QSC Audio CX Series - Manual (Page 20)

19
Part 2—CX Series Application Guide
W
ith the helpful advice and input from contractors and consultants around the world, engineers at
QSC designed the CX Series amplifiers to be a versatile and reliable foundation for
high-quality installed sound systems.
This applications guide will help you design your sound system properly and utilize your CX amplifier(s) effectively.
VIII. Distributed lines
The best way to distribute audio power from a common amplifier among numerous relatively low-powered
loudspeakers is to use a distributed line, whether all the loudspeakers are meant to receive the same or different
power levels.
A distributed line is usually referred to by its maximum voltage-
25-, 70-, 100-, or 140-volt lines are most common. An amplifier’s
power rating is determined by how much current it can put out
at a standard voltage level; a low-power 70-volt amp has less
current capability than a high-powered 70-volt amp, even though
they can both put out the same voltage. The high-powered amp
can drive more speakers and at higher power levels than the
lower-powered one can. This is somewhat analogous to AC
electrical distribution, where you might have circuits rated for
15A, 20A, 30A, etc., at 120 or 240 VAC, and you can connect any
number of appliances to them as long as you don’t exceed the circuit’s current capacity.
Loudspeakers on a distributed line use tapped
transformers to match their power requirements
to the line. A loudspeaker with a 70-volt trans-
former tapped at 2 watts, for example, will
receive that amount of power when the amplifier
puts out 70 volts on the distributed line. Trans-
former taps are usually set at 3 dB intervals, i.e.,
each successive tap is twice the power level of its
predecessor. An example would be a transformer
tapped at 0.5, 1, 2, and 4 watts. This allows you
to select a power level that is most appropriate
for the situation while still retaining the versatil-
ity of configuring other loudspeakers on the same
line for different power levels.
8 COM
4
8
16
COM
10W
5W
2.5W
1.2W
70V
4.5V
8
8
8
Amplifier
70V
0
COM
SPKR
2W
1W
0.5W
COM
4
8
16
COM
10W
5W
2.5W
1.2W
COM
4
8
16
COM
10W
5W
2.5W
1.2W
A
B
C
Example of a 70 volt
distributed line
Example of a loadspeaker connected to a
distributed line through a transformer