Studio Technologies 5120 2014 - Manual (Page 12)

Issue 5, October 2014 Model 5120 User Guide
Page 12 Studio Technologies, Inc.
Model 5120
Line/IFB Output Module
The LED lights steadily when the IFB’s
DC output is within its normal range. Dur-
ing normal operation the DC level on pin 2
of the IFB output will be approximately 28
volts. The LED will begin to flash on and
off if the level falls below approximately 24
volts DC. This will occur when the current
draw is greater than nominally 120 milliam-
peres. If the fault lasts for more than one
second the LED will flash at a faster rate.
In addition, the DC output will shut down
for a 5-second period.
There’s really only one piece of advice
when it comes to understanding how to
use the status LED: if it flashes there’s a
problem that must be corrected! The most
likely cause will be too many user devices
being connected to the IFB output connec-
tor. It’s also possible that a wiring problem
could cause a partial or full short circuit
between pin 1 (common) and pin 2 (power
with channel one audio). Troubleshooting
should prove quick and easy. Begin by dis-
connecting the IFB user devices. Observe
the status LED and see if the problem
has gone away. If not, review the intercon-
necting cables and find the fault condition.
Within five seconds of the problem being
“cleared” the status LED will stop flashing.
Cable Length
There are no “hard and fast” rules defining
the maximum cable length possible when
connecting user devices to the Model
5120’s IFB output. The maximum cable
length is directly related to the amount of
resistance in the connecting cable; the low-
er the resistance per foot (or meter), the
longer the cable can be (although cable
capacitance affects high-frequency perfor-
mance, resistance is the limiting factor in
most cases). For example, a contemporary
microphone cable is Belden 1172A which
has 18 ohms resistance per conductor pair
per 1000 feet. Since we’re using 2-conduc-
tor pairs to carry the signal (pins 1 and 2)
you’d get 36 ohms per 1000 feet of cable.
By knowing the cable resistance value,
along with the minimum voltage and maxi-
mum load current required by an IFB user
device, a simple “ohms law” calculation will
tell you the maximum cable length.
Let’s use the example of a Studio Tech-
nologies Model 32A Talent Amplifier being
connected to a Model 5120 IFB circuit.
We’ll select Belden 1172A as the inter-
connecting cable. For correct operation,
the Model 32A needs at least 24 volts
DC between pins 1 and 2 of its IFB input
connector. It has a current draw of 35 mil-
liamperes. The Model 5120’s IFB circuit
presents an output voltage of 28 volts
across pins 1 and 2 and can supply a
maximum current of 120 milliamperes. (As
the Model 32A’s current draw is well within
the Model 5120’s capability, this is not a
limiting factor.) The difference between
the voltage supplied by the Model 5120
(28 volts) and the voltage required by the
Model 32A (24 volts) allows a 4 volt maxi-
mum drop over the interconnecting cable.
Using the current draw and maximum
voltage drop figures, the maximum cable
resistance can easily be calculated: 4 volts
divided by 0.035 amperes equals 114
ohms. And finally, with 1172A’s 36 ohms
(total) per 1000 feet of cable, a maximum
of approximately 3100 feet of cable can
be used and still be less than or equal
to 114 ohms. Using this example as a
guide, entering the appropriate values will
allow you to determine the maximum cable
length for your application.