100 volt speaker system

Constant-voltage speaker systems refer to networks of loudspeakers which are connected to an audio amplifier using step-up and step-down transformers to simplify impedance calculations and to minimize power loss over the speaker cables. They are more appropriately called high-voltage audio distribution systems. The voltage is constant only in the sense that at full power, the voltage in the system does not depend on the number of speakers driven (as long the amplifier's maximum power is not exceeded). Constant-voltage speaker systems are also commonly referred to as 25-, 70-, 70.7-, or 100-volt speaker systems; distributed speaker systems; or high-impedance speaker systems. In Canada and the US, they are most commonly referred to as 70-volt speakers. In Europe, the 100 V system is the most widespread, with amplifier and speaker products being simply labeled with 100 V.

Constant-voltage speaker systems are analogous to electrical power transmission methods employed by power companies to transmit electrical power over long distances. Typically, a power company will step up the voltage of the power being transmitted, which correspondingly reduces the current and therefore also reduces the power loss during transmission. The voltage is stepped back down at the destination. Similarly, in a constant-voltage speaker system the amplifier uses a transformer to step up the voltage of the audio signal to reduce power loss over the speaker cable, allowing more power to be transmitted over a given wire diameter. Each speaker in the system has a step-down transformer to convert the voltage back to a usable level.

Each loudspeaker's step-down transformer can be designed for a single power level or it can have multiple taps, one of which is selected to match the desired power level to be applied to the loudspeaker. Transformers with various taps allow the installer to adjust the sound pressure level up or down at an individual loudspeaker. Purpose-built models are available that have the transformer contained within the loudspeaker enclosure. The loudspeaker step-down transformer primary is connected in parallel to the constant-voltage line.

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Wikipedia