Handbook for Sound Engineers

Sound System Design 1305

there is only one audio cable connecting the two
devices, a second ground connection, through the ac
cables of the devices, makes the return connection and
forms a ground loop. Using a telescoping shield breaks
the ground loop and thus helps prevent pickup of
magnetically coupled and common-impedance coupled
noise. In Fig. 34-69C a ground-lift switch has been
installed so that, when opened, the ground connection
through the shield will be interrupted. Never lift the ac
ground on a power amplifier or otherwise defeat the ac
safety ground on any piece of equipment.

34.5.1.9.10 The Pin 1 Problem

Unfortunately, not all professional audio equipment is
properly grounded internally. In some equipment, noise
from the shield of a connecting cable is coupled via
common impedance into the internal signal path
through improper grounding of pin 1 of an XLR con-
nector. Noise caused by this problem will frustrate the
best efforts of an experienced installer because the prob-

lem is inside the equipment and it cannot be solved

externally. The only solution for this problem is to sub-

stitute a different piece of equipment with proper inter-

nal grounding. Chapter 32 has a more detailed

description of this problem.

34.5.1.9.11 Transformers versus Active Balanced Inputs

and Outputs

All attempts to solve hum and noise problems can be

frustrated if even one piece of system electronics has

poor rejection of hum and noise. To avoid this problem,

all system electronics should have balanced inputs and

outputs (except, of course, for the outputs of system

power amplifiers unless they are 70.7 V systems). In

addition, the internal circuit design of each piece of

audio equipment should be optimized in terms of good

grounding and shielding performance.

Figure 34-67. Noise entering a system through a ground
loop. Courtesy Yamaha International Corp.

Figure 34-68. Minimizing hum with unavoidable ground
loops. Courtesy Yamaha International Corp.

Shield carries return signal

Noise signal

enters return

circuit

Center conductor carries signal

Second return path through an ac ground

(or other path) creates “ground loop”

Electromagnetic noise source

Unbalanced feed cable

Path of ground

loop

Path of

ground loop

Unbalanced ”Y” cable

To minimize hum, pickup, keep ”Y” cable

branches physically close together.

To minimize hum, pickup, keep cable

as short as possible.

A. Unbalanced source device.

B. Unbalanced source.

Unbalanced

source

Unbalanced

source

Figure 34-69. Balanced system ground loops. Courtesy

Yamaha International Corp.

Devices with balanced inputs and outputs

3-wire ac cable

Shield connected

at one end only

A. Telescoping shield (lift the shield at the receiving end).

Balanced shielded cable

Mixer 1

Lift shield here to

break ground loop

Ground

loop

path

3 wire ac

cable

Building ac ground

Balanced

microphone

Balanced

shielded cable

Splitter

box

Mixer 2

3 wire ac cable

Do not lift

shield here

B. Avoiding a potential ground loop when using

two mixers and a microphone splitter.

Balanced portable

cable with shield intact

Devices with balanced

inputs & outputs

Ground lift

switch installed

in box or on

rack panel

Main ac ground

C. Use of a ground lift switch.