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2.6
IMPEDANCE AND TERMINATION
Audio engineering
had its
roots
in
the
telephone industry,
and
"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 characteristic 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,
broadcast 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)
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, patching,
etc.
is
simplified because "double loads"
and
"unterminated" bugaboos
are
essentially eliminated.
"Floating"
(ungrounded) transformer outputs minimize ground loop problems,
and
differential transformerless input circuitry
(or
input transformers)
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.
The Model 535 has input impedances of
40
kohms (40,000
ohms)
when used
in
a
balanced, differential input configuration,
and
20
kohms (20,000 ohms) when used unbalanced
(one side
grounded).
This
makes
the 535
suitable
for use
with
any
nominal source impedance,
low or high.
Only when
it
is
used from
a
source which requires
a
low
impedance termination
(such
as
a
600
-ohm transmission line
or
older vacuum -tube equipment) will
a
source termination resistor
be
required
at
the 535 input.
-7-
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