Uploaded by Imeh Thomas

Battery-powered Lecture theatre public Address System

I certify that this research project titled “THE DESIGN AND
carried out by UDOCHUKWU with MATRIC. NO:, AND was conducted
in the Department of Electrical/Electronics, School of Engineering, Albert
Polytechnic, Nwaniba Road, Uyo under my supervision.
Problem Statement
Objectives -
Classifications of Amplifiers
Characteristics of Passive Components
Characteristics of Active Components
Features and Specifications of the Audio Amplifier
Block Diagram
Design Stages
Complete Circuit Diagram
Circuit Analysis and Principles of Operation
Operation of the P.A System
System Performance and Testing
Application of a P.A System
5.1 Conclusion 5.2
5.3 Maintenance
This research work is a design and construction of a public address audio
amplifier for sound re-enforcement in a lecture theater. The device was
designed to be powered by a 220V/50Hz AC Mains supply, with inbuilt 12V,
18AH/10hrs, deep-cycle lead-acid battery to sustain the power on mains
power failure. A 15.5V, 5A DC, Constant Current/Constant Voltage built-in
charger was incorporated to replenish the battery energy on Mains power
and achieve maximum charge time, with minimum damage to the charging
cells. The device was designed to be capable of driving 70 watts of audio
power into an 8-ohm speaker using a suitable amplifier. The design building
block was broken down into five units namely: power supply unit, charger
and power switching unit, Summing preamplifier unit, Tone control unit,
and power amplifier unit. A symmetrical power supply was adopted to
provide both negative and positive voltages to the Operational amplifier
(Op-Am) and other Integrated Circuit Chips (IC) used in pre-amplifier and
mixer stages. Study shows that, a class AB amplifier is the most efficient
and high-power output class of amplifier and this project was designed
based on the criteria of this class of amplifier. The power amplifier
transistors were connected in Complimentary Push-Pull modes to improve
sound amplification. The public address audio amplifier was designed,
developed and tested and the results showed that the amplifier maintained
low distortion and noise. Different stages of the circuit were simulated
using Multisim simulation software, a product of National Instruments. In
addition, the amplifier was found to exhibit strong linear characteristics,
which showed that the developed amplifier worked satisfactorily.
The first transistor was invented in 1947, since then the pace of electronic
technology has gone from the use of bulky tubes to an extensive use of microchip solid-state semiconductors and from miniaturization to microminiaturization with yet seeming limitless possibilities unexplored. All these
development have resulted in an evasive electronic design technique with a
cardinal priority to maximize speed, efficiency and power output at the expense
of minimized space, size and cost.
Electronic devices have the singular ability to control the current flowing through
them by means of another current flowing through or voltage applied across
their terminals; hence they are also called active devices. Whether the device in
question is voltage-controlled or current-controlled, the amount of power
required for the controlling signal is typically far less than the amount of power
available in the controlled current. Because of this disparity between the
controlling and controlled powers, active devices may be employed to govern a
large amount of power. This behavior is known as amplification.
The benefits of electronic amplification cannot be overemphasized as it enables
the designer to detect, manipulate and control small signals which in most cases
constitute the signal of interest. For example, a dynamic microphone that is
picking up a loud voice or instrument may produce an electrical signal
somewhere in the neighborhood of 0.1 volt. This will require some preamplification to a line level signal (usually about 1 volt) before further processing
can be done.
An amplifier, electronic amplifier or (informally) amp is an electronic device that
can increase the power of a signal (a time-varying voltage or current). It is a twoport electronic circuit that uses electric power from a power supply to increase
the amplitude of a signal applied to its input terminals, producing a
proportionally greater amplitude signal at its output. The amount of
amplification provided by an amplifier is measured by its gain: the ratio of output
voltage, current, or power to input. An amplifier is a circuit that has a power gain
greater than one.
An amplifier controls the output to match input signal shape but with larger
amplitude. An idealized amplifier can be said to be “a piece of wire with gain“, as
the output is an exact replica of the input, but larger. Hence the amplifying
element is said to be linear. Real amplifiers (e.g. transistor) are not linear and the
output will only approximate the input. Non-linearity is the origin of distortion
within an amplifier. Any real amplifier is an imperfect realization of an ideal
amplifier. One important limitation of a real amplifier is that the output it can
generate is ultimately limited by the power available from the power supply. An
amplifier can saturate and clip the output if the input signal becomes too large
for the amplifier to reproduce.
An amplifier can either be a separate piece of equipment or an electrical circuit
contained within another device. Amplification is fundamental to modern
electronics, and amplifiers are widely used in almost all electronic equipment.
Amplifiers can be categorized in different ways. One is by the frequency of the
electronic signal being amplified. For example, audio amplifiers amplify signals in
the audio (sound) range of less than 20 kHz, RF amplifiers amplify frequencies in
the radio frequency range between 20 kHz and 300 GHz, and servo amplifiers
and instrumentation amplifiers may work with very low frequencies down to
direct current. Amplifiers can also be categorized by their physical placement in
the signal chain; a preamplifier may precede other signal processing stages, for
A public address system (PA system) is an electronic system comprising
microphones, amplifiers, loudspeakers, and related equipment. It increases the
apparent volume (loudness) of a human voice, musical instrument, or other
acoustic sound source or recorded sound or music. PA systems are used in any
public venue that requires that an announcer, performer, etc. be sufficiently
audible at a distance or over a large area. Typical applications include sports
stadiums, public transportation vehicles and facilities, and live or recorded music
venues and events. A PA system may include multiple microphones or other
sound sources, a mixing console to combine and modify multiple sources, and
multiple amplifiers and loudspeakers for louder volume or wider distribution.
Problem Statement
This project is designed to overcome communication problems between the
lecturers and students in an overflow lecture theater, as shouting during lectures
may be very exhausting. This simple project may also be used to bridge
communication gap that usually occur in areas where it is hard to find a device for
making important audible and intelligible announcements or speeches so that
everyone can listen to what the speaker is talking about. The project also seeks to
enable the speaker to be able to communicate uninterruptedly for at least 6 hours
during mains power failure, especially in remote areas where constant power
supply is an issue.
Project Objectives
The main objective of this project is to study, design and develop a dual
powered sound reinforcement device (amplifier) that can be used in small and
medium-sized environments. It also must be simple and can be carried around
easily by its user, using outlet power and inbuilt D.C. supply. Other objectives
1. Acquaint with the use of basic Electronic Components and devices.
2. Improve electronic design and constructional skills
3. Strengthen capacity for a better audience
4. Increase or magnify a low energy level signal (in this case sound).
5. Improve communication
6. Understand Basic Principles of Amplifiers.
1.4 Scope of Work
This work is concerned with the design and construction of a 70Watts
public address audio amplifier for sound re-enforcement in a lecture theater,
using simple electronic components and transducers. The device was designed to
be powered by a 220V/50Hz AC Mains supply, with inbuilt 12V, 18AH/10hrs,
deep-cycle lead-acid battery to sustain the power for at least 6 hours of lecture
period on mains power failure. The device is not meant for commercial or
domestic music amplification system.
History of Amplifiers and PA Systems
An amplifier, electronic amplifier or (informally) amp is an electronic device that
can increase the power of a signal (a time-varying voltage or current). It is a twoport electronic circuit that uses electric power from a power supply to increase
the amplitude of a signal applied to its input terminals, producing a
proportionally greater amplitude signal at its output.
