Crown CTS1200 Reference Manual

GUIDE TO CONSTANT-VOLTAGE SYSTEMS
by the Crown Engineering staff
Electric-power companies have a good idea which 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 I
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 P.A., paging, or low-SPL background music.
BACKGROUND
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 speaker has a step-down transformer that matches
the 70V line to each speaker's impedance. The primaries of all the speaker 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 70V 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 70V, 1.41 amperes, and 50 ohms.
How did the 70V line get its name? The intention was to have 100V peak on the line, which is 70.7V rms.
The technically correct value is 70.7V rms, but "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.
2
R heating of the power cables.