Model 42A User Guide
Issue 2, December 2013
Studio Technologies, Inc.
Page 17
reduce the signal-to-noise ratio (raising the
perceived noise floor) and can prevent the
connected user devices from operating op-
timally. Applying signal levels significantly
higher than +4 dBu will reduce the head-
room and greatly increase the chance of
reaching audio “clipping.” Obviously, these
cautions are not unique to the Model 42A,
but apply to most audio equipment. The
front-panel level meters provide an easy
means of confirming that the Model 42A
is being presented with the correct audio
levels.
To confirm correct IFB output operation at
locations away from where the Model 42A
is installed, it’s possible to use the Model
72 Level Meter/Interface, also available
from Studio Technologies. The Model 72
is a compact, portable device that plugs
directly into IFB or intercom circuits
and provides two useful functions. Two
5-segment LED meters display the audio
levels present on pins 2 and 3 of the con-
nected circuit. In addition, two “dry” line-
level audio outputs are provided. Complete
information on the Model 72 is available
on the Studio Technologies website.
Maintaining Correct IFB
Output Current Draw
Each of the four IFB outputs is designed
to provide up to 220 milliamperes of DC
current. By design, the IFB outputs are
protected so that an overload condition,
or even a complete short circuit, should
not cause damage. Exceeding 220 mil-
liamperes for more than a few seconds will
typically cause the auto shut-down mode
to become active. A continuous overload
condition will cause the IFB output volt-
age to cycle through a 4-seconds-on/10-
seconds-off sequence. Restoring the IFB
output load to be within the maximum 220
milliamperes will allow the output to again
operate normally. In extreme cases, such
as when the Model 42A is located in an
environment with elevated temperatures,
a few minutes may be required from the
time an overload condition is removed
to when normal operation will again take
place. Please don’t test the Model 42A’s
ability to sustain frequent overload or
short-circuit conditions! The long-term reli-
ability of the unit can be impacted by the
stress caused by these fault conditions.
The four status LEDs make it simple to
know if an excessive load, or a short cir-
cuit, is being placed on one or more of the
IFB outputs. The LEDs provide a direct in-
dication of the IFB outputs’ DC output volt-
age. Each output voltage is directly related
to the amount of current being drawn, as
well as indicating when its IFB output cir-
cuit has entered its protection mode and
has essentially shut down. During normal
operation the DC level on pin 2 of the con-
nectors associated with each IFB output
circuit will range from approximately 28 to
30 volts. An LED will begin to flash on and
off if the level falls below approximately 24
volts DC. This will occur when the current
draw is greater than approximately 220
milliamperes. As previously discussed, if
the fault lasts for more than four seconds
the LED will flash at a faster rate. In addi-
tion, the DC output will shut down for
a 10-second period.
There’s really only one piece of advice
when it comes to understanding how
to use the under-voltage status LEDs:
if they’re flashing there’s a problem that
must be corrected! The most likely cause
will be a wiring fault that creates a partial
or full short circuit between pin 1 (com-
mon) and pin 2 (power with audio) on
