[sdiy] Silvered mica caps in VCO core integrators

Simon Brouwer simon.o at brousant.nl
Mon Apr 18 16:46:43 CEST 2016


That is correct, I was talking Hz, not kHz.

I'll explain how I calculated:

In the sawtooth core, the capacitor is not set or reset to a fixed voltage for
extended time. So the soakage effect will be a tendency for the capacitor
voltage to go towards the average voltage on the capacitor, which is halfway
between the reset voltage and the threshold voltage. For simplicity let's call
the reset voltage 0.

The graph in the Bob Pease article indicates, for mica caps, an error voltage of
around 0.3% the charging voltage after 50 ms discharge, i.e. 0.15% of the
threshold voltage. Note that in the second half of the sawtooth the error
voltage buildup reverses, but as the soakage is a trailing effect, this won't
fully compensate for the buildup during the first half. I assumed the worst
case, no compensation at all. Thus, the error voltage would cause the capacitor
voltage to reach the threshold voltage 0.15% early (at 50ms period = 20 Hz)

And now that I think of it more, even assuming no reversal of the error voltage
buildup during the second half of the sawtooth, no continued buildup at all will
occur during this second half either! So a 0.15% frequency error at 10Hz, not
20Hz, is already a worst case value.

Best regards
Simon

> On 18 April 2016 at 15:57 Steve Lenham <steve at bendentech.co.uk> wrote:
> 
> 
> I have no idea who is wrong or right on this, but Simon has calculated 
> the effects at tens of HERTZ, not tens of KILOHERTZ, so it is already 
> low frequencies that he is talking about.
> 
> S
> 
> On 18/04/2016 14:42, Ian Fritz wrote:
> > Simon --
> >
> > Thanks for taking the time to look at the numbers.
> >
> > However, please remember (as mentioned before) that soakage is a low
> > frequency problem, not a high frequency one. Extrapolating your result
> > to lower frequencies gives errors of 0.3% at 10 kHz, 0.6% at 5 kHz, 1.2%
> > at 2.5 kHz and so on. Perhaps this extrapolation isn't perfectly
> > accurate, but it gives an indication of fairly significant effects.
> >
> > 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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