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2.4 GROUNDING AND SHIELDING
Grounding in modern broadcast consoles is more critical than with older devices of more limited band-
pass capabilities. Achieving low system ground impedance with a small piece of equipment is relatively
easy. However, the problem becomes progressively more difficult as the system becomes larger. In
designing the BMX III, much thought was given to system grounding requirements and the elimination
of DC path ground loops.
The shield pins on each console connector are connected directly to the console mainframe ground, and
the only location where the console mainframe ground meets the console's "audio common" point is
the power terminal strip on the console mainframe. Therefore, the station's "technical ground" should
be connected to either of the two screw terminals labeled "AUDIO COMMON" on the console power
terminal strip.
NOTE: Do not connect the station's "technical ground" to any terminals on the power supply terminal
strip.
A preferred method of connecting the line shields in a system is to connect both ends of every shield
to all affiliated equipment. However, this method is only satisfactory if every component shares a
common earth ground. This can be accomplished using isolated ground receptacles tied to the station's
"technical ground".
If isolated ground receptacles are not available when grounding the BMX III, observe the following
guidelines:
A) Shields of cables connecting the console to auxiliary equipment should be connected at the
console end only, and should not be terminated to the ground of the auxiliary equipment.
B) Ensure that the auxiliary equipment is connected to a "clean" ground by its power cord
assembly, or by the addition of a separate ground wire connected between the chassis of the
auxiliary equipment and the station's "technical ground".
NOTE: Buzz pickup is generally electrostatic, due to capacitive pickup between an audio line and a
power line. When shielded lines are used this should be no problem, unless the audio lines are run in
the same wireways or area as a power line. Radio-frequency interference can also manifest itself as a
buzz in the program audio. RFI is minimized by the extensive RF bypassing and ground-plane
techniques used in the BMX III, and the shielded lines external to the unit.
2.5 POWER CONNECTION
A separate power outlet should be assigned exclusively to the BMX III power supply. Confirm that the
outlet supplies 120 VAC, ± 8%, 50/60 Hz, and that the voltage does not sag under a load of up to 5
amperes. The third pin "U-ground" on the power connector must be left intact and connected to a
properly installed three way AC outlet. For safety, the "U-ground" wire is connected to the chassis of
the power supply and the cores of the power transformers.
PACIFIC RESEARCH & ENGINEERING
14
BMX SERIES III CONSOLE
REVISION C
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