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JBUUREI
MODEL
7110
2.6
Using
Floating or
Balanced Cables
for
Longer
Puns
Shielded cables are
required
for
all
input
and
output
signals.
If
the output cable lengths are
greater
than
4.5
m
to
6
m
(15
to
20
ft),
consider
installing isolation
transformers (600
ohm:600
ohm)
at
each end
of
the cable. This allows dual-conductor shielded cable
to
be used
in
a
floating
mode,
greatly
reducing the
possibility of
radio
frequency
Interference (RFI) or
hum.
If
the
equipment
to
which
the
7110
output
is
connected has a balanced
or
floating input,
only
one
transformer
need
be
installed
.
An
accessory
output transformer
is
available
for
the
7110
which
wires
directly to
the
circuit
board
inside
the
unit.
See
Section 2.10
for details of
installation
and
wiring.
Similarly,
longer
input
cables should
be balanced
or
floating to
reduce
susceptibility to
RFI
and hum.
If
the output
of
the device feeding the
7110
Is
balanced
or
floating,
no
transformer
is
required
at
all;
simply
use a
dual-conductor shielded
cable.
If
the
input
source
is
unbalanced,
an
isolation
transformer should
be used
at
the
source
device's
output.
With
shorter cables,
particularly
where
interconnected
equipment
is
mounted
in
a
single
rack
or
is
powered
by
the
same
AC
receptacle, there
is
less
need
for
balanced
or
floating input
and
output cables.
If
the
unit
is
modified
to
incorporate the optional output transformer,
it
is
important
to
remember
that
connection
must be
made
to
both
"+"
and
terminals
of
the
output.
The
output
will
drive
any
load
of
600
ohms
or greater
and does
not require
a
resistive
termination.
If
the
7110
is
driving
a
long
line
(in
excess
of
30
meters
[100
feet])
a
620
ohm
termination
resistor
at
the other
end
of
the
line
from the
71
1
0,
in
combination
with
the
use
of
balanced
lines,
will
reduce
the
possibility of
noise pickup
in
those
lines.
2.7
Impedance and
Termination
for
600
Ohm
Lines
In
the
USA
the
early history
of
the audio
industry
is
very
closely
tied to
the
history
and
technology
of
the
telephone
industry.
Much
early
equipment used
for
public
address
systems,
recording,
broadcast
and
reproduction
of
sound
was
either
designed by
or heavily influenced
by
the
scientists
and
engineers
at Bell
Telephone
Laboratories.
The
technology
that
they
and
others
developed has had a
lasting
influence
on
the design
and
specification
of
all
audio
products.
Among
their
contributions
was
the
600
ohm
transmission
line.
The
600
ohm
line
was
developed because
of
a
need
for
a
standardized
impedance
for
long
distance transmission
lines.
Transmission
and
reception
equipment
using
vacuum
tubes,
transformers,
and
passive
equalization
and
mixing
networks
require
known
source
and
load
impedances
to
achieve
predictable
results.
Because
much
of
the
early
professional
audio
equipment
was
designed by telephone
company
people
or
used
similar
types
of
equipment,
it
is
not surprising
that
the
600
ohm
line
became
a
standard
in
the professional audio
industry.
Correct
use
of
the
600
ohm
transmission
line
requires
a
signal
source
with
an
exact
600
ohm
source impedance,
and a
receiving
device
which
also
has a 600
ohm
input
impedance.
If
a
device
does
not
have
the correct
impedance,
it
must be
modified
until
it
does.
In
the
case
of
a
device
with
a
lower than
required
source
impedance
this
would
require addition
of
a
series
buildout resistor
(or
two
in
the
case
of
a balanced
line). In
the
case
of
a
higher
impedance
input
a
resistor
across
the
input
will
suffice.
Other
variations require either
a
resistive
network
or
transformer
to
match impedances.
Page 7
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