Note that the voltage is not constant, as its name seems to imply, but does actually fluctuate with the audio signal. The main implication is that primary wiring and Class 2 wiring must never be placed in the same conduit or junction box. Class 2 covers typical speaker wiring installations. Modern codes break audio and signaling voltages into various classes. Many electrical codes of the time stipulated that voltages of 100 or more Volts must be installed in conduit. The reason the North American audio industry settled on 70 Volts is so all audio wiring can be installed without conduit. In some cases, a constant voltage system is used to save money on speaker wire. Usually this system is used to distribute audio to a large venue with many speakers. Similarly, we can apply a high voltage audio signal to a speaker line as long as there is a transformer at the speaker end to reduce the voltage to appropriate values for the drivers. Because lower currents are required at high voltage, the resistance of the line does not cause a loss of power because of heat, and smaller diameter wires can be used. The voltage is stepped down to hundreds of Volts at the building where the power is used. The power company puts power on a line that carries it a long distance with thousands of Volts. This method of getting audio from amplifiers to speakers is similar in principle to power transmission methods used to distribute electrical power. What is generally called a 70V Line speaker wiring system is also known as a constant voltage speaker wiring system.
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