A public address system (PA system) is an electronic system comprising
microphones, amplifiers, loudspeakers, and related equipment. It increases the
apparent volume (loudness) of a human voice, musical instrument, or other
acoustic sound source or recorded sound or music.
The first practical device that could amplify was the triode vacuum tube,
invented in 1906 by Lee De Forest, which led to the first amplifiers around 1912.
Vacuum tubes were used in almost all amplifiers until the 1960s–1970s when the
transistor, invented in 1947, replaced them. Today, most amplifiers use
transistors, but vacuum tubes continue to be used in some applications.
The development of audio communication technology in form of the telephone,
first patented in 1876, created the need to increase the amplitude of electrical
signals to extend the transmission of signals over increasingly long distances. In
telegraphy, this problem had been solved with intermediate devices at stations
that replenished the dissipated energy by operating a signal recorder and
transmitter back-to-back, forming a relay, so that a local energy source at each
intermediate station powered the next leg of transmission. For duplex
transmission, i.e. sending and receiving in both directions, bi-directional relay
repeaters were developed starting with the work of C. F. Varley for telegraphic
transmission. Duplex transmission was essential for telephony and the problem
was not satisfactorily solved until 1904, when H. E. Shreeve of the American
Telephone and Telegraph Company improved existing attempts at constructing a
telephone repeater consisting of back-to-back carbon-granule transmitter and
electrodynamic receiver pairs. The Shreeve repeater was first tested on a line
between Boston and Amesbury, MA, and more refined devices remained in
service for some time. After the turn of the century it was found that negative
resistance mercury lamps could amplify, and were also tried in repeaters, with
little success.[6]
The development of thermionic valves starting around 1902, provided an entirely
electronic method of amplifying signals. The first practical version of such devices
was the Audion triode, invented in 1906 by Lee De Forest, which led to the first
amplifiers around 1912. Since the only previous device which was widely used to
strengthen a signal was the relay used in telegraph systems, the amplifying
vacuum tube was first called an electron relay. The terms amplifier and
amplification, derived from the Latin amplificare, (to enlarge or expand), were
first used for this new capability around 1915 when triodes became widespread.
The amplifying vacuum tube revolutionized electrical technology, creating the
new field of electronics, the technology of active electrical devices. It made
possible long distance telephone lines, public address systems, radio
broadcasting, talking motion pictures, practical audio recording, radar, television,
and the first computers. For 50 years virtually all consumer electronic devices
used vacuum tubes. Early tube amplifiers often had positive feedback
(regeneration), which could increase gain but also make the amplifier unstable
and prone to oscillation. Much of the mathematical theory of amplifiers was
developed at Bell Telephone Laboratories during the 1920s to 1940s. Distortion
levels in early amplifiers were high, usually around 5%, until 1934, when Harold
Black developed negative feedback; this allowed the distortion levels to be
greatly reduced, at the cost of lower gain. Other advances in the theory of
amplification were made by Harry Nyquist and Hendrik Wade Bode.
The vacuum tube was virtually the only amplifying device, other than specialized
power devices such as the magnetic amplifier and amplidyne, for 40 years. Power
control circuitry used magnetic amplifiers until the latter half of the twentieth
century when power semiconductor devices became more economical, with
higher operating speeds. The old Shreeve electroacoustic carbon repeaters were
used in adjustable amplifiers in telephone subscriber sets for the hearing
impaired until the transistor provided smaller and higher quality amplifiers in the
The replacement of bulky electron tubes with transistors during the 1960s and
1970s created another revolution in electronics, making possible a large class of
portable electronic devices, such as the transistor radio developed in 1954.
Today, use of vacuum tubes is limited for some high power applications, such as
radio transmitters.
Beginning in the 1970s, more and more transistors were connected on a single
chip thereby creating higher scales of integration (small-scale, medium-scale,
large-scale, etc.) in integrated circuits. Many amplifiers commercially available
today are based on integrated circuits.
For special purposes, other active elements have been used. For example, in the
early days of the satellite communication, parametric amplifiers were used. The
core circuit was a diode whose capacitance was changed by an RF signal created
locally. Under certain conditions, this RF signal provided energy that was
modulated by the extremely weak satellite signal received at the earth station.
Advances in digital electronics since the late 20th century provided new
alternatives to the traditional linear-gain amplifiers by using digital switching to
vary the pulse-shape of fixed amplitude signals, resulting in devices such as the
Class-D amplifier.
From the Ancient Greek era to the nineteenth century, before the invention of
electric loudspeakers and amplifiers, megaphone cones were used by people
speaking to a large audience, to make their voice project more to a large space or
group. Megaphones are typically portable, usually hand-held, cone-shaped
acoustic horns used to amplify a person’s voice or other sounds and direct it
towards a given direction. The sound is introduced into the narrow end of the
megaphone, by holding it up to the face and speaking into it. The sound projects
out the wide end of the cone. The user can direct the sound by pointing the wide
end of the cone in a specific direction.
A short time later, the Automatic Enunciator Company formed in Chicago order
to market the new device, and a series of promotional installations followed. In
August 1912 a large outdoor installation was made at a water carnival held in
Chicago by the Associated Yacht and Power Boat Clubs of America. Seventy-two
loudspeakers were strung in pairs at forty-foot (12 meter) intervals along the
docks, spanning a total of one-half mile (800 meters) of grandstands. The system
was used to announce race reports and descriptions, carry a series of speeches
about "The Chicago Plan", and provide music between races.
In 1913, multiple units were installed throughout the Comiskey Park baseball
stadium in Chicago, both to make announcements and to provide musical
interludes, with Charles A. Comiskey quoted as saying: "The day of the
megaphone man has passed at our park." The company also set up an
experimental service, called the Musolaphone, that was used to transmitted
news and entertainment programming to home and business subscribers in
south-side Chicago, but this effort was short-lived. The company continued to
market the enunciators for making announcements in establishments such as
hospitals, department stores, factories, and railroad stations, although the
Automatic Enunciator Company was dissolved in 1926.
In the 1960s, an electric-amplified version of the megaphone, which used a
loudspeaker, amplifier and a folded horn, largely replaced the basic cone-style
megaphone. Small handheld, battery-powered electric megaphones are used by
fire and rescue personnel, police, protesters, and people addressing outdoor
audiences. With many small handheld models, the microphone is mounted at the
back end of the device, and the user holds the megaphone in front of her/his
mouth to use it, and presses a trigger to turn on the amplifier and loudspeaker.
Larger electric megaphones may have a microphone attached by a cable, which
enables a person to speak without having their face obscured by the flared horn.
Simple PA systems are often used in small venues such as school auditoriums,
churches, and small bars. PA systems with many speakers are widely used to
make announcements in public, institutional and commercial buildings and
locations—such as schools, stadiums, and passenger vessels and aircraft.
Intercom systems, installed in many buildings, have both speakers throughout a
building, and microphones in many rooms so occupants can respond to
announcements. PA and Intercom systems are commonly used as part of an
emergency communication system. The term “sound reinforcement system” is
The simplest, smallest PA systems consist of a microphone, an amplifier, and one
or more loudspeakers. PA systems of this type, often providing 50 to 200 watts of
power, are often used in small venues such as school auditoriums, churches, and
coffeehouse stages. Small PA systems may extend to an entire building, such as a
restaurant, store, elementary school or office building. A sound source such as a
compact disc player or radio may be connected to a PA system so that music can
be played through the system. Smaller, battery-powered 12 volt systems may be
installed in vehicles such as tour buses or school buses, so that the tour guide
and/or driver can speak to all the passengers. Portable systems may be battery
powered and/or powered by plugging the system into an electric wall socket.
