Capacitors – What They Are For

A capacitor can store electrical energy – typically for a short time – and at the same time acts as a frequency-dependent component. This makes it possible to couple signals in circuits (separating DC components, passing on AC components) and to decouple them (diverting interference or AC components to ground). Capacitors are also used to filter music signals in a frequency-dependent manner and to shape them deliberately, for example in filters and crossovers. Likewise, they can buffer current peaks in the power supply and thus help to stabilize a system dynamically.

For initial orientation: the base unit 1 farad (named in honor of Michael Faraday) corresponds to a very high capacitance of a kind that practically never occurs in audio circuits. Capacitance is typically specified in µF (microfarads), smaller capacitance values also in nF (nanofarads).

Basic Capacitor Types

Metallized film capacitors

Metallized film capacitors consist of a plastic film onto which a very thin metal layer is deposited. This construction enables compact dimensions and is therefore widespread in many audio applications. A capacitor built this way is not polarized and can therefore be used flexibly in signal paths and filter circuits. Small to medium capacitance values are typical, depending on size and application. For high capacitance values, such as those found in power supply circuits as reservoir capacitors, film capacitors are as a rule physically too large. They are therefore used for such purposes only in special applications – for example as high-voltage capacitors for tube circuits. In practice, film capacitors are often the balanced standard solution when stable properties at a reasonable size are required.

Metal foil capacitors

Metal foil capacitors work with solid metal foils as electrodes, not with a vapor-deposited metal layer. As a result, they are usually larger for the same capacitance, but follow a very “direct” and robust construction principle. They are likewise not polarized and are used where a very stable, long-lived basic character is preferred. As capacitance values in µF increase, the size grows considerably. They are thus a consistent choice when space is available and the required capacitance values are not extremely high.

Electrolytic capacitors

Electrolytic capacitors enable comparatively very large capacitance values in µF in a small space and are therefore the typical choice when “a lot of capacitance” is needed. Because the very thin dielectric is formed by an electrochemical process (anodic oxidation) and would be degraded by polarity reversal, they are fundamentally polarized and are therefore used primarily in applications in which the electrical polarity is fixed. Their typical use is in the power supply, where they buffer energy and absorb load changes. In the signal path, polar electrolytic capacitors are only used if the circuit is designed for them. Their main advantage is the very high capacitance in a compact design.

Bipolar electrolytic capacitors (audio-frequency electrolytics)

Bipolar electrolytic capacitors are the AC-capable special form of electrolytic capacitors and are therefore suitable for music signals. They have no fixed plus/minus polarity and can be used where the polarity reverses during operation. Typical applications are large capacitance values in µF in filters and loudspeaker crossovers, when film capacitors of the same capacitance would become too large. Here, too, the core benefit is: high capacitance at a practical size. They are used deliberately when space and capacitance requirements make this economically sensible.