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PICTORIAL VIEW OF
BALANCED (3-CONDUCTOR) SHIELDED CABLE
NOTE THAT EQUAL NOISE VOLTAGE
IN BOTH INNER CONDUCTORS
CANCELS HERE, BUT SIGNAL
IS OPPOSITE POLARITY AND
DOES NOT CANCEL.
(4)
ELECTROSTATIC
OR RADIO NOISE
(+)
NT
—
SIGNAL
GE
NOISE VOLTA
SIGNAL (-.)
—
(-)
"7
SCHEMATIC VIEW OF BALANCED
(3-CONDUCTOR! SHIELDED CABLE
TRANSFORMER
(WORKS SIMILAR TO BALANCED,
DIFFERENTIAL INPUT AMP)
Fig. 8 NOISE REJECTION IN A BALANCED
LINE
LLL
———————
—Á———ML
a atii
—
|)
The
ВЕ!
(radio frequency
interference)
cuts across both
conductors, inducing equal voltages in the same direction.
These
voltages "meet"
in the transformer
(or differential
amplifier), and cancel out, while the signals generated by the
microphone
flow in opposite directions in each conductor,
and hence do not cancel out. Thus, in a theoretically perfect
balanced
system, only the desired signal gets through the
transformer or differential amplifier.
Why input transformers sometimes are used
There are a number of reasons why input transformers might
be used, We'll discuss just a few of them.
In the case of
certain audio equipment which has an unbalanced input (not
these
amplifiers),
a transformer
converts
the unbalanced
input to a balanced
input. When the transformer is used in
this way, primarily for ground isolation and to obtain the
benefits of a balanced line, it is said to be an "isolation"
transformer or a "matching" transformer. If the transformer
is also used to prevent a low impedance input from overload-
ing a high impedance output, it is known as a "bridging"
transformer (not to be confused with the "bridged" connec-
tions of a stereo power amp output in MONO mode).
Since
they already have balanced
inputs, these amplifiers
have no need for input transformers,
There
are instances
where absolute isolation of the grounds between. the power
amp and the other equipment must be obtained, and in such
cases, there is no viable substitute for a transformer; in such
cases, the commercial versions of these amplifiers ("С" suffix)
may
be
used
since
they
transformers. Alternately, an external input transformer may
be used, such as one of the in-line types that fit in a case not
much larger than an XLR connector.
NOTE:
There are other ways to achieve isolation. One can
digitize the audio signal and transmit it by means of modulat-
ed light in fiberoptics, but this is much more expensive than
include
sockets
for
optional.
using a transformer, with no great performance
advantage.
`
One can use the audio signal to modulate a light, and pick up
the
light with
an
LDR
(light dependent
resistor), thus
achieving
isolation at the expense
of increased
noise and
distortion.
There are also instances where
they is an extremely
large
amount of radiated noise energy near the input cables to the
amplifier
—
for example
near
an SCR-operated
lighting
dimmer panel. In such cases, the common mode rejection of
the
balanced
differential
amplifier
input
may
not
be
adequate to cancel out the very high peak noise voltages, and
only a good quality
input transformer can do the job. In
most
of the applications
for which
these
amplifiers
are
intended,
you
don't
need
a transformer,
and you're not
paying for К.
10
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