Short intro to film caps:
1) The dielectric (he insulator) is a plastic film. The most common films are polyester (a.k.a. nylon; Mylar is a brand) and polypropylene. Some are more linear than others, which is thought to be an audible source of distortion. The actual measured evidence is pretty thin, so you have to use your ears too. Teflon is the most linear, followed by polystyrene, polypropylene, then polyester. An older technology is to use paper impregnated with various oils or waxes (which dominate the performance). You will see the term paper in oil, abbreviated PIO. Some plastic film caps are also bathed in oil, though the oil does not absorb into the plastic itself.
2) The conductive metal can be deposited in an ultra-thin layer on one side of the film (called metalized film) or it can be a separate foil (called film and foil). Aluminum is most common, though copper, silver, and tin are also used. Metalized film is self-repairing - a short between layers will vaporize the metal -. Foil is thicker and needs a thicker film since it can't repair itself, so they are much larger. The foil is thought to reduce internal vibration.
3) There are electrical forces between the layers of a capacitor, and the dielectric itself will respond mechanically to electric fields. This causes capacitors to vibrate in response to the signal voltages, which in turn modulates the signal - causing audible distortion. Some caps are more prone to thisthan others, and the importance of this effect like so many others is still debated.
Bottlehead mostly uses well-made but inexpensive metalized polypropylene when we have film caps. We think these are the most cost-effective choices.
