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.
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
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.
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
PHASING OF LOUDSPEAKERS / UNITS:
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.
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.
- Make connections right and tight.
- Switch off power supply when not in use.
- 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.