Studio Technologies 82 - Manual (Page 12)

Issue 1, January 2000 Model 82 User Guide
Page 12 Studio Technologies, Inc.
observing the selection boxes to the left
of the AC mains connector on the back
panel. Whenever mains power is con-
nected the front panel power present LED
should light. If the LED is not lit confirm
that AC mains power is active (“hot”) and
that the cord is securely mated with the
connector on the Model 82's back panel.
For safety in the event of a major internal
failure or the connection of incorrect AC
mains voltage, the Model 82 contains a
fuse inside its cabinet. The fuse will open
(“blow”) if the failure of an internal compo-
nent causes excessive current to be drawn
from the internal power supply. The fuse
will also open should 220-240Vac be
connected to a Model 82 that is config-
ured for 100 or 120Vac operation. The
fuse is intended to be replaced only by a
competent service technician. This person
will have the training to safely access the
“guts” of the Model 82 and identify where
a problem is located.
Incorrect Output Levels
You must correctly set the input trim pots
so that the four stereo outputs can provide
precise –10dBV or +4dBu nominal output
levels. These trim pots are used to match
the nominal level of the audio input signal
with the Model 82’s internal circuitry. Refer
to the Configuration section of this guide
for detailed instructions.
Technical Notes
Definition of Level—dBu
Whenever possible, Studio Technologies
has opted to use the dBu designation as
it seems to be quite rational. Using dBm
was fine when all audio line outputs were
terminated with 600 ohm loads. In this way
it was easy to say that 0dBm is 1 milliwatt
dissipated in the known load (i.e., 0dBm
across 600 ohms will measure 0.775V). In
contemporary situations an output is rarely
terminated with 600 ohms; generally 10k
ohms or higher. The dBu designation is
better because it refers to dB referenced
to 0.775V, with no reference to load im-
pedance. This takes into account today’s
audio scene where signals have a low
source impedance, and a high input
impedance. The dBu designation is be-
coming the standard for the professional
audio industry.
–10dBV Outputs
Note that a user monitoring, by means of
a level meter (e.g., a VU meter), one of the
Model 82’s outputs might expect to see a
14dB drop when changing from the +4 to
the –10 DIP switch position. In fact, the
output level will drop by only 11.78dB.
This is because the output level is not only
switching between "+4" and "–10," but it's
changing from a dBu to a dBV reference
as well. As –10dBV is the same as
–7.78dBu, the expected 14dB drop never
occurs!