ABSTRACT
It has become inevitable that there would be communication and due to the inherent limitation of the human voice, the public address system came to being. A public address or “P.A SYSTEM” is an electronic amplification system with a mixer ,amplifier and loudspeakers, used to reinforce a given sound , e.g a person making a speech, pre-recorded music ,or message and distributing the sound throughout a venue . The voice signal is passed through a microphone , which converts the sound energy to electrical energy, the energy electrical signal being transmitted is been received and amplified by the amplifier circuit. The amplifier’s output is fed into the loudspeaker which converts the electrical energy back to the original form but amplified sound energy. This project is made up of the power supply unit, the pre-amplifier and power amplifier units, and the tone control unit. This system is capable of delivering 20W of electrical power into a 8 ohm load(loudspeaker).
TABLE OF CONTENTS
Certification Page------------------------------------------------------------------------ i
Dedication --------------------------------------------------------------------------------- ii
Acknowledgement------------------------------------------------------------------------ iii
Abstract------------------------------------------------------------------------------------- iv
Table of contents--------------------------------------------------------------------------- v
List of Table-------------------------------------------------------------------------------- vii
List of figures------------------------------------------------------------------------------ viii
CHAPTER 1:
INTRODUCTION …………………………………………...1
1.1 BACKGROUND OF THE STUDY
1.2 Aims and Objectives……………………………………………..5
1.3 Justification ……………………………………………..6
1.4 Scope of project …………………………………………….7
1.5 System block diagram……………………………………………7
1.6 Project Work Organization ………………………………………8
CHAPTER 2:
LITERATURE REVIEW ……………………………………9
2.1 Ancient medium of information transmission …………………….9
2.2 Modern day medium Approach …………………………………...9
2.2.1 Problems associated with earlier systems…………………..……10
2.2.2 Working principles of the modern systems………………………10
2.3 Public Address System…………………………………………..11
2.3.1 The transmitter system…………………………………………..12
2.3.2 The Receiver system…………………………………………….13
2.3.3 The Antenna……………………………………………………..14
2.3.3.1 Functions of Antenna……………………………………………15
2.3.4 The tone control and mixer stage………………………………..15
2.3.5 The audio amplifier……………………………………………..16
2.3.6 The loudspeaker…………………………………………………17
2.4 Modulation………………………………………………………17
2.4.1 Frequency modulation…………………………………………..18
CHAPTER 3:
SYSTEM DESCRIPTION AND IMPLEMENTATION….19
3.1 Transistor power amplifier……………………………………….19
3.2 MOSFET power amplifier………………………………………..19
3.3 Bipolar junction transistor………………………………………...21
3.4 Operational Amplifier……………………………………………..23
3.4.1 Class A amplifier………………………………………………….24
3.5 Implementation Method…………………………………………..24
CHAPTER 4:
CIRCUIT DESIGN AND ANALYSIS………………………...26
4.1 Power Supply unit design………………………………………….26
4.2 Transmitter design ………………………………………………..27
4.3 Amplifier circuit design…………………………………………....29
4.4 Receiver principles………………………………………………...31
4.5 Demodulation……………………………………………………...32
4.6 Transmitter circuitry…………………………………………….…33
4.7 The input stage…………………………………………………….34
4.8 Mixing stage………………………………………………………34
4.9 Audio power amplifier…………………………………………….36
CHAPTER 5: CONSTRUCTION AND TESTING…………………………37
5.1 The power supply …………………………………………………37
5.2 The output power amplifier………………………………………..37
5.3 Dissipation of excessive heat………………………………………38
5.4 Entire circuit testing………………………………………………..38
5.5 Performance Evaluation…………………………………………...38
5.6 Packaging……………………………………………………….....40
CHAPTER 6:CONCLUSION AND RECOMMENDATION……………..41
6.1 Bill of Engineering measurement and Evaluation (BEME)……….41
6.2 Problems Encountered and Solution……………………………….42
6.3 Audio feedback and prevention……………………………………43
6.5 Conclusion…………………………………………………………44
6.6 Recommendation / Reference ……………………………………44
LIST OF TABLE
Table 1: -----------------------------------------Project Work Organization
Table 2: ---------------------------------------- Bill of Engineering measurement and Evaluation
LIST OF FIGURES
Fig. 1 --------------------------------------A picture of a modern wireless public address system.
Fig. 2 --------------------------------------A transmitter circuit
Fig. 3 --------------------------------------A block diagram of a Public address System.
