[sdiy] Query about best cap type for '2164 stability network

[David G Dixon]

I believe that, in this particular application, you could probably use a
560pF capacitor made from two pieces of tin foil with paper in between, and
it would be just fine.  The capacitor is placed between ground and a
resistor which is itself connected to a virtual ground.  Under normal
operation, this capacitor is never charged.  In other words, it basically
does nothing.  I've often wondered (when placing the 1000th 561 cap in one
of my PCBs) whether this wasn't just some sort of elaborate joke perpetrated
by the chip designers, who had a big laugh thinking about all of us
slavishly putting all those damn caps on our circuits.

What I'd like to know is what this capacitor is actually doing, or under
exactly what circumstances it does anything at all.  Neil has suggested that
the 500R/560pF combo is scaled for the 30k input resistor -- I'd also like
to know what the proper values would be for a 100k input resistor (or any
input resistor).  I just always use 499F/560pF, and never seem to have any
problems with it.  Also, I don't understand why the 500R resistor is
critical.  Most RC networks pass the signal between the R and the C.  I just
don't know enough about the subtle electronics here to understand what this
stupid thing is actually doing, and I've never been able to find any
guidance in any of the many textbooks I own on electronics (or, if I have,
then I wasn't clued-in enough to see it for what it was).


> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Guy McCusker
> Sent: Sunday, May 06, 2018 10:16 AM
> To: *SYNTH DIY
> Subject: Re: [sdiy] Query about best cap type for '2164 
> stability network
> 
> Thanks Richie.
> 
> You've given several reasons that C0Gs are good in general, 
> and that's why I tend to go for C0G for small value caps if I 
> don't have a good reason not to. But in the spirit of trying 
> to learn about this stuff properly, I wondered if there are 
> any reasons particular to the 2164 application that would 
> call for C0G over something else.
> 
> It doesn't seem to me that the capacitance change with 
> voltage or temperature are essential in this application. The 
> high frequency performance may be. Is it?
> 
> I realise that from an actual building standpoint this is a 
> somewhat academic question - we can just use C0G and stop 
> worrying - but I like worrying / learning.
> 
> On Sat, May 5, 2018 at 9:41 AM, Richie Burnett 
> <rburnett at richieburnett.co.uk> wrote:
> > Low capacitance change with voltage and with temperature, 
> low microphony, low loss at high frequency. C0G is the best 
> you can get in small C values, in every respect except maybe 
> size and cost. So no reason not to use it here for DIY unless 
> you're counting every last penny or need to fit hundreds of 
> them into a small space. But just for small cap sizes like 
> hundreds of pF.
> >
> > -Richie,
> >
> > Sent from my Xperia SP on O2
> >
> > ---- Guy McCusker wrote ----
> >
> >>What is the advantage of C0G in this application? Voltage 
> derating and 
> >>temperature coefficient don't seem critical. Could an X7R 
> cap do bad / 
> >>not as good things here?
> >>
> >>David is right that most of the 560pf ceramics at the usual 
> >>distributors are C0G, but some are not, X7R being the next most 
> >>popular. Tayda have some Z5U it seems.
> >>
> >>
> >>
> >>On Fri, May 4, 2018 at 9:32 PM, David G Dixon 
> <dixon at mail.ubc.ca> wrote:
> >>> Yes, that's what I meant to say as well (I believe that virtually 
> >>> all 560pF MLCC caps are NP0/C0G anyway).
> >>>
> >>>> -----Original Message-----
> >>>> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] 
> On Behalf 
> >>>> Of rburnett at richieburnett.co.uk
> >>>> Sent: Friday, May 04, 2018 3:05 AM
> >>>> To: *SYNTH DIY
> >>>> Subject: Re: [sdiy] Query about best cap type for '2164 
> stability 
> >>>> network
> >>>>
> >>>> NP0 / C0G ceramic
> >>>>
> >>>>
> >>>> On 2018-05-04 10:32, Tom Wiltshire wrote:
> >>>> > Hi All,
> >>>> >
> >>>> > What type of capacitor would be most suitable for the 
> 500R/560pF 
> >>>> > stability network on the input of the 2164 VCA? (In
> >>>> practice I mostly
> >>>> > use 560R/470pF - easier values to get).
> >>>> >
> >>>> > It's not directly in the audio path, so film caps seems
> >>>> like overkill.
> >>>> > Instead it's to take interference to ground, so it feels to me 
> >>>> > like ceramic is appropriate.
> >>>> > Is that right, or would I be better off with something
> >>>> else? I realise
> >>>> > any improvement is unlikely to be audible, but I'm looking for 
> >>>> > guidance on "best practice".
> >>>> >
> >>>> > Thanks,
> >>>> > Tom
> >>>> >
> >>>> > ==================
> >>>> >        Electric Druid
> >>>> > Synth & Stompbox DIY
> >>>> > ==================
> >>>> >
> >>>> >
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