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MPA Series User Manual v1.1 9

ideally by the people who will normally make them. The Mic

EQ should be adjusted if necessary to maximise voice clarity.

Microphone Priority

MPA Series mixer-ampliers provide two separate

microphone priority functions. One of these enables the

microphones to have priority over music, the other allows

Mic 1 to have priority over the other microphones.

Microphone over music priority

Fully automatic voice operated priority (VOX function) is

available for the microphone signals. This function is enabled

by internal jumper J6 (default setting is ON). When enabled,

the music signal is automatically attenuated by 30 dB when

a microphone signal is detected, allowing the message to be

clearly heard. Normal music operation is restored smoothly

after the announcement ends.

Note that the presence of a signal on any of the four mic

inputs will operate this function.

Mic 1 over Mics 2-4 priority

If internal jumper J8 is set to ON (the default setting is

OFF), Mic inputs 2, 3 and 4 will be muted whenever the

Microphone Access Input is enabled. This ensures that Mic 1

will always have priority over any other microphones in use

when Mic 1 is used for paging.

Note that if jumper J13 is left at its default setting of ON,

Mics 2, 3 and 4 will be permanently disabled by this function,

leaving Mic 1 as the only active microphone input.

Chime

The MPA Series ampliers have an internal pre-

announcement chime generator. The chime is triggered by

the Microphone Access Input, if enabled by jumper J1 being

set to ON. J1’s default setting is OFF.

An internal preset rotary control is provided to adjust the

chime volume; the front panel level controls have no effect

on the chime level. See g.7 for locations of internal controls.

Power Amplier Stage

and Outputs

Low impedance operation

The MPA Series ampliers are able to deliver their rated

power into a 4 ohm load, as follows:

Model Rated Power

MPA60

60 watts

MPA120

120 watts

MPA240

240 watts

The maximum output power will be reduced with higher

load impedances.

When using multiple low-impedance loudspeakers (normally

8 ohms) with a single amplier, series and/or parallel wiring

should be employed to produce a total load impedance of

not less than 4 ohms.

The low impedance output is available on a 2-pin 5 mm-pitch

screw terminal connector on the rear panel. Note that one

of the output pins is connected internally to 0 V.

Line Output

MPA Series ampliers are tted as standard with an output

transformer that can be enabled by moving an internal

connector. The transformer should be placed in circuit if the

amplier is to be used with a 100 V, 70 V or 25 V line speaker

distribution system.

To convert the amplier to high-voltage line operation, the

low impedance output socket should be unplugged from

CON8 on the main pcb (a short pair of heavy red and black

wires), and replaced by the spare plug connected to the

toroidal transformer nearer the rear of the unit (same wire

types).

Note that the low-impedance output will no longer be

available once the transformer is plugged in. The secondary

(output side) of the transformer is permanently wired to the

line output connector (a 4-pin 5 mm-pitch screw terminal

type), thus the high-voltage output will be active once the

internal connector change has been made. A safety cover is

tted over the connector, which may be removed to connect

the speaker cable. Replace the cover after the connection

has been made.

The transformer secondary has 3 line outputs: 100 V, 70 V

and 25 V; whilst these have a common 0 V connection, the

output is fully oating i.e. it is isolated from the rest of the

amplier. When the transformer is in use the maximum total

combined load should not exceed the rated power for the

amplier model (see table above).

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