[sdiy] Silvered mica caps in VCO core integrators

Simon Brouwer simon.o at brousant.nl
Tue Apr 19 09:42:49 CEST 2016


Hi Ian, 

I'm not sure what you mean by "I don't seem to see that difference coming into
your calculation."

If you look at the model of a capacitor in fig. 4 in Pease's article, then the
capacitor on the left represents the nominal capacitance, and all the others the
soakage effect. How much the latter contribute to the voltage on the capacitor
after a discharge period of e.g. 50 ms can be read for each type of capacitor
from the graph in fig. 7. Whether the soakage is caused by polarization and/or
any number of other physical effects is not relevant to the calculation, we can
simply deal with the capacitor as a black box.

Best regards
Simon


> On 19 April 2016 at 01:19 Ian Fritz <ijfritz at comcast.net> wrote:
> 
> 
> Oh, yes, I read the frequencies wrong.  I still have my super S/H set 
> up, so when I get a chance I can compare mica to polystyrene.  May take 
> a while though.  In the meantime, I'll be thinking more about your 
> calculation. The difference in the regular dielectric charge and the 
> same with the extra polarization charge added might be one way to think 
> about this problem.  I don't seem to see that difference coming into 
> your calculation.
> 
> Ian
> 
> 
> 
> On 4/18/2016 1:15 AM, Simon Brouwer wrote:
> > Hi,
> >
> > Based on the data in the Bob Pease article I did some calculation and I
> > estimate
> > that, in a basic sawtooth core without compensation for the effect, the
> > soakage
> > in mica capacitors would cause a frequency error of less than 0.15% at 20
> > Hz,
> > half that at 40 Hz and so forth. IMO this need not disqualify mica
> > capacitors
> > for a good quality VCO.
> >
> > Best regards
> > Simon
> >
> >> On 18 April 2016 at 02:20 Michael E Caloroso <mec.forumreader at gmail.com>
> >> wrote:
> >>
> >>
> >> Many vintage polysynths with CEM VCOs used mica caps for the charge core.
> >>
> >> Not every VCO design can work with mica caps if the soakage impacts
> >> reset time of the ramp core.  Other designs work fine.
> >>
> >> MC
> >>
> >> On 4/17/16, Ian Fritz <ijfritz at comcast.net> wrote:
> >>> Tony --
> >>>
> >>> Thank for your thoughtful response.  A couple of comments.
> >>>
> >>> What is the relevance of something working as well as it did 17 years
> >>> ago?  Why wouldn't it? And what does that have to do with using a mica
> >>> cap?  I have lots of circuits built 40 years ago that still work fine.
> >>>
> >>> I'm glad that you brought up FM applications, as this is one area where
> >>> good tracking and stability have a big advantage.  And actually, going
> >>> by message board traffic, there has been a fair amount of interest in FM
> >>> recently.
> >>>
> >>> "If it works for you, it works." Well, obviously.  But if you are
> >>> talking about manufacturing something would't you be more concerned
> >>> about your customers' needs? Hmmm..., use a more expensive inferior
> >>> part. Doesn't seem like good business sense to me.
> >>>
> >>> But after all is said and done, this really isn't rocket science.  It's
> >>> basically trivial to achieve the level of performance I mentioned.  I
> >>> demonstrated it many years ago (1998).  It only requires careful choice
> >>> of a few critical components. I never considered mica timing caps,
> >>> because I had read that they have soakage issues and that everyone
> >>> prefered polystyrene. The Bob Pease work certainly confirmed this.
> >>>
> >>> But you still haven't told us exactly how well your mica VCO works.
> >>> Would you by any chance have some data you could share with us?
> >>>
> >>> Thanks for your interest.
> >>>
> >>> Ian
> >>>
> >>>
> >>>
> >>>
> >>> On 4/17/2016 12:29 AM, Tony Clark wrote:
> >>>> tl;dr
> >>>>
> >>>> I wouldn't hesitate AT ALL to use Mica's for VCO's.  I used Mica caps
> >>>> for the VCO circuits I designed 17 years ago and they still work just
> >>>> fine.
> >>>>
> >>>> Unless you are Mr. Chowning and have some new fangled retro analog
> >>>> bent, I wouldn't be too concerned about your VCO meeting some silly
> >>>> "modern standard" (and is it IEEE?).  If it works for you, it works.
> >>>> If it doesn't, pick a different part.  It's not hard to swap out a
> >>>> silly cap.
> >>>>
> >>>> Tony
> >>>>
> >>>
> >>>
> >>> --
> >>> ijfritz.byethost4.com
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> 
> 
> -- 
> ijfritz.byethost4.com



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