16
The Last Best Right Way To Do It
If transformer isolation is not an option, special cable assemblies are a
last resort.
The key here is to prevent the shield currents from flowing into a
unit whose grounding scheme creates ground loops (hum) in the audio path
(i.e., most audio equipment).
It is true that connecting both ends of the shield is theoretically the best
way to interconnect equipment –though this assumes the interconnected
equipment is internally grounded properly. Since most equipment is
not
internally grounded properly, connecting both ends of the shield is not often
practiced, since doing so usually creates noisy interconnections.
A common solution to these noisy hum and buzz problems involves dis-
connecting one end of the shield, even though one can not buy off-the-shelf
cables with the shield disconnected at one end. The best end to disconnect
is the receiving end. If one end of the shield is disconnected, the noisy hum
current stops flowing and away goes the hum — but only at low frequencies.
A ground-sending-end-only shield connection minimizes the possibility of
high frequency (radio) interference since it prevents the shield from acting as
an antenna to the next input. Many reduce this potential RF interference by
providing an RF path through a small capacitor (0.1 or 0.01 microfarad ce-
ramic disc) connected from the lifted end of the shield to the chassis. (This is
referred to as the “hybrid shield termination” where the sending end is bonded
to the chassis and the receiving end is capacitively coupled. See Neutrik’s
EMC-XLR for example.) The fact that many modern day installers still follow
this one-end-only rule with consistent success indicates this and other ac-
ceptable solutions to RF issues exist, though the increasing use of digital and
wireless technology greatly increases the possibility of future RF problems.
If you’ve truly isolated your hum problem to a specific unit, chances are,
even though the documentation indicates proper chassis grounded shields,
the suspect unit is not internally grounded properly. Here is where special test
cable assemblies, shown in Figure 3, really come in handy. These assemblies
allow you to connect the shield to chassis ground
at the point of entry
, or to pin
1, or to lift one end of the shield. The task becomes more difficult when the
unit you’ve isolated has multiple inputs and outputs. On a suspect unit with
multiple cables, try various configurations on each connection to find out if
special cable assemblies are needed at more than one point.
Figure 3. Test cable
TEST
WIRE
GROUND CLIP
FEMALE
MALE
1
C
2
3
1
2
3
RED
BLACK
SHIELD
RED
BLACK
SHIELD
2-CONDUCTOR SHIELDED CABLE
