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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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