Handbook for Sound Engineers

1304 Chapter 34

34.5.1.9.6 Grounding to Reduce External Noise Pickup

After safety, the second reason to pay attention to
grounding is that, although proper grounding won’t
always reduce external noise pickup, poor grounding
can unquestionably increase external noise pickup.
One myth about grounding is that a piece of equip-
ment must be earth grounded to avoid noise pickup.
Anyone who owns a portable MP3 player or CD player
knows that this isn’t true. Good grounding practice is
primarily a matter of proper connections between
devices, avoiding ground loops and using equipment
that does not have the “Pin 1 Problem.”

34.5.1.9.7 Definitions: Ground, Earth, Common

A common connection in an audio system is some point
where a group of circuits (usually shields or other
zero-signal circuit lines) connect. Ground in an audio
system is the primary zero signal reference for the sys-
tem. In a typical system, there may be a common con-
nection of all audio signal shields and a separate
common connection of all power-supply negative termi-
nals. At some physical point in the system, these com-
mons are connected together. That point becomes the
system ground and is called the zero signal reference
potential because that is the place where a voltmeter ref-
erence lead is placed. An earth connection is a connec-
tion directly to the earth often made via a copper rod
driven into moist, salted soil. The system ground is
physically connected to the earth at this copper rod.
These terms are not fixed in meaning, however, and the
term ground. for example, is often used in place of
either of the other two terms, or grounding may be used
as a general term to describe the practice of external
noise reduction. In addition, outside the United States,
the term earth is often used in place of the term ground
to indicate the system zero signal reference potential.

34.5.1.9.8 Ground Loops in Unbalanced Systems

A couple of examples will help explain what ground
loops are and how to avoid them (also see Chapter 32).
In Figs. 34-66, 34-67, and 34-68, the loop is between
two audio cables that connect a line-level device to a
power amplifier with unbalanced inputs. These are
examples of unavoidable ground loops. The best way to
deal with this type of ground loop is to keep the cables
as short as possible and bundle them physically as close
together as possible (lace or tape them together if the
setup will allow it). This reduces the area enclosed by
the loop, which will reduce the pickup of external noise.

Alternately, use an appropriate transformer to balance

the connection and allow a telescoping shield to inter-

rupt the ground loop (see the next section).

Unfortunately, this only reduces the pickup of

magnetically coupled external noise. It does not reduce

the pickup of common-impedance coupled external

noise. Balanced connections and good grounding prac-

tice are the only ways to successfully reduce this

common noise problem. See Chapter 32 for a more

extensive discussion of these problems.

34.5.1.9.9 Ground Loops in Balanced Systems and

Using the Telescoping Shield Connection

By using balanced connections between two pieces of

audio equipment, the shield at the receiving end can be

lifted (disconnected) to interrupt one type of ground

loop, Fig. 34-69A. This lifted ground results in what is

known as a telescoping shield. Since, in a balanced line,

the shield does not carry audio signal, it can be discon-

nected at one end without interrupting the audio signal

(and without disrupting the effectiveness of the shield).

Unfortunately, this is not a very practical solution to the

problem in a portable audio system because it would

require special cables that have the shield disconnected

on one end (also see Chapter 32).

In Fig. 34-69B the ground loop occurs between a

microphone splitter and the two mixers. Even though

Figure 34-66. Two possible ground loops in an audio

system. Courtesy Yamaha International Corp.

A. Mixer or other device with unbalanced output.

B. Mixer or other device with stereo unbalanced outputs.

Chassis

connections

Unbalanced cable

Shield

3-wire

ac power

cable

Ground loop path, follows

shield, through device chassis and

ac ground in building

Ground loop plan

Ac ground

(part of main ac system)

3-wire

ac

power

cable

Chassis

connections Ground loop path

Shield

Shield

Ground loop path follows shield

of one cable to chassis of P2100

back through shield of other cable and through

mixer chassis.