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Background — Wellspring
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 stepup
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.
U.S. Standards— Who Says?
This scheme became known as the constant-voltage distribution
method. Early mention is found in Radio Engineering, 3rd
Ed. (McGraw-Hill, 1947), and it was standardized by the American
Radio Manufacturer’s Association as SE-101-A & SE-106,
issued in July 1949{1}. Later it was adopted as a
standard by the EIA (Electronic Industries Association), and today
is covered also by the National Electric Code (NEC){2}.
Basics — Just What is “Constant” Anyway?
The term “constant-voltage” is quite misleading and
causes much confusion until understood. In electronics, two terms
exist to describe two very different power sources: “constant-current”
and “constant-voltage.”
Constant-current is a power source that supplies a fixed amount
of current regardless of the load; so the output voltage varies,
but the current remains constant. Constant-voltage is just the opposite:
the voltage stays constant regardless of the load; so the output
current varies but not the voltage.
Applied to distributed sound systems, the term is used to describe
the action of the system at full power only. This is the
key point in understanding. At full power the voltage on the
system is constant and does not vary as a function of the number
of loudspeakers driven, that is, you may add or remove (subject
to the maximum power limits) any number of loudspeakers and the
voltage will remain the same, i.e., constant.
The other thing that is “constant” is the amplifier’s
output voltage at rated power – and it is the same voltage
for all power ratings. Several voltages are used, but the most
common in the U.S. is 70.7 volts rms. The standard specifies that
all power amplifiers put out 70.7 volts at their rated power. So,
whether it is a 100 watt, or 500 watt or 10 watt power amplifier,
the maximum output voltage of each must be the same (constant) value
of 70.7 volts.
Figure 1 diagrams the alternative series-parallel method, where,
for example, nine loudspeakers are wired such that the net impedance
seen by the amplifier is 8 ohms. The wiring must be selected sufficiently
large to drive this low-impedance value.
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