Electric power companies have a good idea that has been applied to audio engineering. When they run power through miles of cable, they minimize resistive power loss by running the power as high voltage and low current.
To do this, they use a step-up transformer at the power station and a step-down transformer at each customer’s location. This reduces power loss due to the I2R heating of the power cables.
The same solution can be applied to audio communications in the form of a constant-voltage system (typically 70 volts in the U.S. and 100V overseas).
Such a system is often used when a single power amplifier drives many loudspeakers through long cable runs (over 50 feet). Some examples of this condition are distributed speaker systems for PA, paging, or low-SPL background music.
The label “constant voltage” has been confusing because the voltage is really not constant in an audio program. A better term might be “high impedance.”
A typical high-impedance system is shown in Figure 1. A transformer at the power amplifier output steps up the voltage to approximately 70 volts at full power.
Each loudspeaker has a step-down transformer that matches the 70-volt line to each loudspeaker’s impedance.
The primaries of all the loudspeaker transformers are paralleled across the transformer secondary on the power amplifier.
Figure 1. A typical high-impedance system using a step-up transformer on the amplifier output.
There are three options at the power-amp end for 70-volt operation:
• an external step-up transformer
• a built-in step-up transformer
• a high-voltage, transformerless output
These options are covered in detail later in this article.
The signal line to the loudspeakers is high voltage, low current, and usually high impedance. Typical line values for a 100-watt amplifier are 70 volt, 1.41 amperes, and 50 ohms.
How did the 70-volt line get its name? The intention was to have 100-volt peak on the line, which is 70.7 volts rms.
The technically correct value is 70.7 volt rms, but “70-volt (or “70V”) is the common term. There are 70 volts on the line as maximum amplifier output with a sine wave signal. The actual voltage depends on the power amplifier wattage rating and the step-up ratio of the transformer. The audio program voltage in a 70V system might not even reach 70V. Conversely, peaks in the audio program might exceed 70V.
Other high-voltage systems might run at other voltages. Although rare, the 200V system has been used for cable length exceeding one mile.