Capacitors

What do capacitors do?

In short, capacitors story a given amount of electricity. Depending on the circuit, all the energy can be released at once, or it can be slowly released. Capacitance is measured in farads (abbreviated F on capacitor packaging), and the larger this value, the more energy it can store. In general, physically larger capacitors hold more energy - beware of them when taking apart equipment.

A farad is a big value so capacitance is usually measured in uF(1/1,000,000F), nF, pF etc...

example: a 120mF(120,000uF)/15V electrolytic capacitor is 15cm high and 7,5cm in diameter

Capacitors will block lower frequencies (and DC) but higher frequencies will pass with ease, depending on their capacitance.

Capacitors are generally used to isolate DC, filter noise from power rails, filter high frequencies in crossover networks and filter RF in amplifier circuits.

Here's something to note:

Capacitors store energy in the same way that you build up static charge shuffling across the carpet on a cold day.


Dielectrics

To make a capacitor, the dielectric is sandwiched between two plates of conductive material, so that a controlled charge can be applied directly.

If you've experienced trying to throw a piece of tape away and having it statically cling to your fingers, you've encountered a good dielectric.

Capacitor distortion, or Nonlinearity

It would be nice if all capacitors fit ideal specifications, but it just ain't so. [this needs to be expanded a bit more]

http://members.aol.com/sbench102/caps.html (sorry, this link is down. It was an excellent article, and gave direct scope shots of the capacitors! Paper/oil was very linear, as I recall)
EDIT: Keantoken, May it be this link? http://greygum.net/sbench/sbench102/caps.html

If you are seeking to improve the sound of an amplifier, in general it is only worth it to change capacitors in the signal path. Also you should know that whether or not capacitors actually affect the sound to a significant degree is a debated topic, so really it's your experience that matters more than anyone else's opinion.

Electrolytic capacitors loose their capacitance over time, so its good to change them in older equipment, if u get around to it.


There are two main types:

Unipolar:
These capacitors have polarity (+-). If you get the polarity wrong, count on having something go POW and They can be axial (+/- leads on both ends) or radial ( +/- both leads on one end).
Negative side marked by a -, a stripe, or a dot(tantal).

Electrolytics: best when smoothing PSU ripple, high capacitance and affordable
There are bipolar verions of them too.

Tantalum: typically used where space is important for their high size to capacitance ratio.
can explode violently it used wrong(are unipolar)

Triple layer(super) capacitors: very high capacitance, useful in high current and pulsed designs(class d car amps)
limited voltage(2.7V usual, but can be put in series)

Next are all bi-polar, they do not have polarity:

Plastic Film: not used anymore cuz they absorb moisture and degrade

Paper and Oil: not used normally cuz they absorb moisture and degrade(only big, high voltage versions are being made for industry use)

Polyethylene: plastic as dielectric, most common, usually square shaped, 65V to 2000V(most i saw)

Polypropylene: also plastic, usually considered best for audio

Mica(silver mica): not used anymore, considered a holy grail by some audiofiles due to their high tolerance. a mineral as dielectric(if i remember) and expensive

Ceramic: not usable in hifi audio due to changing of capacitance with voltage(u can also hear them "sing" as they expand and subtract) but good enough for home stereo
most found in radio circuits

on a personal note: i sold allot of these and from what i found out i conclude:
It is better to put more "normal" electrolytic's in the PSU section than a couple of low ESR(equivalent series resistance) ones(low ESR tipes are usually for very high frequencies(GHz), are biger and more expensive), to put them on a pcb can have some benefits(wont call Ghandi to join u thou).
All "block" capacitors are excellent for sound.(although some1 i trust sayd that they can have a "directional" resistance (in mOhm))