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bkspero 20th December 2006 09:04 PM

What affects sizes of PP film capacitors?
I've tried to find the answer to this question and haven't been successful. Its probably so elementary that everyone knows it (but me) and doesn't have to discuss it. But I have to ask.

What affects the physical size of a Polypropylene film capacitor other than its capacitance. I ask because my son and I are moving on to build a 2nd Direct Input box. For the first one we used the capacitor/resistor parts kit sold by Jensen Transformer. For the 2nd one we thought that we'd try to get some experience sourcing the parts ourselves.

We ordered parts online, and when the PP capacitors arrived they were about double+ the physical size of those supplied with the Jensen kit. Capacitance is correct. We have room to fit them (we are direct wiring). But I am curious what could make the size difference?

My only guess is that the new capacitors may have a higher voltage limit than those supplied by Jensen. And that could require thicker film and then more plate area to achieve the same capacitance. Is that the reason? Or is it something else?

Also, is it a problem using a capacitor with a high voltage limit (250V) in a low voltage circuit like a DI box?

By the way, the first DI box came out great. Thanks to those who provided grounding suggestions.

Jonathan Bright 20th December 2006 09:25 PM

Voltage rating will almost certainly be one factor and the type of construction may be another. They can be "stacked" or "rolled" (I think, I'm not sure about the second term). I think that the stacked type is the prefered one for the passage of audio signals but I forget why; possibly lower inductance. This is not my field of expertise but I know some of these caps are advertised with self healing properties when they are damaged. This may also be a factor with size. There is no problem with using a higher voltage rating than the original circuit specs or parts. There may well be a prob' if using a lower rating.

EC8010 20th December 2006 09:33 PM

All modern plastic capacitors have the same inductance; it's just that of the lead-out wires. Whether the capacitor plates are made of foil or by metallizing the dielectric makes a big difference to size. Most plastics have pretty well the same dielectric constant (between about 2.5 and 3.0), but some require a thicker film to achieve a given voltage rating, meaning that they need more area as well, and that pushes the volume up. In general, from smallest to largest (for the same value of capacitance and voltage rating); polyester, polycarbonate, polypropylene, polytetrafluoroethylene, polystyrene.

Cobra2 20th December 2006 10:06 PM

which translates (for audiophiles); bigger is better...:D

Arne K

jean-paul 20th December 2006 10:46 PM

Probably not only for audiophiles. There are enough plastic surgeons out there that can testify...

bkspero 21st December 2006 12:48 AM

Thanks all
I think that both the capacitors supplied in the Jensen parts kit and the ones we purchased are wound. We had almost purchased some in small rectangular boxes with short, circuit board solder leads before we caught the mistake and purchased the ones we did. I think the ones we almost, mistakenly, purchased are stacked.

Glad to hear that a higher voltage rating is not an issue. And it sounds like the most likely explanation is the higher voltage rating requiring thicker film and, as a result, more film area for the same dielectric.

Thanks again. Now if my son ever gets back from school we can start DI box #2.

AMV8 21st December 2006 11:42 AM


Our knowledgeabvle moderator said;
"Whether the capacitor plates are made of foil or by metallizing the dielectric makes a big difference to size."
In effect metalised plastic capacitors are smaller than those using foil. Metalised types usually have a "K" in the name such as "MKP" for metalised polypropylene. Normally non metalised sound better - but that is just my humble opinion.
Hope this helps.

EC8010 21st December 2006 12:11 PM

Actually, I think it's "F" for foil and "M" for metallized, as in FKP1 (Wima foil polypropylene) or MKP (metallized polypropylene). And yes, foil capacitors are much bigger. Oh, and I'd agree, I prefer foil too.

jacco vermeulen 21st December 2006 12:17 PM

K is for Kunststoff, German for plastic.

AMV8 21st December 2006 12:29 PM

Thanks EC8010 and Jacco.

I clearly need to move to Rotterdam and become multilingual.


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