These may also be used for by people addressing smaller groups such as
information sessions or team meetings. Battery-powered systems can be used by
guides who are speaking to clients on walking tours.
2.2 Amplifier Types and Classes
There are many forms of electronic circuits classed as amplifiers, from
Operational Amplifiers and Small Signal Amplifiers up to Large Signal and
Power Amplifiers. The classification of an amplifier depends upon the size of the
signal, large or small, its physical configuration and how it processes the input
signal, that is the relationship between input signal and current flowing in the
The type or classification of an amplifier is given in the following table:
Type of Signal
Frequency of
Small Signal
Large Signal
Common Emitter
Common Base
Class A Amplifier
Class B Amplifier
Common Collector
Class AB Amplifier
Class C Amplifier
Direct Current (DC)
Audio Frequencies
Radio Frequencies
Table 2.1: classification of Amplifiers and Signals
Generally, amplifiers can be sub-divided into two distinct types depending upon their power or
voltage gain. One type is called the Small Signal Amplifier which include pre-amplifiers,
instrumentation amplifiers etc. Small signal amplifies are designed to amplify very small signal
voltage levels of only a few micro-volts (μV) from sensors or audio signals. The other type are
called Large Signal Amplifiers such as audio power amplifiers or power switching amplifiers.
Large signal amplifiers are designed to amplify large input voltage signals or switch heavy load
currents as you would find driving loudspeakers.
2.2.1 Power Amplifiers
The Small Signal Amplifier is generally referred to as a “Voltage” amplifier because they
usually convert a small input voltage into a much larger output voltage. Sometimes an amplifier
circuit is required to drive a motor or feed a loudspeaker and for these types of applications
where high switching currents are needed Power Amplifiers are required.
As their name suggests, the main job of a “Power Amplifier” (also known as a
large signal amplifier), is to deliver power to the load, and as we know from
above, is the product of the voltage and current applied to the load with the output
signal power being greater than the input signal power. In other words, a power
amplifier amplifies the power of the input signal which is why these types of
amplifier circuits are used in audio amplifier output stages to drive loudspeakers.
The classification of an amplifier as either a voltage or a power amplifier is made
by comparing the characteristics of the input and output signals by measuring the
amount of time in relation to the input signal that the current flows in the output
circuit. With the introduction to the amplifier of a Base bias voltage, different
operating ranges and modes of operation can be obtained which are categorized
according to their classification.
These various mode of operation are better known as Amplifier Class. Audio
power amplifiers are classified in an alphabetical order according to their circuit
configurations and mode of operation. Amplifiers are designated by different
classes of operation such as class “A”, class “B”, class “C”, class “AB”, etc.
These different Amplifier Classes range from a near linear output but with low
efficiency to a non-linear output but with
a high efficiency. No one class of operation is “better” or “worse” than any other
class with the type of operation being determined by the use of the amplifying
circuit. There are typical maximum efficiencies for the various types or class of
2.2.2. Class A Amplifier Operation
In Class A amplifier operation 100% of the input signal is used (conduction angle
Θ = 360°). The active element remains conducting all of the time. The input signal
amplitude is constantly at sufficiently low level in order to stop operation from
entering the non-linear region of the transistor characteristics therefore distortion
is reduced and the output signal waveform closely resembles the input signal.
Class A Amplifier – has low efficiency of less than 40% but good signal
reproduction and linearity.
Fig. 2.1:
Output Waveform of Class A Amplifier
Advantages of Class A
1. Good Signal Reproduction
2. Good linearity
Disadvantages of Class A
Higher dc power loss at its output.
Poor efficiency.
Impedance matching is poor.
2.2.3 Class B Amplifier
In Class B amplifiers operation, 50% of the input signal is used (Θ = 180°); the
active element carries current half of each cycle. The Class B Amplifier uses two
complimentary transistors (either an NPN or a PNP) for each half of the output
waveform. One transistor conducts for one-half of the signal waveform while the
other conducts for the other or opposite half of the signal waveform. This means
that each transistor spends half of its time in the active region and half its time in
the cut-off region thereby amplifying only 50% of the input signal. Class B
Amplifier – is twice as efficient as class A amplifiers with a maximum theoretical
efficiency of about 70% because the amplifying device only conducts (and uses
power) for half of the input signal.
Fig 2.2:
Output Waveform of Class B Amplifier
High efficiency of about 70%
Crossover distortion is high.
Self-bias cannot be used.
Supply voltage must have good regulation.
2.2.4 Class AB Amplifier
The Class AB Amplifier is a compromise between the Class A and the Class B
configurations above. While Class AB operation still uses two complementary
transistors in its output stage a very small biasing voltage is applied to the Base of
the transistor to bias it close to the Cut-off region when no input signal is present.
An input signal will cause the transistor to operate as normal in its Active
region thereby eliminating any crossover distortion which is present in class B
configurations. A small Collector current will flow when there is no input signal
but it is much less than that for the Class A amplifier configuration. This means
then that the transistor will be “ON” for more than half a cycle of the waveform.
This type of amplifier configuration improves both the efficiency and linearity of
the amplifier circuit compared to a pure Class A configuration.
Fig. 2.3:
Output Waveform of Class AB Amplifier
The other classes of amplifiers not discussed here are class C, D, E, F, G and H.
2.2.5 Amplifier Characteristics
Any amplifier is said to have certain parameters or characteristics. These are the
particular properties that make the amplifier perform in a certain way, and so
make it suitable for a given task. Typical amplifier parameters are described
1. Gain
Amplifier gain is the ratio of the signal measured at the output with the signal
measured at the input. There are three different kinds of amplifier gain which can
be measured and these are: Voltage Gain (Av), Current Gain (Ai) and Power Gain
(Ap) depending upon the quantity being measured
Voltage Gain (Av):
Current Gain (Ai):
Power Gain (Ap):
The power Gain or power level of the amplifier can also be expressed in Decibels,
(dB). The Bel (B) is a logarithmic unit (base 10) of measurement that has no units.
Since the Bel is too large a unit of measure, it is prefixed with deci making it
Decibels instead with one decibel being one tenth (1/10th) of a Bel. To calculate
the gain of the amplifier in Decibels or dB, we can use the following expressions:
Voltage Gain in dB: Av = 20 log Av
Current Gain in dB: Ai = 20 log Ai
Power Gain in dB: Ap = 10 log Ap
2. Output Dynamic Range
Output dynamic range is the range between the smallest and largest bushel
levels and usually given in decibels (dB). Since the lowest bushel
level is limited by output noise, this is quoted as the amplifier dynamic range.
3. Bandwidth and Rise Time
The bandwidth of an amplifier is usually
defined as the difference between
the lower and upper half points. This is also known as the -3 dB Bandwidth for
other response tolerance are sometimes quoted (-1 dB, -6dB).
4. Settling Time
It is the time taken for the output to settle to within a certain percentage of the
final value. This is usually specified for oscilloscope vertical amplifiers
and high accuracy measurement systems.
5. Slew Rate
Slew rate is the maximum rate of change of output variable, usually quoted in
volts per second and sometimes microsecond.
