Handbook for Sound Engineers

Grounding and Interfacing 1213

to magnetic fields. Magnetic coupling is also reduced

by separation distance, cables crossing at right angles

rather than running parallel, and shielding with

magnetic material such as steel EMT conduit.

Pay attention to cable shield grounding. As discussed

in Section 32.5.3, the shield must be grounded at the

driven end, it may be grounded at both ends, but never

grounded only at the receive end. As a standard prac-

tice, grounding at both ends is recommended for two

reasons:

1. If the device input has marginal RF suppression,
   grounding the shield at the input will usually
   reduce problems,


2. It doesn’t require the use of a specially wired cable
   that might find its way into another system and
   cause unexpected problems. If special cables are
   made—to deal with a pin 1 problem, for
   example—be sure they are clearly marked.

Don’t terminate to reduce noise. Nearly every prac-
tical audio system should use unterminated audio
circuits. This is standard professional audio practice
worldwide. While a 600: termination resistor at an
input may reduce noise by up to 6 dB or more,
depending on the driver output impedance, it will also
reduce the signal by the same amount, so nothing is
gained. If noise is caused by RF interference, installa-
tion of a suitably small capacitor at the input may be
much more appropriate.

Use ground isolators to improve noise rejection. As
discussed in Section 32.4.1, common balanced input
circuits have generally unpredictable noise rejection in
real-world systems. Actual in-system CMRR can be as
little as 30 dB when using balanced sources and as little
as 10 dB when using unbalanced sources. A quality
transformer-based ground isolator can increase the
CMRR of even the most mediocre balanced input to
over 100 dB.

Beware of the pin 1 problem. As much as 50% of
commercial equipment, some from respected manufac-
turers, has this designed-in problem. If disconnecting
the shield at an input or output reduces a hum problem,
the device at one or the other end of that cable may be
the culprit. See Section 32.5.3 for test methods. Loose
connector-mounting hardware is a major cause of pin 1
problems. Never overlook the obvious!

32.6.3.7 Tips for Unbalanced Interfaces

Keep cables as short as possible. Longer cables in-

crease the coupling impedance. Serious noise coupling

is nearly certain with 50 ft or 100 ft cables. Even much

shorter cables can produce severe problems if there are

multiple grounds. And never coil excess cable length.

Use cables with heavy gauge shields. Cables with

shields of foil and thin drain wires increase the

common-impedance coupling. Use cables with heavy

braided copper shields, especially for long cables. See

Section 32.7.4 for a recommended high-performance

cable. The only property of cable that has any signifi-

cant effect on audio-frequency noise coupling is shield

resistance, which can be measured with an ordinary

ohmmeter.

Bundle signal cables. All signal cables between any

two boxes should be bundled. For example, if the L and

R cables of a stereo pair are separated, nearby ac

magnetic fields will induce a current in the loop formed

by the two shields, causing hum in both signals. Like-

wise, all ac power cords should be bundled. This will

tend to average and cancel the magnetic and electro-

static fields they radiate. In general, keeping signal

bundles and power bundles separated will reduce

coupling.

Maintain good connections. Connectors left undis-

turbed for long periods can oxidize and develop high

contact resistance. Hum or other interference that

changes when the connector is wiggled indicates a poor

contact. Use a good commercial contact fluid and/or

gold-plated connectors to help prevent such problems.

Don’t add unnecessary grounds! Additional

grounding almost always increases circulating noise

currents rather than reducing them. As emphasized

earlier, never disconnect or defeat the purpose of safety

or lightning ground connections to solve a noise

problem—the practice is both illegal and very

dangerous!

Use ground isolators at problem interfaces. Trans-

former-based isolators magnetically couple the signal

while completely breaking the noise current path

through the cable and connectors. This eliminates

common-impedance coupling and can improve immu-

nity to RF interference as well.

Predict and solve problems before an installation. For

systems that consist mostly of devices with two-prong

power cords, some very simple multimeter measure-

ments on each system device and cable makes it