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Why always high voltage on MKP, film and foil caps?

Zombie

pfm Member
On everal occassions I have wanted to replace bipolar elcos with a non-elco, but I have always stumbled on that most of the MKP and good quality film/foli caps are huge if the values get over 20 uF.
Why are those caps only available +200 V values? Is it of technical reasons or just habit?
 
Both... plus commercial reasons... Film caps simply don't lend themselves to being made in high capacitance values but are easily made to handle 200V or a lot more.
As with eg SIT power fets and many of the very low noise high Gm small signal fet's, there is simply not the commercial demand for them to make it profitable for big companies.

Where there is demand, such as motor start capacitors and power factor correction capacitors, there are film caps of 1000uF and more available but they are very expensive and very big.
 
The limitation of film cap's is film thickness - even at the lower limits of film thickness that can be made, you are looking at a large number of V before the film insulator fails.

There are also economic considerations. I worked for ICI on Melinex (PETP - polyester) and Propafilm ( OPP - orientated polypropylene) and the economics of making very thin films (low V) for cap' manufacture were grim at that time (1980s). At that time, the financially attractive lower limits of thickness were around 50 microns PETP and 20 microns OPP - way too thick for cap' manufacture. It is all very well making a mint from a tonne of 10 micron of either, made over several days, but if that satisfies the world demand for Lord knows how long, it is not attractive business.
 
I see that MKS are made in lower voltage at relatively high capacitances, up to tens of uF. They're obviously better than bipolars, but how would they stack up against MKP in audio?
 
Polyester is generally reckoned to be less good than polypropylene but as you say, anything is better than electrolytics! Having said that, IMO way to much emphasis is placed on capacitor types and even more so brands.
 
Horses for courses Jez.
Hifi has very few limitations in reality, which is what we are talking, after all.
If you want cheapness, use ceramics, if you want high cap', better ESR, and cap' v. frequency, use aluminium electro's, if you want small case size, low ESR and great cap' v. frequency, use tantalum electro's. If you want huge cap' at modest price, irrespective of case size, go for film.

You pays yer money...………………………..
 
Read somewhere that if the elco has a couple of VDC "bias" over it, it has no negative impact on the sound signal compared to without the "bias".
 
Horses for courses Jez.
Hifi has very few limitations in reality, which is what we are talking, after all.
If you want cheapness, use ceramics, if you want high cap', better ESR, and cap' v. frequency, use aluminium electro's, if you want small case size, low ESR and great cap' v. frequency, use tantalum electro's. If you want huge cap' at modest price, irrespective of case size, go for film.

You pays yer money...………………………..

Well... there's rather more to it than that but yes it's certainly horses for courses. Manufacturers will generally use the cheapest and smallest that will do the job adequately. Hi fi (and to a lesser extant guitar amplifiers) are rather different commercially when we get to the more "hi end" in that there are many boutique types out there, claimed to be made from pure silver which was smelted on a full moon and rolled by shamans chanting spells:rolleyes:

There was a thread just a few days ago with a good link to an interesting article on this subject. As I said there I've made my own comparisons of capacitor types as coupling caps at line level and can report that differences are minuscule to the extent of wondering if you are imagining it or not...

A very few applications for caps do seem to be very much effected by cap choice though.

One will often see posts about capacitor choices and makes etc in these pages for applications where there is bugger all difference to be had and some waste a lot of money on this!

Rather like foo fuses and mains leads it's something people see a lot of discussion about in hi fi mags and internet forums etc and so they think it must be true and important if so many people are talking about it... when in fact lots of people are talking about such things because lots of other people are advertising such things everywhere you see hi fi being discussed!
 
Read somewhere that if the elco has a couple of VDC "bias" over it, it has no negative impact on the sound signal compared to without the "bias".

There are measurable differences yes but non polarised types for crossovers are designed not to need any bias...
In amplifiers etc where a non polarised (bipolar) electrolytic is needed then it has been done to use two back to back electrolytics and take the centre point to a voltage source via a high value resistor thereby biasing both caps.... it's very rarely done due to being a PITA and costing more. In a speaker you don't have a convenient DC voltage source to do this anyway!
 


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