Studio Technologies 770 - Manual (Page 39)

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 39
Model 770
the most likely place to find nonstandard
plugs—we even found them on some
“world-class” headphones that we use in
our lab!
Definition of Level
Studio Technologies has opted to use the
dBu designation as it seems to be quite
rational. Using dBm was fine when all
audio outputs were terminated with 600
ohm loads. In this way it was easy to say
that “0dB” was 1 milliwatt dissipated in the
known load (i.e., 0dBm across 600 ohms
will measure 0.775V). Today, 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 load impedance not a
factor. This takes into account the contem-
porary audio scene where most equip-
ment has a low output source impedance,
and a high input impedance.
Faux VU Meter
In the spirit of George Washington (and to
make our mothers proud of us) we have
to “tell the truth” and admit that while the
meter selected for the Model 770 is of very
good quality, it doesn’t meet the specifica-
tions of a “true” VU meter. So why was a
less-than-perfect meter selected for the
Model 770? The answer is easy and not
surprising—bucks! While the Model 770’s
existing meter is already quite expensive,
being built in England by a top-notch
meter manufacturer, the cost for a “true”
VU meter is about five times as much!
The technical difference between the
meter used on the Model 770 and a “true”
VU meter lies in the ballistic characteris-
tics, i.e. how the pointer accelerates and
decelerates in response to audio signals.
The Model 770’s meter pointer moves very
close to, but not exactly, in the manner
required to be “perfect.”
For the Model 770 operator, the selected
meter provides a completely acceptable
indication of level. But as a technically
oriented company, we wanted to “come
clean” about our actions. Too bad so
many other audio companies don’t tell you
that their “VU meters” are fakes—many
don’t even come close to meeting the
specs!
VU Meter Calibration
From the factory the VU meters are cali-
brated so that a +4dBu signal level, as
measured on main output 1, results in a
reading of 0 VU on the meter. However,
it may be observed that the meter may no
longer be precisely calibrated when the
monitor source button is set to something
other than main. A difference of up to
0.5dB from the calibrated value could be
expected. This is due to tolerance varia-
tions in the components of the monitor
circuitry. In reality, only when steady tones
are present on the monitor sources will
this variation become apparent. During
actual operation it’s unlikely to become
a factor.
Model 32 and 33 Output Level
By design, the output level on the Model
32 and Model 33 Talent Amplifiers cannot
be set to fully “off.” While the output level
can be substantially attenuated, it never
can be set for full attenuation. This was
to ensure that talent personnel could never
accidentally be fully “isolated” from their
program or IFB source. There may be
special cases where full attenuation is
desired. This might be especially true with
the Model 33 Talent Amplifier, where a mix
of the two IFB channels can be achieved.