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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.
Capacitors will block lower frequencies (and DC) but higher frequencies will pass with ease, depending on how they are used.
Capacitors are generally used to isolate DC or to filter noise from power rails.
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.
The dielectric of a capacitor is what holds the electrical charge. 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.
To make a capacitor, the dielectric is sandwiched between two plates of conductive material, so that a controlled charge can be applied directly.
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.
There are two main types:
Electrolytics: 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).
Bi-polar: These do not have polarity. Often used in loudspeaker crossovers.
These are either bipolar or electrolytic, but use different dielectric materials. Certain dielectrics have advantages over others.
Paper and Oil:
Polypropylene: Usually considered best for audio
Tantalum: Typically used in RF applications, or where you need to filter EMI from a power supply. Usually they have very high ESR, on the order of 5 to 10 ohms, and are very nonlinear, so they are not best suited for analog applications.
Ceramic: Generally a bad idea to use in the signal path but they have their advantages for filtering in power supplies. These are the little disc-shaped orange one that usually have something like "10" 0r "561" labeled on them. There are different types, such as NP0, etc.
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