Manual

Model 45A User Guide Issue 3, August 2015
Studio Technologies, Inc. Page 17
It’s important to remember that if an external
source of 24 volts DC is connected it will
always be used as the Model 45A’s power
source. Even if switch 1 is in its on (up)
position, power will be drawn from the con-
nected intercom circuit only 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 automatically revert to pulling power
from the intercom circuit.
Some additional details might be helpful.
Be certain to allow only the connected inter-
com 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 supply-
ing 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 correctly. 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 func-
tion 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 7. 2-wire (PL) power source switch
settings
Auto Null Button Operation
A pushbutton switch, located on the
Model 45A’s front panel, is used to acti-
vate the auto nulling functions. A confi
gu-
ration parameter allows selection of the
button’s operation. The choices are dual
auto null mode and independent auto
null mode. When dual auto null mode is
selected a single “tap” (press and release)
on the pushbutton switch initiates a rou-
tine that begins with channel 1’s auto
null sequence, followed immediately by
channel 2’s auto null sequence.