1)
In
an audio system, the chassis
of
each piece
of
equipment
should
be
connected to a
good common ground point (ideally a cold water
pipe
or a rod
driven
into the
earth)
by one
and
only one
wire.
2)
Meanwhile,
there
must
be one
and
only
one circuit
ground path
between each
piece of
equipment.
It is when these
two
requirements become
confused, omitted,
or redundant that
problems
develop.
If
there is a
connection
missing, hum and noise will result. If
more
than
one
ground
path exists,
then a "ground
loop"
may
develop.
A
ground
loop
can
be
viewed as a single
turn
of
a giant
transformer.
Because
60Hz AC
magnetic fields exist in
every area
served by
mains
power,
a
ground loop
will have
a hum
current
induced in it by
stray
AC magnetic fields.
Because
a
ground wire
has appreciable impedance,
this induced current
will cause
a hum
voltage to
appear between different
parts
of
the ground
system.
If
great care is
not
taken,
this
hum voltage
can
intrude
on
the
audio signal.
How grounding is
accomplished
depends on whether the
equipment to be
interconnected is
balanced or unbalanced.
An unbalanced
connection
uses
two terminals:
"hot" and ground. Wires used
in such
connections
are typically
single-conductor
shielded.
(RCA plugs
and two-conductor
phone plugs are
often
used to terminate such
cables.)
If
because
of
stray fields
or
ground loops, a hum
voltage
appears between
"hot"
and ground, then this hum will
be mixed
into the
desired signal since the unbalanced
connection cannot distinguish
between the
desired
signal and hum.
In the case of
balanced connections,
audio is applied to the
"+"
and
terminals;
the input responds
to
the difference between the
voltages at the two terminals.
A
third
terminal is
connected to
chassis ground
and
is
available
for the connection
of
the
shield of
the
two-conductor shielded
wire
that
would be used (Belden
8451,
for
example).
If
a
hum
voltage is developed
between the shield and
both
audio wires,
then the
balanced input would
reject
this
"common mode" voltage, since the input
responds
only to
the difference in voltage
between the audio wires. This ability to
reject hum
and noise is the primary advantage
of
a
balanced configuration.
Referring back
to
the
ELECTRICAL INSTALLATION section,
notice how these
rules
are applied
in
the table and
diagram.
For involved systems such
as
arena-type sound reinforcement, professional
recording studios,
or
large broadcasting
facilities,
a
formal and
systematic
"transmission
ground system"
should be worked out for the
entire system.
See
Section
24 of
The Audio
Cyclopedia for details (mentioned earlier).
(Interesting digression:
The
"balanced" technique
was
first
perfected by the
telephone
company,
which has
to
run
miles
of
unshielded cables close to each other
and
also to high voltage
AC
lines without
pickup
of
excessive hum
or
crosstalk
from other
circuits. Originally,
telephone
circuits were unbalanced on single wires,
with the
earth
providing the ground
return.
As soon as
electric lighting
became
popular and power
lines
were placed on telephone
poles,
the power
lines
interfered
with the
telephone
service
so
badly
as
to render
conversation
impossible.
The
telephone
company embarked on
a
research program
which led to
both the
balanced
line
technique, and to
the choice
of
the familiar
600 ohm
impedance as
the
optimum
compromise
between rejection
of
electrical and
magnetic
interference.
Today, the telephone company's
specification for maintenance
of line
balance is
extremely tight, as proper
common
mode
rejection
is
vital
to
the
success
of
the
entire system.)
41
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