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I dont know much about capacitors. What is the difference between the two? What applications are each suited for? Thanks, David
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The dielectric in a capacitor is the part that keeps the two electrodes apart by a small distance, so that the capacitance between the electrodes is controllable and stable over time. Small caps used in audio circuits are typically film types, where the dielectric is a thin sheet of polyester, polypropylene, polystyrene, teflon, or impregnated paper. The electrodes are either metal foils placed on either side of the dielectric film, or metal layers evaporated onto the film.
Electrolytic capacitors are used where the circuit needs a larger amount of capacitance in a small space. Typical applications are for power supply filters and inexpensive speaker crossovers. The dielectric in an electrolytic capacitor is a thin layer of aluminum oxide grown on an aluminum foil electrode. The layer is grown by electrochemistry, and the capacitor contains a paste of chemicals called the electrolyte. The other contacted electrode is another piece of foil, but the electrolyte is the actual capacitor electrode, as it is in intimate contact with the aluminum layer.
As Andy said, this scheme only works where the capacitor is polarized by a DC voltage, as it would be in a power supply. The system breaks down if the bias voltage is reversed.
There are some non-polar electrolytics made for use in speaker crossovers, where there is no DC bias and the AC signal is large enough to destroy a polar capacitor. These are essentially two polar capacitors connected back-to-back, so that one is always able to support the voltage in what it sees as the proper direction.
Electrolytic capacitors have a finite operating life. Once the water dries out from the electrolyte, the internal resistance goes up and the capacitor no longer works the way it should to support the circuit. There are fancy meters to measure the internal resistance, but the simplest way to check a suspect capacitor is to weigh it and compare the weight against a known-good one of the same size. The water loss reduces the weight considerably.
Polarised electrolytic caps have a '+' and a '-' side and you have to connect the right way round - eg. between a DC rail and ground (or they will explode). :-))Non-polarised electrolytic caps - like Black Gate 'N's - don't have this limitation. However, I have heard they do sound better connected the 'right' way round (the longer lead is '+ve').
Then there are non-electrolytic caps (eg. polystyrene or teflon film). These, nominally don't matter which way round they are connected ... but, again, some brands say they sound better connected one way.
Typically, electrolytics are large values used in power supplies. Non-electrolytics are used everywhere else! :-)) Like coupling caps, in passive speaker Xovers or in active Xovers.
Regards,
Andy
Edits: 06/03/09
Andy, Thank you for that thourough explanation. I appreciate it!
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