Studio Technologies 45A - Manual (Page 17)

Model 45A User Guide Issue 2, November 2011
Studio Technologies, Inc. Page 17
if the external source of 24 volts DC is
not present. With that in mind, it’s highly
recommended that unless the application
has been carefully designed, any time an
external source of 24 volts is going to be
connected switch 1 should be placed in
its off (down) position. This ensures that a
failure of the connected 24 volt DC source
won’t cause the Model 45A to automati-
cally revert to pulling power from the inter-
com circuit.
Some additional details might be helpful.
Be certain to allow only the connected
intercom circuit to supply power (switch 1
on) if it’s fully able to do so. Trying to draw
current from a circuit that’s not capable
of supplying the needs of the Model 45A
will typically result in greatly degraded
intercom audio quality. This is because
the Model 45A’s power supply circuit
will always attempt to draw the current it
requires. If it’s able to do so a constant
high-impedance load will be applied to the
circuit and everything will perform correct-
ly. But if the party-line intercom circuit isn’t
capable of providing the required voltage
and current the Model 45A’s circuitry will
become unstable and negatively impact
the intercom circuit’s audio quality. Howls,
squeaks, and squeals are the expected
failure mode! No damage will be done
but intercom performance will be greatly
compromised.
In conclusion, the Model 45A can function
very well using the intercom circuit as its
power source. But this requires that the
circuit can meet the voltage and current
requirements.
Model 45A Provides 2-Wire
Party-Line Power Source
Configuration DIP switch 2 is used to
select whether or not the Model 45A
will serve as an intercom power source,
providing DC power and 200 ohm termi-
nations to the intercom interface. When
switch 2 is in its off (down) position the
Model 45A will not serve as an intercom
power source. This setting is appropriate
when an external intercom power supply
is providing power and terminations to the
connected 2-wire intercom user devices.
When switch 2 is in its on (up) position
the Model 45A can provide the intercom
power source, with the requirement that
an external source of 24 volts DC is con-
nected to the Model 45A.
Figure 6. Details of front panel showing the five configuration DIP switches