1214 Chapter 32
possible to actually predict hum levels and identify the
problem interfaces before a system is installed.^49
32.7 Alternative Treatments and Pseudoscience
The audio industry, especially the high-end segment,
abounds with misinformation and myth. Science,
evidence, and common sense are often discarded in
favor of mysticism, marketing hype, and huge profits.
Just remember that the laws of physics have not
changed! See Fig. 32-57.
32.7.1 Grounding from Technical to Bizarre
In most commercial buildings, the ac outlets on any
branch circuit are saddle grounded or SG-types mounted
in metallic J-boxes. Since SG outlets connect their
safety ground terminals to the J-box, the safety ground
network may now be in electrical contact with
plumbing, air ducts, or structural building steel. This
allows coupling of noisy currents from other loads—
which might include air conditioning, elevators, and
other devices—into the ground used by the sound
system. In a scheme called technical or isolated
grounding, safety grounding is not provided by the
J-box and conduit but by a separate insulated green wire
that must be routed back to the electrical panel alongside
the white and black circuit conductors to keep induc-
tance low. The technique uses special insulated ground
or IG outlet—marked with a green triangle and some-
times orange in color—which intentionally insulates the
green safety ground terminal from the outlet mounting
yoke or saddle. The intent of the scheme is to isolate
safety ground from conduit. Noise reduction is some-
times further improved by wiring each outlet as a home
run back to the electrical panel or subpanel, making each
outlet essentially a separate branch circuit.^50 This tech-
nique is covered by NEC Article 250-74 and its excep-
tions. In many cases, simply adding a new branch circuit
can be just as effective yet far less expensive than imple-
menting a technical ground system.
Many people, believing that the earth simply absorbs
noise, have a strong urge to install multiple earth ground
rods to fix noise. This is desperation-mode thinking.
Code allows extra ground rods, but only if they are
bonded to an existing properly implemented safety
ground system. Code does not allow them to be used as
a substitute soil resistance is simply too high and
unstable to be relied on to divert fault currents.^51
Equipment grounding via the standard power cord
safety ground is logical, easy to implement, and safe.
It’s highly recommended for all systems and is the only
practical method for portable or often reconfigured
systems.32.7.2 Power-Line Isolation, Filtering, and
BalancingMost sound systems use utility ac power. If it is
disconnected, of course, all hum and noise disappears.
This often leads to the odd conclusion that the noise is
brought in with the power and that the utility company
or the building wiring is to blame.^52 Devices claiming to
cleanse and purify ac power have great intuitive appeal
and are often applied without hesitation or much
thought. A far more effective approach is to locate, and
safely eliminate, ground loops that cause coupling of
noise into the signal. This solves the real problem. In
reality, when system designs are correct, special power
treatment is rarely necessary. Treating the power line to
rid it of noise is analogous to repaving all the highways
to fix the rough ride of a car. It’s much more sensible to
correct the cause of the coupling by replacing the shock
absorbers!
First, when any cord-connected line filter, condi-
tioner, or isolation transformer is used, Code requires
that the device as well as its load still be connected to
safety ground as shown in Fig. 32-58. Cord-connected
isolation transformers cannot be treated as separately
derived sources unless they are permanently wired into
the power distribution system per Code requirements.
Sometimes makers of isolation transformers have been
known to recommend grounding the shield and output to
a separate ground rod. Not only does this violate Code,
but the long wire to the remote ground renders the shield
ineffective at high frequencies. It is a sobering fact that,
while a device may control interference with respect to
its own ground reference, it may have little or no effect
at the equipment ground.53,54 Because all theseFigure 32-57. Officer Einsten of the Physics Police.
Courtesy Coil-Craft.