Interconnection-4
Another way to create the necessary isolation is to
use a
direct box.
Originally named for its use to convert
the high impedance, high level output of an electric
guitar to the low impedance, low level input of a re-
cording console, it allowed the player to plug “directly”
into the console. Now this term is commonly used to
describe any box used to convert unbalanced lines to
balanced lines.
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 usu-
ally creates noisy interconnections.
A common solution to these noisy hum and buzz
problems involves disconnecting 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 dis-
connect 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 inter-
ference by providing an RF path through a small ca-
pacitor (0.1 or 0.01 microfarad ceramic 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 install-
ers still follow this one-end-only rule with consistent
success indicates this and other acceptable 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 spe-
cific 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 multi-
ple 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.
See Figure 4 for suggested cable assemblies for your
particular interconnection needs. Find the appropri-
ate output configuration (down the left side) and then
match this with the correct input configuration (across
the top of the page.) Then refer to the following pages
for a recommended wiring diagram.
Ground Lifts
Many units come equipped with ground lift switches.
In only a few cases can it be shown that a ground lift
switch improves ground related noise. (Has a ground
lift switch ever
really
worked for you?) In reality, the
presence of a ground lift switch greatly reduces a unit’s
ability to be “properly” grounded and therefore im-
mune to ground loop hums and buzzes. Ground lifts
are simply another Band-Aid
®
to try in case of ground-
ing problems. It is true that an entire system of prop-
erly grounded equipment, without ground lift switches,
is guaranteed (yes
guaranteed
) to be hum free. The
problem is most equipment is
not
(both internally and
externally, AC system wise) grounded properly.
Most units with ground lifts are shipped so the unit
is “grounded” — meaning the chassis is connected to
audio signal ground. (This should be the best and is
the “safest” position for a ground lift switch.) If after
hooking up your system it exhibits excessive hum or
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
