Studio Technologies 43 - Manual (Page 11)

Issue 1, July 2004 Model 43 User Guide
Page 12 Studio Technologies, Inc.
long-term reliability of the unit can be im-
pacted by the stress caused by these fault
conditions.
If there is concern that an excessive load
is being placed on the IFB circuit, perform-
ing a simple test is recommended. This
can be performed using any good-quality
digital multimeter. Begin by setting the me-
ter to measure DC current. Then place the
meter leads in series with the pin 2 lead of
the XLR-type connector. The easiest way
to measure the pin 2 current is to create
a simple adapter cable using one female
and one male 3-pin XLR-type connector.
Connect pin 1 on both connectors togeth-
er. Connect pin 3 on both connectors
together. Connect separate wires to the
pin 2 leads on both connectors. Then con-
nect the meter leads to these two wires.
The meter will indicate the DC current
being drawn while normal operation of
the connected device(s) takes place. Be
certain to connect the maximum number
of devices that might be powered by the
IFB circuit. That is, measure the worst-
case condition and ensure that the load is
within the rated 200 milliamperes output.
If possible, leaving a 10 or 20% reserve
margin is a good practice.
Technical Notes
Cable Length
There are no hard and fast rules defining
the maximum cable length possible when
connecting user devices to the Model 43’s
IFB circuit. The maximum cable length is
directly related to the amount of resistance
in the connecting cable; the lower the re-
sistance per foot (or meter), the longer the
cable can be. (Although cable capacitance
affects high-frequency performance, resis-
tance is the limiting factor in this case.) For
example, a standard 20 AWG microphone-
type cable is Belden 8412, which has 10.9
ohms resistance per conductor per 1000
feet. Since we’re using two conductors
to carry the signal (pins 1 and 2) you’d
get 21.8 ohms per 1000 feet of cable. By
knowing the cable resistance value, along
with the minimum voltage and maximum
load current required by an IFB user de-
vice, a simple “ohms law” calculation will
tell you the maximum cable length.
Let’s use the example of a Studio Tech-
nologies Model 220 Announcer’s Console
being connected to the Model 43. We’ll
select Belden 8412 as the interconnecting
cable. For correct operation, the Model
220 needs at least 24 volts DC between
pins 1 and 2 of its IFB input connector. It
has a current draw of 125 milliamperes.
The Model 43’s IFB circuit presents an
output voltage of 30 volts across pins 1
and 2 and can supply a maximum current
of 200 milliamperes. (As the Model 220’s
current draw is well within the Model 43’s
capability, this is not a limiting factor.) The
difference between the voltage supplied
by the Model 43 (30 volts) and the volt-
age required by the Model 220 (24 volts)
allows a 6 volt maximum drop over the
interconnecting cable. Using the current
draw and maximum voltage drop figures,
the maximum cable resistance can eas-
ily be calculated: 6 volts divided by 0.125
amperes equals 48 ohms. And finally,
with 8412’s 21.8 ohms (total) per 1000
feet of cable, a maximum of 2200 feet of
cable can be used and still be less than or
equal to 48 ohms. Using this example as a
guide, entering the appropriate values
will allow you to determine the maximum
cable length for your application.