6. Output Impedance
Output impedance, otherwise called the source impedance, or internal impedance
of an electronic device is the opposition exhibited by its output terminals to an
alternating current (AC) of a particular frequency as a result of resistance,
inductance and capacitance. It can be thought of as being the impedance (or
resistance) that the load sees “looking back” into the amplifier when the input is
zero. The generalised formula for the output impedance can be given as: ZOUT
7. Noise
Noise is an undesirable but inevitable product of the electronic devices and
components. It is measured in either decibels or the peak output voltage produced
by the amplifier when no signal is applied.
8. Efficiency
Efficiency is the measure of how much of the input power is usefully applied to
the amplifier’s output. The efficiency of the amplifier limits the amount of total
power output that is available.
Efficiency (ŋ) =
A signal as referred to in communication systems, signal processing, and
electrical engineering "is a function that conveys information about the behavior
or attributes of some phenomenon".
The IEEE (The Institute of Electrical and Electronics Engineers) Transactions on
Signal Processing states that the term "signal" includes audio, video, speech,
image, communication, geophysical, sonar, radar, medical and musical signals. In
electronics, a signal is an electric current or electromagnetic field used to convey
data from one place to another. The simplest form of signal is a direct current
(DC) that is switched on and off; this is the principle by which the early telegraph
worked. More complex signals consist of an alternating-current (AC) or
electromagnetic carrier that contains one or more data streams.
input signal - signal going into an electronic system (MP3, microphone,
output signal - signal that comes out of an electronic system (loudspeaker, oscilloscope,
another amplifier...)
2.4 Sound and Sound Wave
Sound is basically a waveform of energy that is produced by some form of a
mechanical vibration such as a tuning fork, and which has a “frequency”
determined by the origin of the sound for example, a bass drum has a low
frequency sound while a cymbal has a higher frequency sound.
A sound waveform has the same characteristics as that of an electrical waveform
which are Wavelength (λ), Frequency (ƒ) and Velocity (m/s). Both the sounds
frequency and wave shape are determined by the origin or vibration that
originally produced the sound but the velocity is dependent upon the medium of
transmission (air, water etc.) that carries the sound wave.
Sound waves have three important characteristics that will be considered in this
project. They are the frequency, the velocity and the wavelength.
Frequency – is the number of wavelengths per second in Hertz, (ƒ)
Velocity – is the speed of sound through a transmission medium in m/s-1
(300,000,000 m/s)
Wavelength – is the time period of one complete cycle a sound wave completes
in Seconds, (λ)
Fig. 2. Sound wave
Sound is the generalized name given to “acoustic waves”. These acoustic waves
have frequencies ranging from just 1Hz up to many tens of thousands of Hertz
with the upper limit of human hearing being around the 20 kHz, (20,000Hz)
range. The sound that we hear is basically made up from mechanical vibrations
produced by an Audio Sound Transducer used to generate the acoustic waves,
and for sound to be “heard” it requires a medium for transmission either through
the air, a liquid, or a solid. Audio Sound Transducers include both input sensors,
that convert sound into and electrical signal such as a microphone, and output
actuators that convert the electrical signals back into sound such as a
Characteristics of Passive and Active Components
2.5.1 Passive Components
Generally, in electronics passive components are devices, components or circuits
that do not have directional functions. Such components do not introduce “gain”
into the system. Pure resistance, capacitance, inductance or a combination of these
three will be designated as PASSIVE since they do not provide any amplification
of signals. In a public address system, resistors and capacitors are the common
passive components.
Resistors are passive, two-terminal electrical components that help control the
flow of current in a circuit by implementing electrical resistance. A high
resistance means there is less current available for a given voltage. Resistance is
the property of a conductor, which determines the quantity of current that
passes through it when a potential difference is applied across it.
Inside a resistor, electrons collide with ions, slowing the flow of electricity and
lowering the current while producing heat.
Resistors obeys ohm’s law at all times, implying that the current I flowing through
a given resistor is directly proportional to the potential difference V, applied
across it, provided the temperature and other physical factors are constant. In this
project resistors were selected based on this property.
I = V/R …………. Ohm’s law, where I is current through the conductor in unit
amperes, V is the potential difference measured across the conductor in unit volts,
and R is the resistance of the conductor in unit ohms.
Two types of resistors are available for use in all electrical circuits namely:
Fixed resistors: Fixed resistors are by far the most widely used type of resistor.
Their resistance values are fixed and cannot be varied. Different resistor
materials are used for fixed resistors. For all resistor types the used
materials have influence on the resistor properties like the tolerance, cost
and noise. The three common kinds are:
Carbon composition
Film resistor; and
Wire-wound resistor
In these resistors, a thin film of conductive (though still resistive) material
is wrapped in a helix around and covered by an insulating material.
Variable resistors: These resistors consist of a fixed resistor element and a
slider which taps onto the main resistor element. Their resistances can be
varied to any desired value by adjusting the slider to manage current flows
at and below a specific level.
Variable resistors: This type is used to vary the amount of resistance in a
circuit. The most common variable resistor are the potentiometers and
Fig. 2.11: Resistor Types: Fixed value and Variable Resistors Resistor Symbols
In this project the two types of resistors were used at different points, due to their
availability it was relatively easy to get all the resistors needed.
Fig. 2.4:
Symbols of Resistors
2.5.2 Capacitors
A capacitor (originally known as a condenser) is a passive two-terminal electronic
component that stores electrical energy in an electric field. The effect of a
capacitor is known as capacitance measured in Farads, F). Unlike a resistor, a
capacitor does not dissipate energy. Instead, a capacitor stores energy in the
form of an electrostatic field between its plates. When capacitors are connected
across a direct current DC supply voltage, they become charged to the value of
the applied voltage. They function like temporary storage devices and maintain
or hold this charge indefinitely as long as the supply voltage is present.
Capacitors could be fixed or variable. Fixed capacitors have their capacitances
fixed throughout their use in the circuit. Variable capacitor is a capacitor whose
capacitance may be intentionally and repeatedly changed mechanically or
Though there are many types of capacitors named according to their type of
dielectric, in general, there are 3 types of capacitors that will be available in the
values that are appropriate as AC coupling in most signal paths: electrolytic,
tantalum and ceramic. Each has strengths and weaknesses. Electrolytic
capacitors are generally the best performing for this purpose.
Fig. 2.: Capacitor Types-Fixed and Variable
The Electrical charge stored by a capacitor is equal to ½CV2 joules. This charge is
measured in coulombs and has a symbol Q. A simple capacitor consists of two
conducting plates or semiconducting plates separated by a dielectric material. The
capacitance of capacitor is dependent mainly on three factors:
The area of the plates (i.e. the greater the area, the greater the capacitance
and vice versa).
The distance between the plates (i.e. the closer the distance, the greater the
capacitance and vice versa).
The di-electric material. Energy stored by the capacitor in form of
electrostatic field within its di-electric. Capacitance
The capacitance (C) of the capacitor is equal to the electric charge (Q)
divided by the voltage (V):
the capacitance in farad (F)
the electric charge in coulombs (C), which is stored on the
the voltage between the capacitor's plates in volts (V)
Capacitance of Plates Capacitor
The capacitance (C) of the plates capacitor is equal to the permittivity (ε)
times the plate area (A) divided by the gap or distance between the plates (d):
the capacitance of the capacitor, in farad (F).
the permittivity of the capacitor's dialectic material, in farad per
meter (F/m).
the area of the capacitor's plate in square meters (m2].
the distance between the capacitor's plates, in meters (m).
