A power amplifier is a device that sits between a sound source and a passive loudspeaker. Its job is to take a line-level signal and make it loud enough to fill a room or a venue with sound. Power amps can feature one, two, or more channels. For example, a mono power amp would have one channel, and a stereo power amp would have two channels. While designs vary, a standard feature on most power amplifiers is a power switch, which engages the device, and a volume control, which determines the device’s output level.