[sdiy] Silvered mica caps in VCO core integrators

Ian Fritz ijfritz at comcast.net
Mon Apr 18 15:42:12 CEST 2016


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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