Specifications

4
as much as 28.3 volts, as determined by Ohm’s Law, while an amp that
does 200 watts into 8 ohms can put out 40.0 volts.
This is where the true concept of “constant voltage” comes in; it helps
simplify system design by converting one of the variables into a constant
value. But you can’t just connect typical 8-ohm speakers across a 70-
volt line because they’ll want to draw about 625 watts each. How then
do you plan and control the amount of power to each speaker when you
have a defined maximum line voltage? The answer: through transform-
ers. Each speaker has a transformer that converts the line voltage to
another value (almost always lower) to actually drive the speaker. Taps
on the transformer allow you to select the power level the speaker
receives when the line voltage reaches its maximum of 70 volts. It is
somewhat analogous to AC electrical service, in that you can plug a 100-
watt appliance and a 50-watt one into outlets carrying the same 120
VAC; you don’t have 120 volts for one and 85 volts for the other.
Regular low-impedance amplifiers are perfect for systems with one,
two, three, or four speakers per amp channel, with each speaker getting
the same amount of power. But if you need to power more speakers,
or provide different power levels to some or all of them, you would often
have to do some complicated series-parallel calculations and wiring.
And even then if a speaker fails, is removed, or must be added, it would
alter the power distribution among the rest. A distributed line elimi-
nates the need for such calculations and considerations. It lets you
forget about impedances. And it also lets you substitute amplifier
models as needed without having to re-calculate power distribution
among the loudspeakers. For example, if expanding a distributed
speaker system or increasing some power taps requires you to upgrade
a 150-watt 70-volt amplifier to a 200-watt model, you can do so without re-calculating or reconfiguring all the
other speaker taps, although you would have to match the gain of the new amp to that of the old one.
Why 70 volts?
If 70 volts seems like an odd number to become a de facto standard for distributed line voltage, how about 70.7 volts?
That’s the actual figure used in design of distributed lines, although it suggests a lot more precision that you
should hope to measure on an audio voltage. The number 70.7 came about for two reasons. First, as we’ve seen
already in this book, many loading and impedance calculations involve squaring the voltage. The approximate square
of 70.7 is 5000, which was easy to remember and work with in the days before pocket calculators. The second
reason is that versions of the National Electrical Code (NEC) before 1999 classify signal circuitry of 100 volts or higher
as Class 1, requiring a higher grade of wiring. Settling on 70.7 volts allowed a distributed line circuit to be deemed
a Class 2 circuit, with a safety margin of exactly 3 dB to allow for loading variations, audio peaks, etc.
Distributed line voltages other than 70 volts are common in some areas and applications. In Europe, 100-volt lines
are prevalent instead of 70 volts. And in the United States, 25-volt lines are common in public school buildings. In
applications where distributed lines have to run very long distances, 140- and 200-volt lines carry the audio power at
a high ratio of voltage to current (a high-impedance line, in other words) to minimize losses due to wire resistance.
voltage
current
resistance
power
Ohm’s Law
Nearly two centuries ago a German scientist named Georg Ohm
discovered that the current through a load is directly propor-
tional to the voltage across it, and also inversely proportional
to the resistance of the load. This relationship is called Ohm’s
Law, and the scientific community honored ohm by naming the
unit of resistance after him. In its basic form, Ohm’s Law is
expressed as the equation
E = I × R
where E is voltage (in volts), I is current (in amperes), and R is
resistance (in ohms).
You can also use Ohm’s Law to calculate the power in the load,
which is equal to voltage times current. The properties of
power, voltage, current, and resistance are all interrelated, so
if you know the value of
any two of them, you
can calculate the
other two. This
Ohm’s Law
“wheel” shows
how to solve for
any of the four
properties.