Originally Posted by Radio Wrangler
Let's split capacitors into electrolytic ones and all the rest.
A capacitor has two conductive metal plates facing each other with some sort of dielectric material between them. You get more capacitance if the area of the plates is bigger. You get more capacitance if the plates are closer together. Dome dielectric (insulator in other words) materials also give you more capacitance.
A microfarad is quite a large amount of capacitance, and a trick was devised to make capacitors smaller and cheaper. They used an anodised layer on the face of aluminium foil as a dielectric, and the foil as one of the electrodes. By matting the face of the foil and having a thin anodised layer, they could pack in an awful lot of microfarads into a small capacitor BUT the thin anodising made the voltage rating low. With the thin anodised layer following every detail of the matt foil, we run into a problem... the other electrode plate has to follow the finest detail of this craggy surface without any gaps. The way they did it comes as a surprise. They used an electrically conductive liquid! It's like a jelly impregnated into some permeable paper used to hold the stuff in place. A second aluminium foil with no anodising is used to connect to the liquid. Some people think the liquid is chemically active and the thing works like a battery, but they've missed the point.
Because the liquid is water-based, there is slow evaporation and loss of vapour through leaky seals. Eventually, electrolytic capacitors run out of water, dry up and stop working properly. But they're really small, and they're cheap!
Because one foil is naked aluminium, and the other has a built-up oxide (anodised) layer, they rely on there being some bias voltage. Without bias voltage, the oxide layer degrades. Worse, with reversed voltage it is driven away. As Craig says, they made 'Non polarised' electrolytics to look after this issue while handling AC signals in speakers. They work, but they aren't very good.
So, you may want to dodge these components, even though alternatives are larger and more expensive.
The other sort are just two plain, naked metal foils with a thin plastic or oiled paper film between them. Different people have different views on the various plastics, but they're nicer than electrolytics. The values you get are more accurate, and they don't have the limited lifetime set by their water content and quality of seals.
Polypropylene film capacitors should be fine. Go for the right number of microfarads. Go for the same voltage rating or a bit more, but look out for increased size.
The audio world is full of magic fairy dust and inflated prices. There are special 'audiophile' capacitors with all sorts of claims of audible improvements. If you believe in these claims, you may find yourself able to hear the improvements, especially if you paid a lot and put yourself in a position where you had to hear something to justify the cost. If you don't believe in the claims, you won't be able to hear the improvements, and you may feel you've wasted your money.
I used to design radio gear for linking aircraft into automatic anti-collision systems and air traffic control. You might suspect that I was very careful in my choice of components, and that they got the living daylights tested out of them. Hot, cold, fierce vibration, crash tests and explosive decompression, salt spray, you name it. Capacitors from good mainstream manufacturers were just fine.... and they work for audio too. So don't rush into premium priced 'audiophile' parts. Some are perfectly fine but overpriced, some are re-packaged ordinary stuff from scammers, some are just crap. Stick with a mainstream brand, and buy them from a main distributor. There are fakes of all sorts of components on the go. Nothing is so cheap that someone isn't faking it. 13 amp fuses! Jet engine parts!
David
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