Studio Technologies 220 2008 - Manual (Page 31)

Model 220 User Guide Issue 4, October 2008
Studio Technologies, Inc. Page 31
were fine, being reasonably close to the
desired –10 dBu. But some were much too
low, while others were much too “hot.” We
observed one unfortunate baseball “color”
commentator being sent interrupt audio
signals so “hot” relative to program audio
as to almost make his ears bleed! This
situation should not have been allowed
to happen.
In defense of field technical personnel,
measuring the audio level of an IFB circuit
hasn’t traditionally been an easy proposi-
tion. But that situation has now changed.
After experiencing this condition in the
“field,” Studio Technologies’ engineers
were motivated to design the Model 72
Level Meter/Interface. This compact
device plugs directly into IFB and intercom
circuits and provides two useful functions:
level meters and “dry” audio outputs. Two
5-segment LED meters allow direct obser-
vation of the audio signal levels present on
IFB or intercom circuits. The display range
is optimized for the signal levels found on
these circuits, rather than traditional “VU”
scaling.
The Model 72 also provides two
transformer-coupled “dry” audio outputs,
one for each IFB or intercom channel.
These outputs are useful for a variety of
production and testing applications. For
example, the outputs can serve as the
interface between a traditional “wet” IFB
system and a wireless in-ear monitor sys-
tem. The outputs can also be connected
to a monitor panel, allowing visual and
aural monitoring of the IFB audio signals.
In conclusion, we’re sorry for this shame-
less promotion of the Model 72 Level
Meter/Interface! But necessity was defi-
nitely the “mother” when it came to the
unit’s invention. Working “in the field”
without such a device, we felt “blind” when
connecting to IFB circuits. That no longer
has to be the case and we think that you’ll
find owning one a very worthwhile invest-
ment. For further information please refer
to the Studio Technologies website.
Phantom Power
The Model 220 provides a 48 volt nominal
source of phantom power to support con-
denser-type microphones. It’s designed
to meet the P48 requirements as specified
in the IEC 61938 standard. The circuitry is
very simple: two 6.81 k ohm resistors pro-
vide a path from a 48 volt source to pins 2
and 3 of the microphone input connector.
The resistors and the power source work
together to provide 48 ±4 volts, up to a
maximum current of 10 milliamperes.
Symptoms of Insufficient
Power
A core part of the Model 220’s internal
circuitry is a switch-mode power supply
that produces +48 volts, +12 volts, +5
volts, and –12 volts. This power supply
circuit works very well as long as it is “fed”
with sufficient input voltage and current.
“Sufficient” is defined as a minimum of 24
volts on the IFB input and 20 volts on the
external 24 volt DC input. The necessary
current, 125 milliamperes for the IFB input
and 80 milliamperes for the external input,
must be supplied over their respective
voltage ranges.
It’s worth discussing what will happen
if either power source falls below its speci-
fied minimum. Typically, if the Model 220
is being powered by an external 24 volt
nominal power source, normal operation
will continue until the input falls to the
18-20 volt range. As the input voltage
drops below this range the Model 220’s
internal power supply will have reduced