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Applying constant-voltage principles results in Figure 2. Here
is seen an output transformer connected to the power amplifier which
steps-up the full-power output voltage to a value of 70.7 volts
(or 100 volts for Europe), then each loudspeaker has integrally
mounted step-down transformers, Figure 2. 70.7V Transformer-Coupled
Constant-Voltage Distribution System converting the 70.7 volts to
the correct low-voltage (high current) level required by the actual
8 ohm speaker coil.
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It is common, although not universal, to find power (think loudness)
taps at each speaker driver. These are used to allow different loudness
levels in different coverage zones. With this scheme, the wire size
is reduced considerably from that required in Figure 1 for the 70.7
volt connections.
Becoming more popular are various direct-drive 70.7 volt
options as depicted in Figure 3. The output transformer shown in
Figure 2 is either mounted directly onto (or inside of) the power
amplifier, or it is mounted externally.
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In either case, its necessity adds cost, weight and bulk to the
installation. An alternative is the direct-drive approach, where
the power amplifier is designed from the get-go (I always wanted
to use that phrase, and I sincerely apologize to all non- American
readers from having done so) to put out 70.7 volts at full power.
An amplifier designed in this manner does not have the current capacity
to drive 8 ohm low-impedance loads; instead it has the high voltage
output necessary for constant-voltage use — same power; different
priorities. Quite often direct-drive designs use bridge techniques
which is why two amplifier sections are shown, although singleended
designs exist. The obvious advantage of direct-drive is that the
cost, weight and bulk of the output transformer are gone. The one
disadvantage is that also gone is the isolation offered by a real
transformer. Some installations require this isolation.
Voltage Variations — Make Up Your Mind
The particular number of 70.7 volts originally came about
from the second way that constant-voltage distribution reduced costs:
Back in the late ’40s, UL safety code specified that all voltages
above 100 volts peak (“max open-circuit value”) created
a “shock hazard,” and subsequently must be placed
in conduit – expensive – bad.
Therefore working backward from a maximum of 100 volts peak (conduit
not required), you get a maximum rms value of 70.7 volts (Vrms =
0.707 Vpeak). [It is common to see/hear/read “70.7 volts”
shortened to just “70 volts” – it’s sloppy;
it’s wrong; but it’s common – accept it.] In Europe,
and now in the U.S., 100 volts rms is popular. This allows use of
even smaller wire. Some large U.S. installations have used as high
as 210 volts rms, with wire runs of over one mile!
Remember: the higher the voltage, the lower the current, the smaller
the cable, the longer the line. [For the very astute reader: The
wire-gauge benefits of a reduction in current exceeds the power
loss increases due to the higher impedance caused by the smaller
wire, due to the current-squared nature of power.]
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