For many applications, it is necessary to drive many speakers with a single amplifier, giving greater coverage to multiple rooms or over a given area. Constant voltage systems (usually 100V line) employ a transformer in the amplifier to convert the output to high impedance and a corresponding transformer in each speaker to tap of the required power from the amplifier.

Theoretically, this allows a large number of speakers to be driven by one amplifier, so long as the overall power being tapped by the speakers is within the amplifier’s output capacity (usually 90% of the amplifier’s maximum rated output).

In addition, the transformers in each speaker may have tappings at different points along the primary coil, which effectively draw different Wattages from the amplifier. For example, a 10W speaker may also have settings for 5W or 2.5W.

The speaker output may also be varied using remote volume controls, known as attenuators, but these should be considered to be at their maximum volume output when choosing an amplifier to drive the speakers.

If more speakers are required beyond 90% of the 100V amplifier’s maximum output, a slave amplifier may be employed to drive these.

The slave amplifier is simply linked from the line output of the main amplifier, to which another series of speakers are then linked to, in the same way the speaker are from the main amplifier.