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VCO polystyrene capacitor question
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author VCO polystyrene capacitor question
Blake Smith
Hey there,

I'm building an Oakley VCO and I'm wondering if anyone has an opinion as to whether or not I can get away with substituting a wima polypropylene type 1% capacitor for the polystyrene 1% indicated in the builders guide for C14, or if the different dielectric wouldn't perform correctly.

Thanks in advance.
eljay
I've substituted polystyrene for polypropylene and vice versa in a few Oakley modules without any problems, I think as long as the specification is as required there shouldn't be an issue. On this occasion (for the VCO) I used a 1000pF 1% polystyrene capacitor from Farnell part number 9520236. This is square and has leads across the diagonal that fit with the PCB layout.
Synthbuilder
Polypropylene are good capacitors but their capacitance varies with temperature roughly twice as much a polystyrene of a similar value. Thus polystyrene is more stable and better for a VCO. The small change in capacitance produces a similarly small change in absolute tuning. However, it may not be noticeable in practice. Absolute tuning errors are less objectionable to scaling errors which can't be easily tweaked during play.

These days a C0G or NP0 ceramic may be better than polystyrene - the SVCO uses these.

Tony
Blake Smith
So a C0G ceramic will do the trick as long as the tolerance is 1%? I'll see if the supplier carries those.

Thanks.
Synthbuilder
Blake Smith wrote:
So a C0G ceramic will do the trick as long as the tolerance is 1%?


No need for 1% tolerance, the C0G or NP0 type ensures that the device won't be a poor part. The 1% is simply the tolerance of the recommended polystyrene capacitor. Generally speaking any part with a tolerance of 1% is a good part - but the key specification with a timing capacitor is stability rather than its absolute value.

Tony
Blake Smith
That makes it easy, thanks.
Dr. Sketch-n-Etch
I've used WIMA poly caps in VCOs, and they work very well. I also often use silvered mica caps in VCOs, and they also work very well.

I've never used a polystyrene cap, because I can't buy them locally. Also, they are big, and easy to damage during soldering.
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