An amplifier is integrated into the device. An audio amplifier takes a small signal and makes it bigger without making any other changes in it which results in better sound, so any sound source can be connected to speakers.
Denon Heos 3
65% have it
9. has an AUX input
An auxiliary input allows you to play other audio sources by connecting them through a simple audio connection like a 3.5mm jack, i.e for MP3 or portable DVD/CD player.