Constant-voltage is the common name given to a general practice begun in the late 1920s and early 1930s (becoming a U.S. standard in 1949) governing the interface between power amplifiers and loudspeakers used in distributed sound systems. Installations employing ceiling-mounted loudspeakers, such as offices, restaurants and schools are examples of distributed sound systems. Other examples include installations requiring long cable runs, such as stadiums, factories and convention centers. The need to do it differently than you would in your living room arose the first time someone needed to route audio to several places over long distances. It became an economic and physical necessity. Copper was too expensive and large cable too cumbersome to do things the home hi-fi way.
Stemming from this need to minimize cost, maximize efficiency, and simplify the design of complex audio systems, thus was born constant-voltage. The key to the solution came from understanding the electric company cross-country power distribution practices. They elegantly solved the same distribution problems by understanding that what they were distributing was power, not voltage. Further they knew that power was voltage times current, and that power was conserved. This meant that you could change the mix of voltage and current so long as you maintained the same ratio: 100 watts was 100 watts — whether you received it by having 10 volts and 10 amps, or 100 volts and 1 amp. The idea bulb was lit. By stepping-up the voltage, you stepped-down the current, and vice-versa. Therefore to distribute 1 megawatt of power from the generator to the user, the power company steps the voltage up to 200,000 volts, runs just 5 amps through relatively small wire, and then steps it back down again at, say, 1000 different customer sites, giving each 1 kilowatt. In this manner large gauge cable is only necessary for the short direct run to each house. Very clever.
Applied to audio, this means using a transformer to step-up the power amplifier’s output voltage (gaining the corresponding decrease in output current), use this higher voltage to drive the (now smaller gauge wire due to smaller current) long lines to the loudspeakers, and then using another transformer to step-down the voltage at each loudspeaker. Nothing to it.
Read More: Constant-Voltage Audio Distribution Systems: 25, 70.7 & 100 Volts.
