Constant-Voltage-
Stemming from this need to minimize cost, maxi-
mize 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 volt-
age 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 (Elec-
tronic Industries Association), and today is covered
also by the
National Electric Code (NEC)
2
.
Basics — What is “Constant” Anyway?
The term “constant-voltage” is quite misleading and
causes much confusion until understood. In electron-
ics, two terms exist to describe two very different
power sources: “constant-current” and “constant-volt-
age.” Constant-current is a power source that sup-
plies 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 volt-
age 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.
