JBL LSR28P, Техническое и сервисное руководство

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A WORD ON BALANCED AND UNBALANCED INPUTS
In order to achieve an undistorted sound, the installation technician has to connect the
LSR28P properly. The introduction of noise, hums, pops and whistles in the sound media
detracts from the audience’s understanding, and ultimately, their comprehension whether that
media is of a vocal or musical nature. So, it is of critical importance that the Audio System be
correctly installed.
There are three methods of connecting the source output to the source input of the LSR28P
System. Each method has its own attributes and the NUETRIK Combo connector can
accommodate all methods of hookup, however, the quietist and most universally accepted
method of hookup for the professional environment is the first method or “balanced input.”
The first method of hookup, called a “balanced input”, of which the LSR28P was designed, uses
XLR connectors throughout the system. The actual reason why this is a more accepted method
of hookup is the fact that XLR connectors utilize three conductors. Each conductor has a
purpose—one conductor for the hot lead, one conductor for the audio ground and, lastly, one
conductor for the chassis ground, which is connected to the shield of the input cable. This third
conductor, the shield, which is terminated at the LSR28P, eliminates extraneous noise that would
ordinarily be induced into the input leads or ground plane and carried onward to the
amplification process.
The second method of hookup, called an “unbalanced input”, is by using ¼” phone plug which
is predominantly used within the semiprofessional environment.
The phone plug can be plugged into the center of the Nuetrik connector. Again, each method
will work but the ¼” phone plug method is more susceptible to inducing the annoying hum that
is commonly referred to as a ground loop due to the fact that the chassis ground and audio
ground are hard wired together. This condition will occur especially if a large distance separates
the monitor and preamplifier allowing differing levels of voltage potential to reside upon the
ground plane. The unwanted signal is admitted into the system via the input circuitry and, or the
power supply.
If the unwanted signal enters the input circuitry, it is usually through input cable induction and,
most of the time, the hum that will be induced is a 60cycle hum. If the unwanted signal enters
through the power supply the entire amplifier will be affected. In order to avoid this scenario,
the technician should use a 3-conductor cable, preferably shielded R58 coaxial cable. Again,
two of the conductors carry the signal and the other conductor is connected to the chassis
ground.
If the “unbalanced input” method must be used, termination of the ground must be done at
the input to the LSR28P. This will avert the above possible problems by preventing a complete
circuit forming and possible internal electronic circuit damage resulting from a malfunction or
internal a.c. short of either equipment.