Constant-Voltage-
8Ω
8Ω
POWER
(LOUDNESS)
TAPS
7O.7 V @
FULL POWER
AMP
AMP
IN
IN
AMP
8
Ω
8
Ω
7O.7 V @
FULL POWER
POWER
(LOUDNESS)
TAPS
Figure 1. Low-Inpedance Series-Parallel 8Ω Direct Drive
Figure 2. 70.7V Transformer-Coupled
Constant-Voltage Distribution System
Figure 3. 70.7V Direct-Drive Constant-
Voltage Distribution System
IN
Z = 8
Ω
8
Ω
8
Ω
8
Ω
8
Ω
8
Ω
8
Ω
8
Ω
8
Ω
8
Ω
AMP
Figure 1 diagrams the alternative series-parallel
method, where, for example, nine loudspeakers are
wired such that the net impedance seen by the ampli-
fier is 8 ohms. The wiring must be selected sufficiently
large to drive this low-impedance value. 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, converting the 70.7 volts to the correct
low-voltage (high current) level required by the actual
8 ohm speaker coil. It is common, although not univer-
sal, 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 Fig-
ure 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. 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-im-
pedance 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 sec-
tions are shown, although single-ended 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 isola-
tion offered by a real transformer. Some installations
require this isolation.
