Appendix
A:
Interconnections
and
Grounding
Small systems usually
come
together
easily because cable runs are
usually
short
and
the interconnections
between various
pieces of equipment
are not terribly
complex. Therefore, do not
be
intimidated
by the seeming complexity of
the
discussion on
interconnections
and grounding below. This is more
information than
most people will ever need
to
successfully install
a small
system; we have included
it
in
case things don't work right and
you
need to find out why.
DRIVING
THE
424A
FROM
HIGH IMPEDANCE/
HIGH
LEVEL SOURCES
Both
"+"
and sides of the inputs are bypassed
to
chassis
ground
for
RF through
lOOOpF
capacitors. To assure common mode rejection, and to assure
that these
capacitors do
not
affect
the
frequency
response
of
the system,
the
output
impedance
of the equipment driving the 424A should be 600 ohms
or
less.
Most
professional and
semi-professional
sound equipment will satisfy this requirement.
The
424A
can
be driven
by
unbalanced sources of
up
to
10,000
ohms
(such
as
the
outputs of some vacuum tube preamps)
by removing
the lOOOpF capacitors
from
the
"+"
inputs, and driving these inputs from the hot side of the driving equipment's
outputs.
(See
the section
below on Grounding for an
explanation of balanced and
unbalanced connections.)
If
the
lOOOpF capacitors are left
in
place and
the source
impedance
is
I0K,
the
capacitors will
cause
a
high frequency rolloff which
is 3dB
down at
1
6kHz, and
which
rolls
off
at 6dB/octave thereafter.
The absolute clipping level
of
the
424A
input
is +20dBm.
If levels
greater than
+
20dBm are
expected,
an external loss pad must be used before the input.
The
Audio Cyclopedia
,
Section
5,
contains instructions for making
such pods. (Tremaine,
H.M.:
The
Audio Cyclopedia
,
Second Edition, Indianapolis,
Howard
W.
Sams &
Co.,
Inc.,
1969).
GROUNDING
Grounding
serves two
purposes: it joins
the
common references
of
various
pieces
of
electronic equipment,
and it shields the electronics from various electric fields
(RFI
and
hum).
(Interference
caused
by
magnetic fields
is
not
decreased by conventional shielding,
and special magnetic shielding
materials must be used for this type of problem. In
audio, such
shielding is
ordinarily used with low-level
magnetic transducers like
tape
heads, magnetic phono
cartridges, and dynamic microphones,
and
with low-level
transformers.
Line-level processors such
as
the
424A are not normally
sensitive to
this sort of interference.)
There are two types
of
ground:
circuit and chassis.
Circuit
ground
serves as a
common reference
for
the
electronics. Chassis ground permits
use of
the
chassis
as
a shield
in
the
same way that the shield
on shielded cable
protects the inner
conductors.
Whether the circuit and chassis grounds
are
identical,
are
separate, or
are
separable
depends on the
type of equipment and the interconnecting
scheme.
In
professional systems correct
grounding is important. The general principles are
these:
40
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