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To study specifications and working of Public Address System.

Object: To study specifications and working of Public Address System.

Apparatus: Microphones, loud speakers and 100W amplifier kit, connecting wires, multimeter.

Theory: A public address system amplifies low level sound to a higher level so that it can be heard by large number of people gathered and at a considerable distance. A basic public address system has input source is a microphone, which pickups low level sound like a human speech. Examples of some other input sources are a cassette player for playback of recorded music, or compact disc player.


It is an electronic circuit in which sound low level signal to higher level signal and improves the signal gain. The gain depends upon the number amplifiers are used in cascaded stages. This determines the AF power levels. This amplifier consists of 3 or 4 microphone auxiliary circuit at input level with controlling switches for each microphone. Each microphone has got volume controllers to control audio signal levels. Special electronic circuit provides an excellent sound quality. Cut and Boost type Bass and Treble controls, master control, 4 input volume controls and a special feature of adjusting tape sensitivity through tape Hi/Lo switch have been provided. A led array gives a visual indication of the output level. Line output in addition to speaker outputs of 4,8,16 ohm and 100W is provided.


Fig 2.1: Front panel controls of an amplifier.


Player Section:


       Track system               :           2 Track Monomial

       Tape speed                 :           4.75 Cm/Sec

       FF/Rewind Time        :           190 seconds with C.60 tape

      Tape speed accuracy   :           within +2% - 1%

      Wow and flutter          :           0.35% Nominal

Amplifier section:

         Output power             :           100W (RMS) max

                                                       75W RMS at 10% THD

                                                       70W RMS at 5% THD (Rated)

        Frequency response   :           55Hz - 15000 Hz 3dB

        S/N Ratio                   :          > 55dB

        Input channels            :         4 X Mic 0.6 mV (Lo - Z) 4.7 K

                                                      1 X Aux 100mV / 470 K

        Speaker outputs         :          4,8,16 and 100W

        Line output                :          1000mV / 3.5 K

       Tone controls             :           Bass - 10dB + 7 dB at 100Hz

                                                      Treble - 10dB + 7 dB at 10 KHz


       Power supply mains   :           220 - 240V Ac 50/60 Hz

                                                      140 VA at rated output

        Battery                        :         12 - 14V dc, Drains 5A/12V

        Semiconductors          :         17 Transistors, 11 Diodes,

                                                      2 Zener diode

                                                      6 LED’s 2IC

        Protections                  :         Ac fuse (2Amps, 240V)

                                                      DC fuse (10Amps)

        Indicators                    :         Pilot LED

                                                      5 LED Array for output level

      Dimensions Net Weight  :      L 385 x D310 x H130 mm

      Net Weight                      :      9.6 Kg.

Microphone Connection:

 4 independent microphone inputs have been provided through jacks on the front panel. Mic 4 inputs may be used by keeping the Mic/Tape – Lo Tape Hi selector switch at MIC position. Microphone inputs are unbalanced Low impedance usable with microphones of impedance 200 - 600. In case microphones with both HIGH and LOW impedance are used, be some that switch on the microphone is at low position. Microphone with up to 30 meter long cable can be used always use good quality shielded cables. For good intelligibility of sound, the microphone should be placed at least 20 Cm away from the speaker’s mouth. Keeping the microphone too close to the mouth will result in distortion of sound and may even cause damage to the diaphragm of the microphone.


Speaker impedance taps of 4, 8 and 16 have been provided for direct connection of speakers to the amplifier when the distance between the amplifier and speakers is less than 50 meters. Use 23/26 or thicker cable for connections. Be sure that the total impedance of the speakers is equal to or more than the impedance specified on the terminal strip.

Fig 2.2: speakers


When two or more speakers / units are installed in the same area and are facing in the same direction, it is essential that their cones / diaphragms move in unison. Otherwise, the sound output of one speaker will cancel the sound output of other. To avoid any mistake, the terminals of all driver units are marked L1 & L2. For correct wiring refer the sketches.

Observation Table:

Sr. No

i/p voltage

Amplifier O/p











Connect a set of speakers to EITHER any ONE of the low impedance taps 4 & 8 or 16 or to 100V line output. Do not use more than one speaker output connection at a time.


PAS is assembled in college ground observed sound distribution through the ground in public meeting.


  1. Make connections right and tight.
  2. Switch off power supply when not in use.
  3. Handle the equipments carefully.


1. Study the block wise operation and specification of public address system

2. Methods of testing further IF, Cd remains loaded in deck but play does not take place.


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