Headphone Amplifiers.

Multi-Channel integrated AV receivers mainly power speakers in a home cinema system. This is where the term 5.1 comes from. The original digital surround sound amplifiers powered five speakers and connected to a subwoofer (.1) as the discrete bass speaker of the system. These products typically have built-in radio receivers. The term AV receiver is used generally now, though, as radio listening has moved into internet radio listening rather than off an antenna.