16
2120 Audio System Upgrade,
Continued
Class B Risers
In a single channel Class B system, each 2120 amplifier supervises the riser. A monitor point local
to the 2120 amplifier reports a loss of riser signal.
Two channel Class B transponders are subdivided into Type 1 and Type 2 systems (see MUX2-11-
004). Type 1 systems supervise the TALK channel at each 2120 amplifier and the EVAC channel
at an Audio Riser Monitor board (assembly 617-278), with associated monitor and control points
located in the last audio transponder in the system. Type 2 systems supervise the risers at each
2120 amplifier.
Class B audio risers that do not utilize the Audio Riser Monitor Board (single channel and the
EVAC channel of Type 1 dual channel systems) are galvanically isolated from Earth ground and
so do not report earth faults. To meet current UL requirements, these type of risers will connect
the B-line of the Riser NAC to the 0V shield line, allowing earth faults to be reported by the
System Power Supply.
Figure 10. Two Channel Audio System with Class B Riser
Backup Riser
Amplifiers
When the system is configured for backup amplifiers, the Flex amplifier will be configured for 1:1
backup. Two channel audio systems with backup will require two Flex amplifiers.
Notes:
1. Audio Riser maximum distance 10,000 ft with #18 AWG (minimum).
2. Maximum voltage 25Vrms, maximum short circuit current 2.25A (over-current protected).
3. Audio wiring must use twisted shielded wiring when run in the same conduit as other circuits.
4. Install 733-973 1K Resistor between shield terminal on Flex35 and the B-audio line to ensure
correct operation of the earth detect circuit.
Continued on next page
B+
B-
SHLD
Flex-35 25V Amplifier
Riser NACs
1
K
*
B+
B-
1
K
*
SHLD
+
-
+
-
EVAC RISER
TALK RISER
2120 Remote
Amplifier
Shield
*Note 4
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Summary of Contents for 4100-6065
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