Handbook for Sound Engineers

Sound System Design 1301

which would still be within the safe limits for these
power amplifiers.
The easiest way to describe the change in power
output with different load impedances is to take an
example, such as is shown in Fig. 34-64. One manufac-
turer’s professional power amplifier is rated at 440 W
per channel into a 4: load. It has a minimum load
impedance of 2: even though its 440 W power rating
is at 4:, and 440 W into 4: means exactly that.
Connect a 4: loudspeaker to one channel of this power
amplifier, and the amplifier will produce as much as
440 W into that loudspeaker. Connecting two 8: loud-
speakers (in parallel) to one channel of this power
amplifier will, again, result in as much as 440 W into
the resulting total impedance of 4:. Each 8: loud-
speaker in this example will receive exactly one-half of
the total power, or a maximum of 220 W.

Connect a single 8: loudspeaker to one channel of
this power amplifier, and that loudspeaker will still only
receive a maximum of about 220 W. (The actual power
will be slightly higher.) Connect a single 16: loud-
speaker, and it will receive a maximum of about 110 W.
In other words, doubling the load impedance halves the
power output of a power amplifier. Conversely, halving
the load impedance doubles the power output of the
amplifier. Remembering this simple relationship can
help insure that a loudspeaker and power amplifier will
be compatible in terms of impedance and power levels.

34.5.1.8 Digital Audio Level and Impedance Watching

Since most digital audio connections and cabling are
based on computer standards, an audio system designer
or installer may confidently use properly rated computer
cable and connectors to connect digital audio devices.
Pay attention to maximum cable length and, for portable
applications, use connectors and cabling designed for
portable usage. For proprietary or noncomputer-related
digital audio connectors, such as the AES/EBU connec-
tion, consult the device manufacturer for cable and con-
nector recommendations. Also see Chapter 39 for a
detailed discussion of digital audio networking and con-
nections.

34.5.1.9 Grounding and Shielding (also see Chapter 32)

Caution: In any audio system installation, governmen-
tal and insurance underwriters’ electrical codes must be
observed. These codes are based on safety and may vary
in different localities. In all cases, local codes take pre-

cedence over any suggestions contained in the Hand-

book for Sound Engineers.

Note that the ac power discussions in this section

apply specifically to the United States only. The general

discussions of grounding and shielding, however, should

be applicable to audio systems used in any location.

Always obey local and national fire and electrical safety

regulations.

Figure 34-64. Impedance and power transfer. Courtesy

Fender Musical Instruments.

Inputs Outputs

Inputs Outputs

Inputs Outputs

Inputs Outputs

4 7

8 7

16 7

8 7

8 7

D. Two 8 7 loudspeakers connected in parallel

draw a maximum of 440 W total or 220 W

each from the amplifier.

A. One 4 7 loudspeaker draws a maximum

of 440 W from the amplifier.

B. One 8 7loudspeaker draws a maximum

of 220 W from the amplifier.

C. One 16 7loudspeaker draws a maximum

of 110 W from the amplifier.