Handbook for Sound Engineers

Grounding and Interfacing 1199

To the user, symptoms are indistinguishable from
many other noise coupling problems such as poor
CMRR. To quote Neil again,

Balancing is thus acquiring a tarnished reputa-

tion, which it does not deserve. This is indeed a

curious situation. Balanced line-level intercon-

nections are supposed to ensure noise-free

system performance, but often they do not.

In balanced interconnections, it occurs at line inputs
and outputs where interconnecting cables routinely have
their shields grounded at both ends. Of course, grounding
at both ends is required for unbalanced interfaces.
Fig.32-32 illustrates several examples of common
impedance coupling. When noise currents flow in signal

reference wiring or circuit board traces, tiny voltage

drops are created. These voltages can couple into the

signal path, often into very high gain circuitry,

producing hum or other noise at the output. In the first

two devices, pin 1 current is allowed to flow in internal

signal reference wiring. In the second and third devices,

power line noise current (coupled through the parasitic

capacitances in the power transformer) is also allowed

to flow in signal reference wiring to reach the

chassis/safety ground. This so-called sensitive equip-

ment will produce additional noise independent of the

pin 1 problem. For the second device, even discon-

necting its safety ground (not recommended) won’t stop

current flow through it between input and output pin 1

shield connections.

Figure 32-31. Old and new IEC tests for CMRR compared.

Generator

Generator

Generator

Device under test

Device under test

Device under test

IEC Normal-mode test

IEC Common-mode

test 1988 ed 2

IEC Common-mode

test 2000 ed 3

Meter

Meter

Meter

RT and CT are trimmed to

same reading as S2 is

toggled. This “perfect”

reading is used to calculate

CMRR.

S2 is toggled and highest

reading noted. This reading

is used to calculate CMRR.

RG

S2

CT

RG RT

RG

S2

100 7 ±1%

100 7 ±1%

Figure 32-32. How poor routing of shield currents produces the pin 1 problem.

Signal circuitry Signal circuitry Signal circuitry

Power

supply

Power

supply

Power

supply

Sig Ref Sig Ref Sig Ref

Safety ground Noise Safety ground Safety ground

voltage

Noise

voltage

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

A black dot at pin 1 indicates a direct chassis connection.