DC-to-AC power supplies are mostly switching power supplies in which the AC voltage from the power line, the AC main, is rectified to convert it to an unregulated DC voltage. This rectified AC is applied to a DC-to-DC converter that chops up the voltage so that it can be processed at a high frequency, typically 20 to 100 kHz. This allows power voltage conversion using less expensive, smaller capacitors and transformers. Aluminum electrolytic capacitors are typically used at the input as bus capacitors and at the output as output filter capacitors.