GENERAL
I)
424A chassis should
always
be earth-grounded
(i.e. through third wire
in power cord or through rack.)
for maximum protection
from shock.
Floaf
this ground only
as
last
resort.
Because
it
is
not always possible to determine if the
equipment
driving
or being
driven by the
424A has its circuit ground internally connected to
its chassis
ground
(which is always connected to
the ground prong
of the AC line cord, if present),
and because the use of fhe AC
power line
ground often introduces noise or other
imperfections
such as
RFI, hum,
clicks,
and
buzzes,
the
wiring
techniques in the
diagram
are not
universally applicable.
If you follow the
diagram
and
hum or
noise appears,
don't be afraid to
experiment. If
the noise sounds like
a
low-level crackling
buzz, then probably there
isn't enough grounding. Try
connecting the input
of the 424A to
a
chassis
ground terminal
on
the barrier strip
and see
if
the buzz goes away. You can also
try strapping the 424A's chassis and circuit grounds
together, and see
if
this helps.
A
ground loop usually causes a smooth,
steady
hum rather than
a
crackly buzz.
If
you
have
a
ground loop,
you can
often break
it by
disconnecting
the
jumper
between circuit
and
chassis grounds on
the
424A's rear-panel barrier strip. In either
case,
think carefully
about
what
is going on, and keep in mind the general
principle: one and only one circuit ground path should exist between
each piece
of
equipment! (Bear in mind that
the
circuit
grounds
of
the two channels
of
the
424A
dual-channel
unit are connected together internally, and
could conceivably
introduce
a
ground
loop
if
you do not
take
this connection into account in planning your
wiring.)
Wiring The 424A With
Single-Conductor Shielded Cable: Sometimes, particularly
if
you
are using the 424A
with
musical
instruments or home-type equipment,
single-
conductor shielded cable
may be the only
type
immediately available,
making
it
impractical
to
use
the
preferred two-conductor
cable. In this case,
connect the
inner conductors of
the shielded cables
to
the
"+"
sides of
the 424A inputs
and
outputs.
Connect the shield of the
424A
input cable to the input,
and connect
the shield of the
424A output
cable
to the 424A circuit ground terminal on the
rear-panel
barrier strip.
The shield
will
ordinarily receive chassis
ground from
the
external equipment
which it is connecting to the output of
the
424A.
The chassis ground/circuit ground
jumper
on the
rear barrier strip
of the
424A should
be
left in whichever
configuration gives
minimum hum or
buzz. To
minimize hum or buzz, it may be
necessary
to jumper one or
more
shields to chassis ground. Because use of
single-
conductor cables
virtually eliminates any possibility
of
carefully controlling
the
system
grounding scheme,
it
is NOT RECOMMENDED! Even so, it often does
work
adequately.
10
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