Commercial 70V mixer amplifiers are like most other amplifiers, except that they are specially designed to drive many speakers, while simultaneously offering mixing capabilities. Audio distribution systems, especially those used in department stores and warehouses require many loudspeakers to be powered by a single amplifier over long runs of speaker wire. Commercial high impedance speakers are used to deliver sound to many speakers in these environments because they employ a large transformer at the output of the amplifier. A combination mixer amplifier allows for background music as well as microphone input. ProAudio.com offers a great selection of 70V mixer amplifiers for various applications.