Studio Technologies 220 2004 - Manual (Page 30)

Model 220 User Guide Issue 2, August 2004
Studio Technologies, Inc. Page 31
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 IFB circuit. From what could be de-
termined, 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 well. But a
new problem arose when it came to find-
ing an external linear power supply for
Model 220 users worldwide. For locations
that are served by 100 or 120 volts, 60 Hz
a 24 volt DC linear “wall-wart” power sup-
ply was readily available. This is the power
supply that is provided with Model 220
units shipped to North America or Japan.
But as of this writing, 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 universal
input switch-mode power supply is in-
cluded with each Model 220. It’s far from
an ideal solution, leading to noise on the
IFB audio channels when long cable runs
are present. But at least users will have an
“emergency” power source if nothing else
is available.
In conclusion, users where the AC mains
source 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 220’s
current requirement is 80 milliamperes,
a typical medical-grade power supply
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 5.0 mm coaxial jack is used for
the positive (+) connection.
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