter to oscillate reliably in edge cases.
> 
> On Fri, Jan 8, 2021 at 12:51 PM David G Dixon <dixon at mail.ubc.ca> wrote:
>> I have the same experience.  Poly film caps oscillate much easier than C0G caps of the same value.  I have replaced C0G with WIMA polyester to get stable oscillation in filters.
>> 
>> From: Tom Wiltshire
>> Sent: Friday, January 08, 2021 7:26 AM
>> 
>> On 8 Jan 2021, at 16:02, ColinMuirDorward <colindorward at gmail.com> wrote:
>>> Yesterday I did some very non-scientific testing of 3320 4pole BPF built using C0G vs polypropylenes. The test may have been more about the tolerances of the caps rather than the dielectric, since my polys are 2.5% vs the C0Gs at 5%. I noticed that at lower frequencies, the polypropylene version would go into oscillation before the C0G version. Any thoughts on this?
>> 
>> Yes, I think you’re right. That seems more likely to be an effect of the tolerance to me.
>> 
>> Tom

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