Wheatstone SP-8, Техническое руководство

Страница: 122 / 128

Appendix Page - 1 SP-8 / Dec 95
A
PPENDIX
Appendix
BALANCED VERSUS UNBALANCED
INPUT/OUTPUT 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.
SP-8 consoles use a new type of 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
The ability to use output HIGH or
LOW permits an easy phase re-
versal of the console's output
signals, should this be desired.
CHAPTER CONTENTS
BALANCED vs UNBALANCED I/O CONNECTIONS ..................... A-1
Typical Unbalanced Connections (dwg) .................................. A-2
NOTES ON LEVEL MEASUREMENT ........................................... A-3
TROUBLESHOOTING ................................................................. A-3
Basic Procedures .................................................................. A-3
Testing a "Live" Console – Precautions ................................... A-3
Integrated Circuits ................................................................ A-4
OTHER DETAILS ........................................................................ A-5
FURNITURE LEG SET ASSEMBLY INSTRUCTIONS ..................... A-6
SP-8 / Dec 98