Orban 674A - Page 36

In professional
systems correct
grounding is important.
The general principles
are
these:
1)
In an audio
system, the
chassis of
each
piece
of equipment
must
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 at every
point
served by mains power,
the ground
loop will have a hum
current
induced in it by stray
AC
magnetic
fields.
Because
the ground wire has
appreciable impedance,
this
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
(
op.
cit.)
.
(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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