Studio Technologies M233-01151 - Manual (Page 38)

Issue 5, November 2014 Model 233 User Guide
Page 38 Studio Technologies, Inc.
Phantom Power
The Model 233 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: 6.85 k ohm, 1%-tolerance
resistors provide a path from a 45 volt DC
source to pins 2 and 3 of the microphone
input connector. The resistors and the
power source work together to provide
the required 48 ±4 volts, up to a maximum
current of 10 milliamperes.
Symptoms of Insufficient
Power
A core part of the Model 233’s internal cir-
cuitry is a switch-mode power supply that
produces +45 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. “Suf-
ficient” is defined as a minimum of 24 volts
on the intercom input and 20 volts on the
external 24 volt DC input. The necessary
current, 125 milliamperes for the intercom
line and 90 milliamperes for the external
source, must be supplied over their respec-
tive voltage ranges.
It’s worth discussing what will happen if
any of these power sources fall below their
specified minimum. Typically, if the Model
233 is being powered by an external 24
volt nominal power source, normal opera-
tion will continue until the input falls to the
18-20 volt range. As the input voltage drops
below this range the Model 233’s internal
power supply will have reduced stability,
operating in this manner until its low-volt-
age shutdown circuit halts operation. Note
that as the input voltage moves down from
24 volts the input current will rise propor-
tionately to make up for the loss of power.
Using the intercom line to provide Model
233 power shouldn’t prove to be a prob-
lem. Power supplies associated with
broadcast and production intercom
systems are designed to support multiple
beltpack and related devices. In the “big
scheme of things,” connecting a Model
233 shouldn’t add a significant load.
LED Colors
As previously described, two LED indica-
tors are associated with the main output
and are located directly above the main
output pushbutton switch. The red LED,
located on the left, is lit whenever the
main output is muted. The green LED,
located on the right, is lit whenever the
main output is active. The thought pro-
cess behind the color choices was that
red would relate to the main output being
muted (“stop”) while green would relate
to the main output being active (“go”). It’s
possible that these color choices may not
meet the needs of all users and applica-
tions. For example, it’s reported that one
European broadcaster typically uses these
two colors in the opposite fashion. Their
choice is to have the red LED lit whenever
the main output is active, warning the tal-
ent that they are “on-air.” The green LED
is lit whenever the main output is muted,
indicating to the talent that it’s “safe” to
say whatever they wish, about whomever
they wish to say it about!
For consistency, the LEDs associated
with the talkback buttons were selected
to be green. They light whenever their
associated talkback functions are active.
It’s possible that some applications may
benefit from revising these LED colors too.
While red is certainly one possible choice,
other colors are also a possibility, includ-
ing amber, orange, blue, or white—these
days there are lots of choices available.