[sdiy] VCO saw integrator cap questions

[Carsten Tönsmann]

In my opinion all those cap discussions are a bit exaggerated. Low leakage 
for sample and hold is good, but as you don't build S&Hs for scientific 
purposes but for musical applications this is not too important, I would 
say. Temparature stability of caps for VCOs I haven't heard of yet as this 
is solved by temperature resistors or transistor arrays as I know.

AND IF YOU WANT REAL MATHEMATICAL STABILITY USE A DCO INSTEAD OF AN VCO. Its 
just curious. All those people out there spending tons of effort searching 
for analogue precision to solve a mathematical problem: The exact doubling 
of frequencies and the constant generation of them.

Not to be misunderstood: Sound forming modules like filters work best as 
analogue solutions, as the non mathematical nonlinearities make the specific 
sounds of them, and the wondeful timbre of a moog ladder just before self 
oscillation cannot be computed by a DSP. But some mathematical tasks like 
octave control should be solved digitally in my opinion.

Carsten

----- Original Message ----- 
From: "David G. Dixon" <dixon at interchange.ubc.ca>
To: "'Synth DIY'" <Synth-diy at dropmix.xs4all.nl>
Sent: Monday, September 21, 2009 8:21 AM
Subject: [sdiy] VCO saw integrator cap questions


> I've got a couple of stupid questions about caps, specifically for a
> saw-core VCO integrator.  It is difficult for me to obtain polystyrene 
> caps,
> so I would like to go with metallized polypropylene.  I have read that 
> their
> leakage properties are even superior to polystyrene.  So here are my
> questions:
>
> 1) What are the most important properties for a VCO integrator cap, and 
> why
> is polystyrene considered to be so superior?  Is metallized polypropylene
> just as good?  Better?  What about C0G monolithic ceramics?  They are
> supposed to be extremely stable with temperature.  Why don't we use them 
> for
> VCOs?
>
> 2) (This is probably a really dumb question, but) I bought some metallized
> polypropylene caps from my local electronics shop which are little yellow
> boxes with 0.2" lead spacing.  However, I see pictures of synth modules on
> the web with these massive yellow boxes which appear to have about 0.4" 
> lead
> spacing.  Is one superior to the other?  What type of caps are these large
> yellow boxes, and why are they so big?
>
> 3) I get ceramic caps from my school, and they are these pretty little 
> blue
> things, some only about 3 mm wide and maybe 1.5 mm thick, with white
> markings (I think they're EPCOS, supplied by Digikey).  They are C0G in
> smaller values (up to 2n2), and typically have 0.1" lead spacing, although
> some are 0.2".  However, I have never noticed these in any of the pictures
> of synth modules I have seen on the web, but I usually see caps which are
> much larger.  Is this just coincidence, or should I be avoiding these caps
> for some reason?  I use them for coupling, bypassing, and speed-up
> applications, but never for timing (integrators, filters, etc).  Could I?
> Should I?  They seem to work perfectly well.
>
> After 18 months of synth diy, I must confess that I'm still fairly 
> confused
> about caps in general.  People say to use this or that cap for this or 
> that
> application, but I'm not really sure why.  I understand that one would 
> want
> low leakage for a sample and hold application, but other than that, I'm 
> more
> or less in the dark.  Is there a good source of straight dope on the
> selection of caps with specific properties for specific applications?  For
> example, I read on a previous [sdiy] thread that one should use either
> polystyrene or metallized polypropylene for VCOs, but polypropylene for
> VCFs.  The author failed to mention why, however.  Does the metallized
> variety have some deleterious effect on VCFs?  I've used silvered mica in 
> my
> filters, and they seem to work extremely well.
>
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