Orban 424A Operating Manual page 45

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.)
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