Model 233 User Guide Issue 1, September 2007
Studio Technologies, Inc. Page 37
The power supplied by an IFB circuit,
normally in the range of 28 to 32 volts DC,
is usually sufficient to operate the Model
233’s circuitry. The acceptable input range
is 24 to 32 volts, with a required current
of 125 milliamperes. Note that the speci-
fied input voltage is given when measured
directly at the Model 233’s IFB input con-
nector, not at the source of the IFB circuit.
In North American field and in-studio
broadcast applications it is common to
find RTS 4000-series IFB equipment be-
ing used to provide the IFB circuits. The
Model 233 can be directly connected to,
and function correctly with, one of these
circuits. For reliable operation, especially
when using lengthy cable runs, it’s strong-
ly recommended that no other device be
connected to a 4000-series IFB circuit that
is specified for connection to a Model 233.
This requirement is due to the current-
limited DC source that is supplied by the
4010 IFB Controller.
Phantom Power
The Model 233 provides a 48 volt nominal
source of “phantom power” to support
condenser-type microphones. It’s de-
signed to meet the P48 requirements as
specified in the IEC 61938 standard. The
circuitry is very simple: two 6.81 k ohm,
1%-tolerance resistors provide a path from
a 48 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.
External Power Sources
As has been previously discussed, an
external source of 24 volt DC can be used
to power the Model 233. While develop-
ing the 200-series of announcer console
products, an interesting phenomenon was
discovered regarding acceptable sources.
To meet worldwide requirements, using
a compact switch-mode “universal input”
power supply seemed to be an excellent
solution. Supplying one of these with each
Model 233 would have allowed operation
anywhere in the world. Whether connect-
ing to 100 volts, 60 Hz in Japan or 240
volts, 50 Hz in Australia, all would be well.
Unfortunately, things did not work out as
planned! It turned out that all of the com-
pact switch-mode power supplies that
were tested induced a great deal of noise
into their DC output. This noise, especially
noticeable in the negative lead of their DC
output, fed right into the common lead of
the intercom circuit. From what could be
determined, noise current would travel
from the power supply, through intercom
input pin 1, and on to the intercom line’s
source’s ground connection. Making the
problem more insidious was the fact that
only when the intercom line’s intercon-
necting cable was sufficiently long did
the problem become noticeable. In the
“lab” where 10-foot long test cables were
used, the audio from the intercom line was
extremely quiet. But testing with 500 or
1000 feet of interconnecting cable resulted
in an annoying “buzz” on the intercom
audio channels.
Many hours were spent trying to eliminate
this problem. But, unfortunately, no solu-
tion was found. It was a humbling expe-
rience that only a few visits to the local
tavern made us feel better about.
The solution turned out to be very simple,
but not without other ramifications. By
changing to a low-cost transformer-based
(“linear”) 24 volt DC power supply the
problem disappeared. With no high-
frequency switching noise to get into