Orban 622B - Appendix: Interconnections and Grounding; Driving the 622 from High Impedance; High Level Sources

Appendix:
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 622
Both
"+"
and
sides of
the
622 inputs
are bypassed to
chassis
ground for
RF
FROM
HIGH
IMPEDANCE/
through
lOOOpF capacitors.
To assure
common
mode rejection,
and to assure
that
LEVEiL.
SOURCES
these
capacitors do
not
affect the
frequency
response of the
system,
the output
impedance
of the
equipment
driving
the
622 should be
600 ohms or less.
Most
professional
and
semi-professional sound
equipment
will satisfy
fh is
requirement.
The 622
can be
driven by
unbalanced
sources 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
622
input
is
+26dBm.
If
such
clipping
occurs, it
will cause
the
OVERLOAD lamp to
flash on
and
off
regardless of
the setting of
the
GAIN
control.
If levels greater
than
+26dBm are
expected, an
external loss pad
must be used
before
the 622 input.
The Audio
Cyclopedia
,
Section
5,
contains
instructions
for
making such
pads.
(Tremaine,
H.M.:
The
Audio Cyclopedia
,
Second
Edition,
Indianapolis,
Howard W.
Sams & Co.,
Inc., 1969).
GROUNDING
Grounding
serves
two purposes:
it joins
the
ground
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
where hum is a
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
equalizers such as
the
622 are not
normally
sensitive to
this
sort
of interference.)
There
are
two types
of
ground: circuit
and
chassis.
Circuit ground
serves
as
a
ground 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
intentionally
joined
depends
on the type
of
equipment
and the
interconnecting
scheme.
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