Interconnection-4
a small capacitor (0.1 or 0.01 microfarad ceramic disc) con-
nected 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 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 specific
unit, chances are, even though the documentation indicates
proper chassis grounded shields, the suspect unit is not in-
ternally 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 con-
nection to find out if special cable assemblies are needed at
more than one point.
See Figure 4 for suggested cable assemblies for your par-
ticular interconnection needs. Find the appropriate 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 immune to ground loop
hums and buzzes. Ground lifts are simply another Band-Aid
®
to try in case of grounding problems. It is true that an entire
system of properly 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 buzzing, there is an
incompatibility somewhere in the system’s grounding con-
figuration. In addition to these special cable assemblies that
may help, here are some more things to try:
1. Try combinations of lifting grounds on units supplied
with lift switches (or links). It is wise to do this with the
power off!
2. If you have an entirely balanced system, verify all chassis
are tied to a good earth ground, for safety’s sake and hum
protection. Completely unbalanced systems never earth
ground anything (except cable TV, often a ground loop
source). If you have a mixed balanced and unbalanced
system, do yourself a favor and use isolation transform-
ers or, if you can’t do that, try the special cable assemblies
described here and expect it to take many hours to get
things quiet. May the Force be with you.
3. Balanced units with outboard power supplies (wall warts
or “bumps” in the line cord) do
not
ground the chassis
through the line cord. Make sure such units are solidly
grounded by tying the chassis to an earth ground using a
star washer for a reliable contact. (Rane always provides
this chassis point as an external screw with a toothed
washer.) Any device with a 3-prong AC plug, such as an
amplifier, may serve as an earth ground point. Rack rails
may or may not serve this purpose depending on screw
locations and paint jobs.
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
Содержание SM 26S
Страница 4: ......
