J. W. Davis & Company DA-120M User Manual - page 10
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XII - CONSIDERATIONS FOR PA SPEAKER LAYOUT
SPEAKER CONNECTIONS
In connecting speakers to a public address amplifier, it is important to present the amplifier with the load
impedance it is designed to handle. Failure to do this can cause overheating and component failure. In
many cases problems can take months to appear in the form of reduced intelligibility and unnecessary
service calls. A load impedance that is too low is especially bad. Strive to have a load impedance of not
less than 80% of the chosen amplifier output impedance. Do not connect a 4-ohm speaker to the 8-ohm
output. Driving a load of higher impedance than rated amplifier output is not as serious, but results in a
power loss proportional to the mismatch and should be avoided. For example, driving a 16-ohm load
through the 8-ohm output will result in a 50% loss in power. The high impedance mismatch should be kept
to less than 200%, especially if it is anticipated that more than 50% of the rated amplifier power will be
required.
(a)
Runs are short (less than 200 ft. (70 m))
There are two methods of connecting groups of speakers to the amplifier. First, using low
impedance (i.e. 4, 8, 16 ohm) outputs. This is preferable where:
(b)
Few horns or speakers are to be used (ie. typically 4-8)
(c)
Same sound levels are required at each speaker
(d)
Low impedance also provides slightly better fidelity and frequency response.
(a)
The runs are long and line losses are to be avoided
High impedance or constant voltage is the second method, and is preferable where:
(b)
Many speakers are to be used
(c)
Different sound levels are required at different locations; for example, indoor speakers and
outdoor horns
(d)
Future expansion possibilities require flexibility in wiring layout.
The following is a more detailed discussion of these two methods.
LOW IMPEDANCE CONNECTION
The speakers must be connected to present a
combined impedance equal to the selected ampli-
fier output impedance; i.e., 4, 8, 16 ohms. The
connections should be arranged in a series/parallel
combination to achieve this according to the
following formula. The impedance should be be-
tween 70% and
200%
of the output impedance
selected. If the amplifier is to be driven anywhere
near its full rated output the impedance should be
well within these tolerances.
SERIES/PARALLEL COMBINATIONS
In larger systems it will be necessary to combine series and parallel connections to obtain the necessary
impedance. The rule for calculating the total effective impedance is to divide the entire circuit into indi-
vidual small series of parallel sub-circuits and apply the foregoing rules to them.
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