Capacitors perform many functions in our stereo systems. They smooth out power supplies, segregate frequencies, block DC, pass AC, and store energy. We encounter them most often in power supplies where their ability to store energy is invaluable, but also in speakers and filters where their frequency dividing characteristics are essential.
A capacitor is a fairly simple device. They are built from alternating layers of insulators and conductors and connected through an input and output lead. They have been around for a very long time.
From Wikipedia: “In October 1745, Ewald Georg von Kleist of Pomerania, Germany, found that an electric charge could be stored by connecting a high-voltage source by a wire to a volume of water in a hand-held glass jar. His hand and the water acted as conductors and the jar as a dielectric (insulator). Von Kleist found that touching the wire resulted in a powerful spark, much more painful than that obtained from an electrostatic machine. The following year, the Dutch physicist Pieter van Musschenbroek invented a similar capacitor, which was named the Leyden jar, after the University of Leiden where he worked. He also was impressed by the power of the shock he received, writing, “I would not take a second shock for the kingdom of France.”
Capacitors have used many types of materials for their two primary elements including oil, beeswax, plastic film, and oxide for the insulators—electrolyte paste and metal foils for conductors. The types of construction materials, in particular, the insulators have a great deal of impact on how they perform and sound in any circuit. Companies that care about sound quality spend a lot of money on quality capacitors.
One thing all capacitors have in common is their tendency to age—some over very long periods of time, others reasonably quickly depending on their operating environment. Heat is their biggest enemy.
Your stereo system is likely filled with these aging contraptions and for those interested in resuscitating vintage audio equipment, knowing when and where to replace them can be a real benefit. I’ve put together a little video on the subject here.