Capacitors can also be used in charge pump circuits as the energy storage
element in the generation of higher voltages than the input voltage. Capacitors
are connected in parallel with the DC power circuits of most electronic devices to
act as reservoirs by smoothening current fluctuations for signal or control
circuits. Audio equipment, for example, uses several capacitors in this way, to
shunt away power line hum before it gets into the signal circuitry. The capacitors
act as a local reserve for the DC power source, and bypass AC currents from the
power supply. This is used in car audio applications. Capacitors are also used as
filters, snubbers and voltage multipliers. Because capacitors pass AC but block DC
signals (when charged up to the applied DC voltage), they are often used to
separate the AC and DC components of a signal. This method is known as AC
coupling or "capacitive coupling. A decoupling capacitor is a capacitor used to
decouple one part of a circuit from another. Noise caused by other circuit
elements is shunted through the capacitor, reducing the effect they have on the
rest of the circuit. It is most commonly used between the power supply and
ground. An alternative name is bypass capacitor as it is used to bypass the power
supply or other high impedance component of a circuit. Capacitor Symbols
Fig. 2.5:
Symbols of Capacitors
In the course of this project capacitors of different capacitances were used as
needed in the construction of a public address system.
2.5.2 Transformers
Transformers (sometimes called "voltage transformers") are devices used in
electrical circuits to change the voltage of electricity flowing in the circuit.
Transformers can be used either to increase the voltage (called "stepping up") or
decrease the voltage-to increase the voltage of an a.c. supply requires a step-up
transformer; to decrease the voltage of an a.c. supply requires a step-down
transformer. Transformers are also used to isolate one a.c. circuit from another
and need not to change the voltage; the purpose is to increase the safety of the
a.c. supply by isolating it from the source, such transformer being known as
isolating transformer.
Transformer make use of electromagnetic induction to transfer electrical energy
from one coil to another. A changing current (Ip) through one coil (Np) (primary
coil) induces a current (Is) in the nearby coil (Ns). The figure below shows how a
changing magnetic field produced by the changing current in the primary coil (Ip)
induces a changing current (Is) in the secondary coil (Ns). The voltage induced in
the secondary coil (Vs) is, at any instant, of opposite polarity to the primary
voltage (Vp). This is an important matter in a detailed study of transformer
action, and for this reason we can say that there is a mutual inductance between
the two windings of the transformer.
Fig. 2.: Transformer Construction
2.5.3: Battery
The word “battery” comes from the Old French word baterie, meaning “action
of beating,” relating to a group of cannons in battle. In the endeavor to find an
energy storage device, scientists in the 1700s adopted the term “battery” to
represent multiple electrochemical cells connected together.
A battery is an electrochemical system that converts chemical energy into
electrical energy. When the chemical reaction are irreversible, it is called a
primary cell but when they are reversible, it is known as a secondary cell. Primary
and secondary cells, although differently constructed, possess the same
components: two electrodes and electrolyte. An electrode is a terminal where
current enters or exit the battery. The electrolyte is the chemical that enables
energy to be stored in the battery cells due to the reactions with the electrodes.
It acts as a catalyst. One electrode chemically “reduces” when current flows and
becomes a positive (cathode) terminal while the other “oxidizes” and becomes
the negative (anode) terminal. The two electrodes are isolated by a separator
and soaked in electrolyte to promote the movement of ions. The anodic material
used for the negative terminal is usually a metal, such as lead, cadmium,
magnesium or zinc. Cathode materials on the other hand are usually chemical
components such as Lead di-oxide (PbO2), magnesium di-oxide (MnO2), MercuryOxide (AgO) or Ag2O). New active materials are being tried, each offering unique
attributes but none delivering an ultimate solution.
There are different kinds of Secondary batteries such as Lead-acid, Nickel
cadmium and others. For the purpose of this project, we will be concerned with
the lead-acid batteries. Invented by the French physician Gaston Planté in 1859,
lead acid was the first rechargeable battery for commercial use. Despite its
advanced age, the lead chemistry continues to be in wide use today. There are
good reasons for its popularity; lead acid is dependable and inexpensive on a
cost-per-watt base. There are few other batteries that deliver bulk power as
cheaply as lead acid, and this makes the battery cost-effective for automobiles,
golf cars, forklifts, marine and uninterruptible power supplies (UPS).
The grid structure of the lead acid battery is made from a lead alloy. Pure lead is
too soft and would not support itself, so small quantities of other metals are
added to get the mechanical strength and improve electrical properties. The
most common additives are antimony, calcium, tin and selenium. These batteries
are often known as “lead-antimony” and “lead-calcium.”
When charging, a buildup of positive ions forms at cathode/electrolyte interface.
This leads electrons moving towards the cathode, creating a voltage potential
between the cathode and the anode. The electrode of a battery that releases
electrons during discharge is called anode; the electrode that absorbs the
electrons is the cathode.
Lead acid
Cathode (positive)
Anode (negative)
Lead dioxide (chocolate
Gray lead, (spongy
when formed)
Sulfuric acid
Lead oxide (PbO2),
electrons added to
positive plate
Lead (Pb), electrons
removed from plate
Strong sulfuric
Lead turns into lead sulfate at the negative
electrode, electrons driven from positive plate to
negative plate
Table 2.: Composition of a lead–acid battery
Weak sulfuric
acid (water-like)
Battery Capacity
The capacity (C) of a battery is an indication of total useful energy available from
a battery. It is stated in Ampere-hours (Ah), which is the product of current in
amperes and time in hours. The figures quoted by manufacturers are usually
typical values and not the minimum guaranteed values. How many ampere-hours
can a battery actually give depends on many factors like discharge rate,
allowable voltage swing from full charge to low cut-off voltage (usually
determined by the circuit), operating temperature and duty cycle. The amount of
current that a battery can deliver depends on the surface area of the electrodes.
Life of a Battery
The service life of the battery is the length of time the battery can supply useful
current to the circuit. The service life of a battery depends on the end-point
voltage of the circuit. (The end point voltage of a battery is the lowest voltage an
equipment or a device can tolerate and still be functional. As a rule, the higher
the end point voltage, the lower the battery service life.
Fig. 2.: Battery Construction and Deep Cycle Lead acid Batteries
2.5.5 Active Components
In electronics, active components are those devices or components that introduce
gain or has directional function in the circuit. In a public address system typical
examples of active components used are transistors, diodes and Integrated Circuits
A transistor is a three terminal semiconductor device that is used to amplify
a given signal, voltage or current. This device is made from semiconductor
materials such as germanium or silicon. A transistor can also be used as an
electronic switch or in a number of other applications.
Transistors are devices with three terminals. The three terminals are called
emitter, collector and base. Transistor is operated in three configurations called as
common base, common emitter and common collector. Transistor is used for
voltage and current amplification according to configurations. At base input signal
of small amplitude is given and magnified output signal is collected at collector.
Thus transistors help in achieving amplification of signal. By passing input current
signal from region of low resistance to region of high resistance amplification is
achieved in transistors.
