Wheatstone TV-80, Техническое руководство

Страница: 98 / 120

A P P E N D I C E S
TV80 / Sep 98
Balanced vs. Unbalanced Connections
By now everyone knows (or should know) that balanced inputs and
outputs are highly desirable - they have an intrinsic ability to reject
hum, noise, crosstalk, and RF, even if the shielding and grounding
leave something to be desired. Telephone companies routinely pack
hundreds of balanced lines into one cable, with no shielding, next to AC
power lines and street lights, and if good balance is maintained, the
individual circuits are completely free of noise and crosstalk.
Not all equipment used in stations is balanced, however, and the
most cost-effective devices often don’t have +4 dBu output levels,
either. Because of these realities, all Wheatstone consoles are designed
to accept balanced or unbalanced sources with levels as low as -10 dBu.
Connecting unbalanced inputs is simple—wire to the console with
typical shielded two conductor cable (like Belden 9451), just as if you
were connecting a balanced source. At the unbalanced machine’s
output, connect the black wire (LOW) to the shield. This “pseudo-
balanced” connection has proven to be the simplest and most trouble-
free way to go. Another plus is that the wiring need not be changed out
if a balanced output machine is subsequently installed in that position.
If the machine has a -10 dBu output, don’t hesitate to turn your input
trimmers as high as is needed.
TV-80 consoles use a balanced output circuit which behaves
exactly like the secondary of a high-quality transformer, with no center
tap—this output is both balanced and floating. Either the HIGH or
LOW side of the output should be strapped to ground, with the output
taken from the other side. (Normally you’d strap LOW to ground, and
take HIGH to feed your unbalanced equipment.)
This type of self-compensating active-balanced output has been
tried before, but it required costly hand-matching of resistors to
maintain stability and low distortion. The 2142 balanced line driver IC
uses laser-trimming of the on-chip resistors, under computer control,
to achieve the desired results. A major advantage over the discrete
component designs is the ability to replace the IC without the need for
hand-picking resistors to restore the performance of the circuit.
The ability to use output HIGH
or LOW permits an easy phase
reversal of the console's out-
put signals, should this be de-
sired.
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