Anodizing capacitors

I had mentioned yesterday that we'll try and cover some aspects of what change happens when we use a capacitor vs. when it's still new in the box. Not only do the capacitor's electrical characteristics change, but the sound of it changes too. This is one of the aspects of equipment break in we can both measure and hear. The easiest to write about are the types of capacitors commonly used in power supplies: electrolytics. But first, let's refresh our memory of what a capacitor is. A capacitor is a simple material sandwich of conductors and insulators. Picture the conductor as the sandwich top and bottom pieces of bread, separated by an insulator. The conductor's material is typically metal and the insulator can be made of any number of items that include: glass, ceramic, plastic film, air, vacuum, paper, mica, and oxide layers. Capacitor types are usually named for their insulating materials, hence: film caps, silver mica, ceramic, paper, etc. Capacitors store energy using an electrostatic field - like the kind of static electricity that zaps you on a dry day. The energy applied to a capacitor doesn't flow in a steady stream like it does through a wire. Instead, energy must be added and subtracted in repeating cycles. Each of the cycles is said to alternate between plus and minus - and you probably know this alternating cycle as AC. This restriction of the capacitor, where steady one-way current does not flow–is what makes capacitors valuable as AC devices. I had mentioned that the names of capacitor types comes from the insulator, but this isn't always the case. One type of capacitor is called an electrolytic. It is named after 1/2 of the conducting material in an unusual sandwich arrangement. In these caps, which are common in power supplies and some audio signal paths, aluminum foil is used for the conductor and aluminum oxide is used for the insulator. To keep the physical size small, two things happen: the second conductor is formed by a thin layer of electrolyte - a sort of conductive paste or goo - and the insulator is formed by applying a voltage to the aluminum foil. When you apply a voltage to aluminum and it is surrounded by appropriate chemicals, an oxide layer forms in a process called anodizing. You've no doubt seen the results of anodizing before. Most high end audio products have an anodized finish to the metal - front panels in particular. The same process I just described is applied to aluminum plates and a hard, electrically insulating finish is formed that covers the aluminum. If the anodizer wants different colors, like a black faceplate, or red, green, or blue (weird, I know), a chemical dye is added to the metal, it's zapped with voltage, and a protective coating of oxide forms. It is this zapping voltage - called a forming voltage - that we'll want to discuss tomorrow.