Studio Technologies 72 - Manual (Page 5)

Model 72 User Guide Issue 2, July 2004
Studio Technologies, Inc. Page 5
Model 72
Level Meter / Interface
They display the level of audio signals
present on pins 2 and 3 of the connected
circuit. The meters are calibrated to clearly
display the signal levels found on typi-
cal IFB and intercom circuits. The meters
display the actual signal levels that are
present on the pins in dBu. (This relates to
the voltage level, in dB, as referenced to
0.775 volts RMS.) The ballistics of the me-
ter is a cross between that of a VU and a
peak (PPM) meter. (We affectionately refer
to it as a “PU” meter!) The way the LEDs
“move” in response to signals should be
comfortable for most users to observe.
Green LEDs are used to display the me-
ters’ lowest four level steps. They light
to indicate signal levels that are typically
present on a correctly functioning IFB or
intercom circuit. The LED that displays
the highest value on the meter scale
(–4 dBu) is yellow in color, indicating that
a relatively “hot” signal is present. While
not necessarily a problem, having the
yellow LED lighting steadily would typi-
cally indicate a signal level that may be of
concern.
Audio Outputs
Two transformer-balanced line-level audio
outputs are provided. One output is as-
sociated with the audio signal that may
be present on pin 2 of the connected IFB
or intercom circuit. The other output is
associated with the audio signal that is
present on pin 3 of the connected circuit.
Whether signal is available on both audio
outputs will depend on the type of IFB or
intercom circuit that is connected. The
actual level on the audio outputs is 14 dB
above its source, i.e., there is 14 dB of
input-to-output gain. This was selected so
that connecting a typical IFB or intercom
circuit that has a nominal level of –10 dBu
will result in a +4 dBu nominal level on the
audio output connectors.
Each audio output is connected to exter-
nal devices using a 3-pin male XLR-type
connector. Mating connectors (females)
should be prepared so that signal high
(+ or hot) is on pin 2 and signal low (– or
cold) is on pin 3. The cable’s shield can
be connected to pin 1, but it will have no
function. To limit the chance of ground-
ing interaction between the Model 72 and
connected equipment, the pin 1 connec-
tions on both audio output connectors are
isolated from any point in the Model 72.
The fact that pin 1 “floats” will minimize
the chance of hums, noise, or buzzes be-
ing present on the equipment connected
to the main output.
For full electrical isolation, the audio out-
puts are transformer isolated. To provide
protection against accidental connec-
tion to cables that have DC power pres-
ent, the audio outputs are also capacitor
coupled. In series with the output leads
are 300 ohm resistors, making the effec-
tive output impedance 600 ohms. These
resistors serve several purposes. They
will limit the maximum amount of output
current that can be drawn in the event of a
cable short. In addition, they allow multiple
audio outputs to be connected together,
creating a passive summing network. By
using a simple “Y” cable the two audio
outputs can be passively summed. A side
effect from using this passive summing
technique is that signal attenuation will oc-
cur. The audio quality won’t suffer, but this
passive summing method does create an
audio “pad.” If the two audio outputs are
connected together a signal attenuation
of 6 dB can be expected.