Electrolytic capacitors

An electrolytic capacitor packs high capacitance into a small volume, making it the basis of power supply filtering and high-value crossover sections. Only bipolar (non-polarised) types belong in a crossover, since audio is an alternating signal.

Electrolytics dry out over time, losing capacitance and gaining ESR, so they are the first parts to replace when refurbishing a 20- or 30-year-old speaker. Bypassing a signal-path electrolytic with a small film capacitor can clean up the treble.