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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.
Most new designs
(all at
UREI)
have high input impedance
to
allow use with various
source impedances.
Most modern audio systems, therefore, utilize amplifiers
and
other active devices which have very low output impedances
and
high
(lOK to 50K)
input impedances.
These products may thus be cascaded
(operated in tandem), or
many inputs may be connected to
a
single
output of
a
proceeding 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, or produced by different
ground potentials
.
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 1178 has an input impedances of 20,000 ohms
when
used in
a
balanced, differential input configuration, and 10,000
ohms
when
used unbalanced (one side grounded).
This makes the
device suitable
for
use
with
any normal 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)
is a
source termination resistor required
at
the 1178 input.
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