JBL 6290 - Page 12

"600
ohm
circuits"
(together
with
their
predecessors,
"500
ohm
circuits")
are
carry-overs
from
telephone
transmission
practices.
Long
audio
transmission
lines,
like
their
video
counterparts,
must
be
properly
sourced
from
and
terminated
in
equipment
which matches
their
character
i
st i c
impedance
if
optimum
frequency
response and
noise
rejection
are
to be
achieved.
However,
transmission
line
theory
and
techniques
are
not only
unnecessary
but
impractical
within
modern
recording
studios,
broad-
cast
studios
and other
local
audio
systems
where
transmission
circuits
are
seldom
more
than
several
hundred
feet
in
length. The
advent
of
negative
feedback
circuitry
and
solid-state
electronics
has
spawned
modern
audio
amplifiers
and
other
signal
processing
devices
having
source
impedances
of
only a
few
ohms.
They
are
essentially
indifferent
to
load
impedances
and, by
varying
their
output
current
inversely
to
changes
in
load
impedance,
maintain
the
same
output
voltage
into
any
load
Impedance
above a
rated
minimum,
with
no change
in frequency
response.
Modern
audio
systems,
therefore,
utilize
amplifiers
and
other
active
devices
which
have
very
low
output
impedances
and
high
(10k
to
50k
ohm)
Input
impedances.
These
products
may
thus
be
cascaded
(operated
in
series),
or
many
inputs
may be
connected
to a
single
output
of a
preceding
device,
without
regard to
impedance
matching.
Switching
and
patching is
simplified
because
double
loads
and
unterminated
bugaboos
are
essentially
eliminated.
Floating
(ungrounded)
transformer
outputs
minimize
ground
loop
problems,
and
differential
transf
ormer I ess
input
circuitry
(or
input
transf
ormers)
minimize
common
mode noise or
interference
which
may
be
induced
into
the
interconnecting
wires
or
cables.
Where
audio
must be
transmitted
through
cables
or
wire
pairs
of
more
than
several
hundred
feet
in
length,
however,
transmission
line
termination
practices
should
still
be
observed.
This
amplifier
has
input
impedances
of 40,000
ohms
when
used
in
a
balanced,
differential
input
configuration,
and 20,000
ohms
when
used
unbalanced
(one side
grounded).
This
makes
the
amplifier
suitable
for
use
with any
normally
encountered
source
impedance,
low
or
high.
Therefore,
there are
only
two
situations
which
will
require
an
input load
at the
amplifier;
1)
when
the
source
requires a
600
ohm load,
such
as a
passive
equalizer,
older
vacuum tube
equipment,
etc.
2)
when
the
source is
a
transmission
line
such as a
telephone
line.
In
some
Instances
it may
prove
beneficial
to
treat
the
input
feed to
the power
amplifier as
a
transmission
line
to
lower
its
impedance
and
its
susceptibility
to
noise
p i ckup
.
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