Transistor Types
Transistors of two types:
Unipolar junction transistor
Bipolar junction transistor
The current condition in unipolar transistor is due to only one type of charge
carriers, majority carriers. The field-effect transistor (FET) is a transistor that
uses an electric field to control the shape and hence the electrical conductivity of a
channel of one type of charge carrier in a semiconductor material. FETs are also
known as unipolar transistors as they involve single carrier- type operation. The
FET has several forms, but all have high input impedance. While the conductivity
of a non-FET transistor is regulated by the input current (the emitter to base
current) and so has a low input impedance, a FET's conductivity is regulated by a
voltage applied to a terminal (the gate) which is insulated from the device. The
applied gate voltage imposes an electric field into the device, this in turn attracts
or repels charge carriers to or from the region between a source terminal and a
drain terminal. The density of charge characters in turn influences the conductivity
between the source and drain.
Fig. 2.: UJT (FET) Symbols
The FET's three terminals are:
Source (S), through which the carriers enter the channel. Conventionally, current
entering the channel at S is designated by IS.
Drain (D), through which the carriers leave the channel. Conventionally, current
entering the channel at D is designated by ID. Drain-to-source voltage is VDS.
Gate (G), the terminal that modulates the channel conductivity. By applying voltage
to G, one can control ID.
In bipolar transistor the current condition is due to both types of charge carriers, holes and
electrons. So it is called bipolar. It is also referred as BJT. These two kinds of charge carriers
are characteristic of the two kinds of doped semiconductor material; electrons are majority
charge carriers in n-type semiconductors, whereas holes are majority charge carriers in p-type
Fig. 2.6:
Symbols of BJT Transistor
Unlike the bipolar Transistor, unipolar transistors such as the field-effect transistors have only
one kind of charge carrier. NPN is one of the two types of bipolar transistors, consisting of a
layer of P-doped semiconductor (the "base") between two N-doped layers. The other type of
BJT is the PNP, consisting of a layer of N-doped semiconductor between two layers of P-doped
Fiug. 2.: Charge carriers in a BJT Transistor
If a p-region is sandwiched between two n-regions like shown in figure (a) below, then its n-p-n
transistor and if a n-region is sandwiched between two p-regions like shown in figure (b), then
its p-n-p transistor.
Fig. 2.: Construction of a BJT Transistor
In the construction of this project the TIP 35, NPN and TIP 36, PNP silicon
transistors were used, and connected in complementary, push-pull mode to
reinforce the audio signal from the integrated circuit driver amplifier output. The
transistors are General purpose transistor, best suited for use in driver stages of the
audio amplifier.
The CENTRAL SEMICONDUCTOR TIP35 and TIP36 series devices are
complementary silicon power transistors manufactured by the epitaxial base
process, designed for high current amplifier and switching applications. They
● 125 W at 25°C Case Temperature
● 25 A Continuous Collector Current
● 40 A Peak Collector Current
● Customer-Specified Selections Available
Fig.2: TIP35/TIP36 Image and Pin Assignment
Technical Specifications
Table 2.: TIP35/TIP36 Technical Specification
2.3.4: Operational Amplifier
As well as resistors and capacitors, Operational Amplifiers, or Op-amps as they are more
commonly called, are one of the basic building blocks of Analogue Electronic Circuits.
Operational amplifiers are linear devices that have all the properties required for nearly ideal
DC amplification and are therefore used extensively in signal conditioning, filtering or to
perform mathematical operations such as add, subtract, integration and differentiation.
An Operational Amplifier, or op-amp for short, is fundamentally a voltage amplifying device
designed to be used with external feedback components such as resistors and capacitors
between its output and input terminals. These feedback components determine the resulting
function or “operation” of the amplifier and by virtue of the different feedback configurations
whether resistive, capacitive or both, the amplifier can perform a variety of different operations,
giving rise to its name of “Operational Amplifier”.
Fig. 2.: Op-amp Symbol
An Operational Amplifier is basically a three-terminal device which consists of two high
impedance inputs, one called the Inverting Input, marked with a negative or “minus” sign, ( - )
and the other one called the Non-inverting Input, marked with a positive or “plus” sign ( + ).
The third terminal represents the Operational Amplifiers output port which can both sink and
source either a voltage or a current. In a linear operational amplifier, the output signal is the
amplification factor, known as the amplifiers gain ( A ) multiplied by the value of the input
signal and depending on the nature of these input and output signals, there can be four different
classifications of operational amplifier gain thus:
Voltage – Voltage “in” and Voltage “out”
Current – Current “in” and Current “out”
Transconductance – Voltage “in” and Current “out”
Transresistance – Current “in” and Voltage “out”.
Fig. 2.: An Equivalent Circuit of an Ideal Op-amp
2.3. Op-amp Parameter and Idealized Characteristic
Open Loop Gain, (Avo)
o Infinite – The main function of an operational amplifier is to amplify the input signal and the
more open loop gain it has the better. Open-loop gain is the gain of the op-amp without positive
or negative feedback and for such an amplifier the gain will be infinite but typical real values
range from about 20,000 to 200,000.
Input impedance, (Zin)
o Infinite – Input impedance is the ratio of input voltage to input current and is assumed to be
infinite to prevent any current flowing from the source supply into the amplifiers input circuitry
(Iin = 0 ). Real op-amps have input leakage currents from a few pico-amps to a few milli-amps.
Output impedance, (Zout)
o Zero – The output impedance of the ideal operational amplifier is assumed to be zero acting
as a perfect internal voltage source with no internal resistance so that it can supply as much
current as necessary to the load. This internal resistance is effectively in series with the load
thereby reducing the output voltage available to the load. Real op-amps have output impedances
in the 100- 20kΩ range.
Bandwidth, (BW)
o Infinite – An ideal operational amplifier has an infinite frequency response and can amplify
any frequency signal from DC to the highest AC frequencies so it is therefore assumed to have
an infinite bandwidth. With real op-amps, the bandwidth is limited by the Gain-Bandwidth
product (GB), which is equal to the frequency where the amplifiers gain becomes unity.
Offset Voltage, (Vio)
o Zero – The amplifiers
output will be zero when the voltage difference between the inverting
and the non-inverting inputs is zero, the same or when both inputs are grounded. Real op-amps
have some amount of output offset voltage.
2.3.3 Integrated Circuits (ICs)
Integrated circuits (ICs) are a keystone of modern electronics. They are the heart
and brains of most circuits. An integrated circuit (IC), is small chip that can
function as an amplifier, oscillator, timer, microprocessor, or even computer
memory. An IC is a small wafer, usually made of silicon, that can hold anywhere
from hundreds to millions of transistors, diodes, resistors, and capacitors etc. The
Operational amplifier described above is a kind of integrated circuit. Other kind of
integrated circuits used in this project are described below.
