Auto Null Button Operation

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 45 to automatically revert to pulling power from the intercom 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 45 will typically result in greatly degraded intercom audio quality. This is because the Model 45’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 45’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 45 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 45 Provides 2-Wire Party-Line Power Source

Configuration DIP switch 2 is used to se- lect whether or not the Model 45 will serve as an intercom power source, providing DC power and 200 ohm terminations to the intercom interface. When switch 2 is

in its off (down) position the Model 45 will not serve as an intercom power source. This setting is appropriate when an exter- nal 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 45 can provide the intercom power source, with the requirement that an external source of 24 volts DC is connected to the Model 45.

A pushbutton switch, located on the Model 45’s front panel, is used to activate the auto nulling functions. A configuration 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 routine that begins with channel 1’s auto null sequence, followed immediately by channel 2’s auto null sequence.

The independent auto null mode allows the auto null function to be initiated for each channel as desired. A single “tap” will start the auto null routine for channel 1; two “taps” will start the routine for channel 2.

When configuration DIP switch 3 is in its off (down) position the dual auto null mode is selected. This is provided spe- cifically for cases where the Model 45’s

2-wire party-line interface will be used with dual-channel intercom user devices. As an example: the RTS TW-series provides two independent audio channels, as well as power, over a single 3-conductor cable. In a case such as this it’s useful for both of the hybrid circuits associated with an

Model 45 User Guide	Issue Preliminary 2, March 2009
Studio Technologies, Inc.	Page 17