TM 55-1930-209-14&P-12
APPLICATION NOTE
M3131 POWER AMPLIFIER
WITH PUSH-TO-PAGE FEATURE
(Nearest Equivalent, Model 4PA-V2)
INTRODUCTION
The M3131 is a general purpose audio power amplifier that Is designed to drive loudspeakers and headset
earphone circuits.
PHYSICAL CHARACTERISTICS
The M3131 module is designed to fit into a standard electrical wiring box that is at least 2 inches deep It occupies
one "space", or the equivalent of one duplex outlet, and may be covered with any cover plate. There is no
electrical connection to the mounting frame. Any number of intercom modules may be nested together using a
multiple gang box.
The M3131 may also be mounted behind a panel by drilling a clearance hole for the potentiometer, and holding the
module to the back of the panel by using a 6-32 screw that would normally hold the cover plate on.
The rear of the amplifier has an 8 terminal strip which gives access to the circuitry. The easiest crimp terminal to
use is an insulated, slotted-tongue flanged spade, such as the AMP P/N 324608 or an equivalent.
Although every effort has beer made to properly ground the amplifier, any high gain circuit may be susceptible to
radio frequency fields. To minimize this effect, the box into which the amplifier is mounted should be connected to
the intercom system ground A short pigtail fastened to a box mounting screw and connected to a ground terminal
on the amplifier usually will work.
ELECTRICAL CHARACTERISTICS
The M3131 operates into a 4-wire intercom line which has +24 Volts DC, ground, a talk line (input to amp), and a
listen line (output from amp). In normal use, headset stations are connected to the amplifier's input, and the output
is connected to loudspeakers or to headset earphones via the headset stations. At no time may the amplifier see a
load across Its output terminals of less than 16 ohms This can be made up of headset earphones, loudspeakers, or
both. Maximum loads might be thirty-two 600 ohm headsets, or nine 150 ohm headsets, or three 45 ohm
speakers, or one 16 ohm loudspeaker. Any combination may be used, as long as the parallel combination of all
loads is not lower than 16 ohms. The M3131 is rated at 2 watts continuous power into a 16 ohm resistive load.
The amplifier is equipped with a volume control If the signal coming from someone's headset station is so strong
that It overloads the amplifier, turn down this volume control. Turning down volume controls on headset stations
will not stop the overloading. The overloading sounds like a harshness or fuzziness, and is due to "clipping" of the
amplifier Since the volume controls in the headset stations appear after the amplifier, they can only turn down a
signal that is already distorted.
The M3131 Amplifier has a push-to-page feature. To utilize this feature, a second M3131 amplifier(in addition to
system amplifier) must be used, and a headset station with a push-to-page button must be connected, (such as our
Models M3142, U3411, and U3211 ) The user at one of these stations pushes his button, which causes his voice to
come over an external speaker running from the second M3131. When the button Is not pushed, the external
speaker is silent.
A push-to-page system is desirable for use in areas where headset stations are not always manned. When the
person hears himself being paged over the loudspeaker, he goes to the nearest headset station to communicate.
The amplifier has two inputs. One is used for the push-to-page application, and the other is used for the "normal"
always-on application Always-on operation is used for powering headsets in a regular intercom system, or for
powering always-on monitoring speakers. The push-to-page input has a diode switch. The switch is turned on
when an activated push-to-page button on a headset station puts DC on the talk line. Because the switch can't
discriminate where the DC is coming from, everybody talking at that moment on the intercom system can be heard
over the paging speaker, even if only one push-to-page button is activated.
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