[sdiy] VCO saw integrator cap questions

[Gerard]

These links also provide some useful information, and also answer some
questions
you have:

http://www.emusic-diy.org/References/Capacitor (nice overview in the
table)
http://my.execpc.com/~endlr/index.html (interesting, very detailed)
http://www.maxim-ic.com/appnotes.cfm/an_pk/3171 (something about
distortion)

Cheers, Gerard

On Tue, 22 Sep 2009 08:48:32 +0200, Carsten Tönsmann
<carsten at analog-monster.de> wrote:
> 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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> 
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