Speaker selec
ti
on
When selec
ti
ng loudspeakers, you must consider a few important factors, such as the loca
ti
on of the mixer ampli
fi
er,
the type of loudspeakers and the need to use a 70V distribu
ti
on system. The following sec
�
ons explain how to connect
loudspeakers or a constant voltage system in a typical installa
ti
on.
Using passive speakers
Before using low impedance speakers (4 or 8 ohms), you must
fi
rst determine the loca
ti
on of your mixer ampli
fi
er. If
the speakers are less than 30 meters away, you can connect the ampli
fi
er directly to the speakers in low impedance
mode.
Be sure to check the impedance of the speakers. The 4 Ohms output allows you to connect a 4 Ohms speaker or two 8
ohms speakers in parallel. If the speakers are more than 30 meters away (approximately), it is best to use a constant
voltage system.
Using a constant voltage system
If your setup has many loudspeakers and long cables, you might consider using a 25 or 70V constant voltage system.
This type of system became standard in the
fi
eld of installa
�
ons in the middle of the 20th century because it o
ffered
an
e
ffi
cient solu
ti
on for the use of mul
ti
ple enclosures and long cables. It is inspired by long distance electrical networks
which use increased voltage and reduced current to carry very high power over long distances with more
a
ff
ordable
and rela
�
vely small diameter cables. The voltage is then reduced and then routed to the consumer on a short, large
diameter cable.
The same principle applies to constant voltage audio systems. A transformer connected to the output of the ampli
fi
er
increases the voltage and reduces the output current, allowing the use of long, smaller-diameter cables to link mul
ti
ple
speakers with step-down transformers. For economy, the 70V standard was adopted in the United States because
electrical conduit is required on lines with voltages greater than 100V peak-to-peak. Commercial sound systems, such
as those installed in schools, o
ffi
ce buildings and restaurants, o
�
en use a constant voltage distribu
�
on system when
mul
ti
ple loudspeakers are connected in di
ff
eren
t areas.
Many loudspeakers with transformers are produced by various manufacturers. Most of these loudspeakers provide tap
points on the transformer to allow power to be distributed to the loudspeakers and their levels to be adjusted
separately. Quest Commercial mixer ampli
fi
ers have a built-in transformer to step up the output voltage and step down
the current so that mul
ti
ple speakers (with transformers) can be connected using long cables. Constant tension systems
allow the use of cables from 1 to 1.6 mm in diameter, depending on their length (long cables must be larger in
diameter).
A simple way to represent a constant voltage is to divide the power in Wa
tt
s of the ampli
fi
er by the number of
loudspeakers used. For example, to connect ten loudspeakers to the output of a PR-130BT ampli
fi
er, use a loudspeaker
whose transformer provides a tap point of approximately 10 Wa
tt
s. 130 Wa
tt
s divided by 10 speakers gives 13 Wa
tt
s
per loudspeaker, but we must always keep a cushion of about 15 to 20% of the power of the amp to make sure not to
overload the amp.
[ Figure 2.2 Constant voltage speaker ]
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