For this project, the uA741 operational amplifier was used in the preamplifier
and tone control circuits, and the TDA 2030 IC was used as a driver to amplify
audio signals from the pre amplifier output.
uA741 Op-amp Image and PIN Assignments
uA 741 IC Pin Functions
Pin Name
Pin No
Non-inverting Input
Inverting Input
No internal Connection
External input offset voltage adjustment
External input offset voltage adjustment
Positive Supply
Negative Supply
Table 2.: uA 741 IC Pin Functions
Fig. 2.: uA 741 Internal Schematics and Components Count
Table 2.: uA741 Technical Specification Integrated Circuit (TDA 2030)
The TDA 2030 is a 20 watt a monolithic Integrated Audio amplifier in PentaWatt
Package of the class AB type designed to deliver a good quality sound output with
minimal distortion. Voltage Gain is 90dB. Low Harmonic Distortion Noise is
0.015%, 1 KHz, 8 ohms and Wide supply range 14V-28V at 4A. Further the
device incorporates an original (and patented) short circuit protection
system comprising an arrangement for automatically limiting the dissipated
power so as to keep the working point of the output transistors within their
safe operating area. A conventional thermal shut-down system is also
Fig. 2.3:TDA 2030 image and Pin Assignment
Table 2.: TDA 2030 IC Technical Specification
2.3.2 Diodes
A diode is a solid state active electronic device, comprising of two
terminals of the p-type and n-type semiconductor, they are generally used as
rectifiers in power supply units that provide direct current. A diode conducts
current when forward biased and its current increases exponentially, when
reversed biased a small leakage current flows until breakdown occurs. Diodes are
commonly made up of silicon or germanium materials. The ability of diodes to
allow the passage of current in the forward direction makes them suitable as A.C
Diode Symbols
Symbol of a Diode
In this project four 1N5404 rectifier diodes were used in the power supply
unit to rectify the alternating current to direct current.
2.4 The Input and Output Transducers of A Public Address System
A transducer is an electrical device which is used to convert one form of energy
into another form. In general, these devices deal with different types of energies
such as mechanical, electrical energy, light energy, chemical energy, thermal
energy, acoustic energy, electromagnetic energy, and so on
Fig 2.: Energy Conversion of a Transducer
The message from the signal source may or may not be electrical in nature. In a
case when the message produced by the signal source is not electrical in nature,
an input transducer is used to convert it into a time-varying electrical signal. For
example, in case of a public address system, a microphone converts the
information or massage which is in the form of sound waves into corresponding
electrical signal. Here we will be concerned with sound transducers which is
divided into sound input and output transducers.
Input transducer
Output transducer
Diagram of Amp with Transducers
In a public address system, the input and output transducers are of key
importance. They input and output transducers are capable of converting energy
from one form to another form. The input transducer, which is the microphone is
fed with mechanical sound energy which it converts into electrical energy, while
the output transducer (loud speaker) on the other hand converts electrical energy
to mechanical sound energy.
Fig. 2. Audio Sound Transducers: a. Speaker and b. a Microphone
2.5.5 Input Transducer (Microphones)
A microphone is an example of a transducer, a device that changes information
from one form to another. Sound information exists as patterns of air pressure;
the microphone changes this information into patterns of electric current. A
microphone produces electrical analog signals that are proportional to the sound
waves acting on its diaphragm. Microphones are classified by the type of
electrical transducer they use. In addition to the transducer, microphone uses
acoustic filters and passages whose shape and dimension modify the response of
the overall system. The output signal from a microphone is an analogue signal
either in the form of a voltage or current which is proportional to the actual
sound wave.
The characteristics of a microphone are both electrical and acoustic. The
sensitivity of a microphone is expressed as mV of electrical output per unit
intensity of sound wave. The impedance of microphone has considerable
importance. A microphone with high impedance has a high electrical output
while the one with low impedance is associated with low output. The high
impedance makes the microphone susceptible to hum pick up. The directionality
of the microphone is also an important factor. If the microphone is used in
sensing of the pressure of sound waves, then it is Omni – directional i.e. it picks
up sound arriving from any direction. A microphone is directional if it responds to
the velocity and direction of the sound wave.
The most common types of microphones available as sound transducers are
Carbon, Dynamic, Moving Iron, Ceramic (Crystal), Electret Condenser (Capacitor),
Ribbon and the newer Piezo-electric Crystal types. Typical applications for
microphones as a sound transducer include Sound reinforcement system (PA),
audio recording, reproduction, broadcasting as well as telephones, television,
digital computer recording and body scanners, where ultrasound is used in
medical applications.
There are several types of microphones that are suitable for P.A system. For this
project, a dynamic microphone will be chosen and described due to some
striking characteristic advantages it has over others such as low noise pick up,
high linearity and low distortion.
The construction of a dynamic microphone resembles that of a loudspeaker, but
in reverse. It is a moving coil type microphone which uses electromagnetic
induction to convert the sound waves into an electrical signal. It has a very small
coil of thin wire suspended within the magnetic field of a permanent magnet. As
the sound wave hits the flexible diaphragm, the diaphragm moves back and forth
in response to the sound pressure acting upon it causing the attached coil of wire
to move within the magnetic field of the magnet.
The movement of the coil within the magnetic field causes a voltage to be
induced in the coil as defined by Faraday’s law of Electromagnetic Induction. The
resultant output voltage signal from the coil is proportional to the pressure of
the sound wave acting upon the diaphragm so the louder or stronger the sound
wave the larger the output signal will be, making this type of microphone design
pressure sensitive. Maximum output occurs when the coil reaches maximum
velocity between the peaks of sound wave so the output is 900 out of phase with
the sound.
The change in the magnetic flux results into an equivalent change in electric flux
hence the electromotive force produced in the coil is a resulting effect of the
sound pressure which is transferred to the amplifier stage of the Public Address
Fig. 2.9: Construction of a Dynamic Microphone: Input sound Transducer
2.4.2 Output Transducers (Loudspeakers)
Output transducers are devices that can convert electrical energy into sound
energy. A loudspeaker is am electro-acoustic device that converts electrical
energy into sound energy, these devices are generally the converse of
microphones. When an analogue signal passes through the voice coil of the
speaker, an electro-magnetic field is produced and whose strength is determined
by the current flowing through the “voice” coil, which in turn is determined by
the volume control setting of the driving amplifier or moving coil driver. The
electro-magnetic force produced by this field opposes the main permanent
magnetic field around it and tries to push the coil in one direction or the other
depending upon the interaction between the north and south poles.
As the voice coil is permanently attached to the cone/diaphragm this also moves
in tandem and its movement causes a disturbance in the air around it thus
producing a sound or note. If the input signal is a continuous sine wave then the
cone will move in and out acting like a piston pushing and pulling the air as it
moves and a continuous single tone will be heard representing the frequency of
the signal. The strength and therefore its velocity, by which the cone moves and
pushes the surrounding air produces the loudness of the sound.
The speech or voice coil is essentially a coil of wire which has an impedance
value. This value for most loudspeakers is between 4 and 16Ω’s and is called the
“nominal impedance” value of the speaker. It is important to always match the
output impedance of the amplifier with the nominal impedance of the speaker to
obtain maximum power transfer between the amplifier and speaker. The human
ear can generally hear sounds from between 20Hz to 20kHz, and the frequency
response of modern loudspeakers called general purpose speakers are tailored
to operate within this frequency.
There are different types of loudspeakers such as the moving coil, electroacoustic and the subdivisions (woofers, mid-range speakers, tweeters). High
fidelity systems makes use of the three speakers, the woofer, the mid-range and
the tweeters, with three speakers the incoming frequencies are split, the low
frequencies (20 – 1000 Hz) are handled by the woofer, the middle frequencies
(800 - 10,000 Hz) are handled by the mid-range speakers while the higher
frequencies ranging from 3.5 – 20 KHz are handled by the tweeters. Public
Address systems mostly use the moving coil “horn type” loudspeaker due to its
high efficiency. A cone-type loud speaker could also be used.
