Studio Technologies 200 - Manual (Page 18)

Model 200 User Guide Issue 3, August 2004
Studio Technologies, Inc. Page 19
noticeable in the negative lead of their DC
output, fed right into the common lead of
the IFB circuit. From what could be deter-
mined, noise current would travel from
the power supply, through IFB input pin 1,
and on to the IFB circuit source’s ground
connection. Making the problem more
insidious was the fact that only when the
IFB circuit’s interconnecting cable was
sufficiently long did the problem become
noticeable. In the “lab” where 10-foot long
test cables were used, the audio from the
IFB circuit was extremely quiet. But testing
with 500 or 1000 feet of interconnecting
cable resulted in an annoying “buzz” on
the IFB 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
the IFB circuit, everything worked fine.
But a new problem arose when it came
to finding an external linear power sup-
ply for Model 200 users worldwide. For
locations that are served by 100 or 120
volts, 60 Hz a 24 volt DC linear “wall-wart”
power supply was readily available. This
is the power supply that is provided with
the Model 200 when it is shipped to North
America or Japan. But as of this writ-
ing, no “perfect” 24 volt DC linear power
source has been located that would serve
220/240 volts, 50 Hz applications. So a
compromise had to be made. For these
applications a small universal input switch-
mode power supply is included with each
Model 200. It’s far from an ideal solution,
leading to noise on the IFB audio chan-
nels when long cable runs are present.
But at least users will have an “emergen-
cy” power source if nothing else is
available.
In conclusion, users where the AC mains
power is 220/240 volts should consider
locating an alternate 24 volt DC power
source. Several options are available that
could make the task simple. It’s expected
that a more sophisticated “medical-grade”
switch-mode power supply will have much
better control of induced noise. It’s highly
likely that using one of these supplies will
provide good results. As the Model 200’s
current requirement is only 50 milliam-
peres, a typical medical-grade power sup-
ply should be able to power multiple units.
Another solution would be to obtain two
of the more commonly available 12 volt
DC linear power supplies, connecting their
outputs in “series” to create a 24 volt DC
source. While not a glamorous solution,
it is technically correct and should prove
cost effective. If this arrangement is imple-
mented remember that the center pin of
the 2.1 x 5.5 mm coaxial jack is used for
the positive (+) connection.
Symptoms of Insufficient
Power
A core part of the Model 200’s internal cir-
cuitry is a switch-mode power supply that
produces +48 volts, +12 volts, and –12
volts. This power supply circuit works very
well as long as it is “fed” with sufficient in-
put voltage and current. “Sufficient” is de-
fined as a minimum of 24 volts on the IFB
input and 20 volts on the external 24 volt
DC input. The IFB input must be capable
of supplying 95 milliamperes of current
over its entire voltage range. The external