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'what capacitors should be used for what applications?'
Author:Francis Vaughan (registered user: 712 posts )
Date: Sun, Oct 28th, 2007 @ 03:07 ( . )

A few more thoughts.

In general those caps that are preferred are those that are simply better at being capacitors, and that have the least in the way of second order weirdness.

Problems with caps come from a range of difficulties that stem from the need to make them out of physical materials. Any cap has an equivalent series resistance - i.e. it looks like a perfect cap with a resistor in series with it. They also appear to have an inductance in series - which means that any cap eventually looks like a tuned circuit, and has a self resonant frequency. What this means for designers and builders is that at some frequency a capacitor actually starts to lose capacitance.

The capacitor plates - i.e. the foil or metal layers mostly contribute to the ESR, the winding of the layers to the ESL. Foil capacitors are generally better than metalised, but they are bulkier and more expensive. Stacked plate capacitors are better than wound for RF applications, but only available in low values. Mica can only be stacked.

Then you get to the dielectric. Dielectrics absorb energy, and also have other weirdo effects, the molecules in any polar material (which means almost any polymer) actually twist slightly in response to the applied electric field. This motion dissipates some of the applied energy, and the time constant in relaxation means that the capacitor actually has a small lag, and can retain a remnant charge. The latter can cause weird distortions. Dielectrics can also be have tribo or peizo electric properties. This is bad, and especially bad in guitar amps, which are not the quietest of places. This means that vibrations in the amp can create signals in the capacitor. In general this sounds bad.

So, types of capacitors.
Electrolytics are about the worst for most of these effects. But some modern ones are surprisingly good. As noted, in general one only tolerates them when one needs a big value. Tantalum are actually worse then electrolytics. Plus they are not tolerant of mistreatment at all, and expensive. They have a low ESR and ESL , which makes them favoured in high frequency applications- and especially where they are needed to improve stability.

Ceramics are the next worst. They are peizo electric, and have desperately bad dielectric absorption and non-linearities. There is scant reason to ever use them - except sometimes some builders find that the edgy sound can actually be used to advantage.

Film caps are generally reasonably benign. In HiFi terms Mylar (polyester) is the least well thought of. But again the sound is something many builders like. The best mojo seems to some with the Mylar/foil ones. I.e. mustard caps, Sozo.
Polypropylene is favourite in HiFi circles, again with a preference for film/foil over metalised foil. As has been noted, the HiFi sound they give might make a amp slightly too brittle sounding. We don't need the DC to blue light response a HiFi system might like. Strangely one of the best possible polymer dielectrics - Teflon - has appalling triboelectric properties. Teflon caps are ruinously expensive anyway, so you will never see them used. Polystyrene is a very high quality dielectric, and makes for a very nice capacitor. Only available in low values, and difficult to find, they have a low melting point and are dissolved by modern production cleaning agents. But worth trying.

Mica is a surprisingly good dielectric, and despite its high cost of manufacture, still favoured for coupling by many. These are almost universally silver metallisation on mica.

Paper/foil and paper in oil/foil have a special place in the HiFi world. But they are big, expensive and again they might not be what you want. I suspect the additional mobility of a liquid dielectric reduces the dielectric absorption and residual charge effects.

In a guitar amp the HiFi rules don't hold in some places. In particular the desire for very low ESR and ESL. We don't need or indeed sometimes even want a super stiff power supply. So HiFi PS caps would be a total a waste of money. On the other hand, whilst guitar amps are renowned for distortion, is is a crafted musical distortion we want. Agents that introduce bad sounding distortion are just as unwelcome here as in a HiFi design. So ceramics are equally disdained.

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Replies:
--* what capacitors should be used for what applications?
10/28/2007 @ 03:34--ogorir
10/28/2007 @ 04:07----Francis Vaughan

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