Fig. 4-------------------------------------- A block diagram of a transmitter system
Fig. 5-------------------------------------- A block diagram of a receiver system
Fig. 6 ------------------------------------- A block diagram of an audio amplifier
Fig. 7 --------------------------------------A typical FM shown with the modulating signal.
Fig. 8 -------------------------------------The symbols of PNP and NPN transistor
Fig. 9-------------------------------------A symbol of an operational amplifier
Fig. 10 --------------------------------- -Power supply unit( a bridge rectifier with a voltage regulator)
Fig. 11 ------------------------------------------Transmitter Circuitry
Fig. 12 -------------------------------Tone control circuitry connected to the amplifier.
Fig. 13 ------------------------------------------System Circuit diagram
Fig. 14 ------------------------------------------ Address system with acoustic feedback
INTRODUCTION
A public address system allows you to broadcast information to a large group of people, whether you are giving a speech or playing live or recorded music.
Public address systems typically consist of input sources, preamplifiers, control and monitoring equipment, and loudspeakers. Input sources refer to the microphones that provides a sound input for the system. These input sources are fed into the preamplifiers. The pre amplified signals are then passed into the audio power amplifiers.
These amplifiers will amplify the audio signals to an adequate speaker line level. In view of the above, one can now say that the public address system is an electronic amplification system used for communication in public areas.
Microphone is a device that converts sound waves into electrical waves. Sometimes colloquially called a micro mike is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, live and recorded audio engineering, in radio and television broadcasting and in computers for recording voice, and for non-acoustic purposes such as ultrasonic checking. The sensitive transducer element of a microphone is called its element. Since a wireless microphone is used in this project; a wireless microphone is one in which communication is not limited by a cable.
A transmitter is extremely important equipment and is housed in the broadcasting station. Its purpose is to produce radio waves for transmission into space. The important components of a transmitter are microphone, audio amplifiers, oscillator and modulator.
It usually sends its signal using a small FM radio transmitter to a nearby receiver connected to the sound system, but it can also use infrared light if the transmitter and receiver are within sign of each other. The transmitter are responsible for taking in the signal from the microphone, modulating it, and transmitting it to the receiver using radio waves.
The first transistor (Q1) is the pre-amplifier for the microphone, and you can omit this circuit if you don’t want to transmit the sound picked up by the mic, for example you can connect your mp3 player directly to C1. The core of this FM transmitter circuit is Q2, a modified Culprits oscillator that the frequency is determined by L1,
C4, C6, and the transistor’s internal base-emitter capacitance. The antenna use 1/16 wave length to compromise between the efficiency and the size. If you want the microphone to be less sensitive, we can replace the R1 by a higher resistor, such as 10k or 22k, and this might overcome the feedback problem if you use this wireless microphone FM transmitter for a public address system
The receiver captures the radio waves sent out by the transmitter, demodulates the signal, amplifies it to an appropriate level, and sends it out to the audio mixer. A receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, and digital data, etc.
Amplifier or simply amp is any device that changes, usually increases, the amplitude of a signal. The “signal” is usually voltage or current. Amplifier is a device for increasing the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with larger amplitude. Routing the low-frequency parts of the signal to an amplifier can substantially improve the clarity of the overall sound reproduction.
In this project, the audio amplifier used is capable of delivering 20watts continuously. The term “power amplifier” is a relative term with respect to the amount of power delivered to the load and/or sourced by the supply circuit.
In general a power amplifier is designated as the last amplifier in a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Power amplifiers have also become lighter, smaller, more powerful and more efficient due to increasing use of Class A amplifiers, which offer significant weight and space savings as well as increased efficiency.
Power amplifier circuits (output stages) are classified as A, B, AB and C for analog designs, and class D and E for switching designs, based upon the conduction angle or angle of flow, Θ, of the input signal through the output amplifying device, that is, the portion of the input signal cycle during which the amplifying device conducts. The image of the conduction angle is derived from amplifying a sinusoidal signal. (If the device is always on, Θ = 360o.) In this project a class A amplifier is used because it offers low signal distortion.
Loudspeaker is an electro-acoustical transducer that converts an electrical signal to sound. A transducer that turns an electrical signal into sound waves is the functional opposite of a microphone.
Since a conventional speaker is constructed much like a dynamic microphone, (with a diaphragm, coil and magnet), speakers can actually work “in reverse” as microphones. The speaker pushes a medium in accord with the pulsations of an electrical signal, thus causing sound waves to propagate to where they can then be received by the ear. The loudspeaker used in this project is a load of 8 Ohms.
