JBL LSR28P Technical & Service Manual

Also see for LSR28P: Owner's manual

12A WORD ON BALANCED AND UNBALANCED INPUTSIn order to achieve an undistorted sound, the installation technician has to connect theLSR28P properly. The introduction of noise, hums, pops and whistles in the sound mediadetracts from the audience’s understanding, and ultimately, their comprehension whether thatmedia is of a vocal or musical nature. So, it is of critical importance that the Audio System becorrectly installed.There are three methods of connecting the source output to the source input of the LSR28PSystem. Each method has its own attributes and the NUETRIK Combo connector canaccommodate all methods of hookup, however, the quietist and most universally acceptedmethod 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, usesXLR connectors throughout the system. The actual reason why this is a more accepted methodof hookup is the fact that XLR connectors utilize three conductors. Each conductor has apurpose—one conductor for the hot lead, one conductor for the audio ground and, lastly, oneconductor for the chassis ground, which is connected to the shield of the input cable. This thirdconductor, the shield, which is terminated at the LSR28P, eliminates extraneous noise that wouldordinarily be induced into the input leads or ground plane and carried onward to theamplification process.The second method of hookup, called an “unbalanced input”, is by using ¼” phone plug whichis predominantly used within the semiprofessional environment.The phone plug can be plugged into the center of the Nuetrik connector. Again, each methodwill work but the ¼” phone plug method is more susceptible to inducing the annoying hum thatis commonly referred to as a ground loop due to the fact that the chassis ground and audioground are hard wired together. This condition will occur especially if a large distance separatesthe monitor and preamplifier allowing differing levels of voltage potential to reside upon theground plane. The unwanted signal is admitted into the system via the input circuitry and, or thepower 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 entersthrough 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 chassisground.If the “unbalanced input” method must be used, termination of the ground must be done atthe input to the LSR28P. This will avert the above possible problems by preventing a completecircuit forming and possible internal electronic circuit damage resulting from a malfunction orinternal a.c. short of either equipment.