[sdiy] Silvered mica caps in VCO core integrators

Simon Brouwer simon.o at brousant.nl
Mon Apr 18 09:15:49 CEST 2016


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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