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 preamplifier 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).
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND OF THE STUDYFig 1: A picture of a modern wireless public address system
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.
46
Microphone is a device that converts sound waves into electrical waves. Some
times 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.
46
Fig2:A transmitter circuit
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
46
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.
46
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.
1.2 AIMS AND OBJECTIVES
Public speaking is the process of speaking to a group of people in a structured,
deliberate manner intended to inform, influence, or entertain the audience. In public
speaking, as in any form of communication, there are five basic elements, often
46
expressed as “who is saying what to whom using what medium with what
effects?â€(W.Kleitz;2005 pg134).
The major objective of this project is to design and construct a public address
system with relatively unique operational characteristics that is capable of delivering
20 watts of electrical power into an 8 Ohms load (loudspeaker). This will be achieved
with adequate tone controls and mixer stages so as to enable communication or
reproduction of speech and recorded music in buildings and institutions.
1.3 JUSTIFICATION
Over the ages, science and technology has been developing with new
inventions in various fields; including the increase in modifications on existing
technology all gearing towards improving effectiveness and reliability of equipment
and achieving to a great degree miniaturization and optimal cost.
This project is backed by my interest in the area of electronics and
communication, and having been groomed to a great extent with introductions into
the various fields of electrical and electronics engineering, with the knowledge in
principles of telecommunications, telecommunication engineering,
basic electronics and lab practice. I chose to combine basic electronics and lab
practice. I chose to combine all these ideas in embarking on this project to develop
46
a special system that utilizes all the above stated knowledge in one system; “the
public address systemâ€.
This system makes use of radio communication system, it utilizes radio
frequency to achieve all its set objectives, like data communication in the operation
of wireless communication between the transmitter and the receiver.
This system is intended to provide a fast and reliable means of communication
in small venues such as school auditoriums, churches, and small bars.
1.4 SCOPE OF THE PROJECT
This system is for public address; it has an estimated power rating of 20watts.
For power supplies, it uses a 220/12Vac transformer for the receiver, amplifier and
loudspeaker units. It has a power On/Off button with volume control.