In Public Address (P.A.) Systems, audio amplifiers are the electronic component that reproduces low-power electronic audio signals, such as the signal from a radio receiver, CD player or microphone, at a level that is strong enough for powering (driving) loudspeakers or headphones. These units are generally either ‘Mixer-Amplifiers’ that allow for multiple source inputs to single or multiple outputs, or ‘Power-Amplifiers’ that have a single input to single or multiple outputs. The outputs are either a stereo signal (8 ohm signal – e.g. home stereo units) or mono 100 Volt Line signal (100V – e.g. school P.A. system). On 100V line systems, many speakers can be added in a variety of layouts, so long as the power output of the amplifier is not exceeded.