Quest Engineering PR-130A, User Manual

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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 ti ons in the middle of the 20th century because it o ff ered 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 aff 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 ft en use a constant voltage distribu ti on sys tem 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-130A 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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PR-130A / PR-240A