An amplifier capacitor is an electronic component that stores excess power. Typically used to provide an extra boost of power during peaks in audio signals, amplifier capacitors increase a circuit's dynamic headroom. Although they are present in just about every amplifier circuit, external amplifier capacitors are especially prevalent in car audio applications.
Most capacitors contain two conductive plates and an insulator between them. Small disc capacitors frequently have two metallic plates and an airspace between them, and they hold an amount of charge measured in micro- or picofarads. A large one-farad capacitor typically consists of a tight roll of two pieces of conductive foil with an insulator between them. Instead of being roughly the size of a child's fingernail, a typical car audio amplifier capacitor with a one-farad capacity is closer to the size of a soda can.
The farad is a measure of charge and is equivalent to the ability to deliver one amp of electrons at one volt for one second. To understand how little power a farad represents, one should consider that a 2,000 milliamp-hour rechargeable AA battery can deliver approximately 8,600 farads of power, based on an output of one amp at 1.2 volts for 7,200 seconds. Although the battery is orders of magnitude more powerful, an amplifier capacitor can be both charged and discharged very quickly, making it perfect for buffering applications. In fact, some signal amplifiers use them just to smooth out power supply anomalies, because a capacitor can absorb extra power when the power supply sends too much energy and can release power when the power supply is running short.
Accessory amplifier capacitor units typically come into play in car audio systems. Compared with wall power, electricity in a car is extremely limited. As such, loud music can draw enough power at peaks to interfere with the car's operation, doing such things as causing headlights to dim. The capacitor draws extra power from the battery when the amplifier does not need it, such as during quiet parts of a song, and discharges its power when peaks in the music demand more power than the battery and alternator can output.
Although car stereos are the most visible users of amplifier capacitors, every amplifier uses them, from the oldest tube circuits to modern solid state amplifier designs. These components allow the amplifier to meet the demands of real-world signals that can vary greatly in amplitude without needing a gigantic power supply, both in terms of capacity and physical size. For amplifiers that have power supply limitations, adding them to the system can be an affordable way to improve performance without incurring the cost of upgrading to a higher-powered amplifier or to a more robust power supply.