Fig. 2.: The Construction of a Speaker: Output Sound Transducer
This stereo amplifier uses low cost, and widely available components. With the
addition of a few passive components it is possible to build a low cost stereo
amplifier featuring a good impulse response, ideally suited for connection to a
small speaker. The project here features a simple design for easy construction; it is
relatively inexpensive. The design is based on a uA741 Op-amp and TDA 2030
amplifier chips. The amplifier system been described here produces 70 Watts
power at very low distortion. The described amplifier has inputs for audio sources
such as Microphones, CD player, MP3 player and FM/AM tuner. Controls for the
described amplifier system are Bass, Treble and Volume control. The amplifier
puts out a surprising amount of power, considering that it runs from a 15V AC 7A
One reason this system performs so well is that it is based on the Micro
Electronics TDA 2030 20W audio amplifier IC. This IC has inbuilt thermal
protection so that even if it is wrongly handled or its output is short-circuited, , it
won’t be damaged. The power amplifier circuits are very close to the commercial
Amplifier system circuit but inevitably there are component differences to provide
different gain and fidelity.
Features and Specifications of the Audio Amplifier System
Total harmonic distortion plus noise:
typically <0.03%
Signal-to-noise ratio:
93dB (96dB A-weighted) with respect to 12W into 8Ω
Channel separation:
–72dB at 1 kHz
Input sensitivity:
500mV RMS for 12W into 8Ω
Input impedance:
Block Diagram
Fig 3.1: Shows the Block Diagram for the Amplifier System
Design Stages
3.4.1 Power Supply
Power supply is part of the electronic equipment that provides the required amount of power at
specified voltage from a primary source which can either be a battery or A.C.mains. Every
electronic equipment needs power supply for their proper operation. The electrical
characteristics of a power supply depend on the circuit being powered, while the size and form
depend the physical requirement of the system in which the circuitry is being used. Power Supply/Charger Block Diagram
Fig. 3.2: Power Supply/Charger Block Diagram
The block diagram in fig. 3.2 above illustrates the power supply and charger stage
used for this project. The Mains input is a 220VAC/50Hz supply which is passed
though an LC filter and an MOS varistor to check AC input surges to the down
transformer transformer whose secondary output voltage is 14-0-14VAC, 7A. The
AC-DC rectifier system is configured to supply a +14VDC, 0V and -14VDC
which is supplied to the charging circuit and different stages of the amplifier. The
limiting stage is to protect the charging circuit from supplying or being supplied a
voltage above the maximum allowable value. The charging system is also equiped
with protective measures to ensure short circuit from the battery terminals and
reverse voltage cutout.
Symetrical Power Supply Circuit Diagram
Fig. 3.3: Circuit Diagram of a Symmetrical Power Supply
The Circuit in fig. 3.3 above illustrates a symmetrical Power supply used for the Amplifier. A
14-0-14V, 7 A step down transformer is used to drop 230 volt AC to 14 volt DC which is then
rectified using the standard full wave bridge rectifier comprising D1 through D4. Smoothing
capacitors C8 and C9 remove the ripples from low volt AC. Two capacitors C10 and C11 are
connected in parallel with C8and C9 to aid in the ripple filtering. D1, D2and D2,D4 help to
split the +14V to both the charging circuit and the amplifier circuit. Fuses F1,F2 and F3 are
provided to open the AC input and the DC output on events of short circuits or overload.
3.4.2 Battery Charger
A battery charger is a device used to put energy into a cell or
(rechargeable) battery by forcing an electric current through it. Lead-acid
battery chargers typically have two tasks to accomplish. The first is to
restore capacity, often as quickly as practical. The second is to maintain
capacity (trickle-charge) by compensating for self-discharge. In both
instances optimum operation requires accurate sensing of battery voltage.
A well designed battery charger must be able to:
 Rejuvenate battery at flat state (0V)
 Charging Voltage must be at least 2.5V above the charged (Battery)
 Discontinue charging at float state (i.e. battery Voltage =Charging
 Automatically switch to trickle charging mode to keep the battery
cells on mild emergency charging.
 Block reversal of current from battery to the charger on power failure
 Cut off charging current when battery or charger terminals are shortcircuited.
Charging a lead acid battery is a matter of replenishing the depleted
supply of energy that the battery had lost during use. This replenishing
process can be accomplished with several different charger
implementations: “constant voltage charger”, “constant current charger” or
a ““multistage" constant voltage/current charger”. Each of these
approaches has its advantages and disadvantages that need to be
compared and weighed
to see which one would be the most practical and realistic to fit with our
Constant Voltage charger:-
Constant voltage charging is one of the most common charging methods
for lead acid batteries. The idea behind this approach is to keep a constant
voltage across the terminals of the battery at all times. Initially, a large
current will be drawn from the voltage source, but as the battery charges
and increases its internal voltage, the current will slowly fold and decays
exponentially. When the battery is brought up to a potential
full charge, which is usually considered around 13.8V, the charging
voltage is dropped down to a lower value that will provide a trickle
charge to maintain the battery as long as it is plugged into the charger.
The best characteristic of this method is that it provides a way to return a
large bulk of the charge into the battery very fast. The drawback, of
course, is that to complete a full charge would take a much longer time
since the current is exponentially decreased as the battery charges. A
prolonged charging time must be considered as one of the issues to this
Solar cells are one of our main portable power sources. Inherently, they
provide a constant current which is dependent on light intensity and other
uncontrollable variability in the environment. This characteristic fits well
with a constant voltage charge design, which does not depend on the
current provided by the input source, which in turn eliminates the
dependence of the charger on external variations like the time of day,
weather conditions or temperature. The effects of the changing voltage are
also minimized since the voltage is being regulated.
Constant Current Source:-
Constant current charging is another simple yet effective method for
charging lead acid batteries. A current source is used to drive a uniform
current through the battery in a direction opposite of discharge.
This can be analogous to pouring water into a bucket with a constant
water flow, no matter how full the bucket is. Constant current sources are
not very hard to implement; therefore, the final solution would require a
very simple design. There is a major drawback to this approach. Since the
battery is always being pushed at a constant rate, when it is close to being
fully charged, the charger would force extra current into the battery,
causing overcharge. The ability to harness this current is the key to a
successful charger. By monitoring the voltage on the battery, the charge
level can be determined, and at a certain point, the current source would
need to be folded back to only maintain a trickle charge and prevent
overcharging. Multi-stage Constant Voltage/Current Charging
Both constant voltage and current approaches have their advantages; that
is the reason multistage chargers have been developed which combine
the two methods to achieve maximum charge time, with minimum damage
to the charging cell.
3.. Stage 1: Deep Discharge Charging Pulse Mode
The Charger starts charging at 0.5V and give pulse current up to 5V.
This has effect of removing loose sulphation formed during deep discharge
state of the battery. Stage 2: Constant Current Mode (CC)
The charger changes to constant current 2.5A. When the battery voltage
reaches up to 14.4V, the charging stage changes from (CC) Constant
Current to CV (Constant Voltage) mode.
50 Stage 3: Constant Voltage Mode (CV)
The charger holds the battery at 14.4V and the current slowly reduces.
When the current reaches at 0.5 C (C= Battery Capacity), this point called
the Switching Point. The Switching Point is one of the great features of this
battery charger that it can adjust the current automatically according to the
battery capacity. Other chargers without microprocessors are not capable
to adjust the current. Stage 4: Standby Voltage Mode
The charger maintains the battery voltage at 13.8V and current slowly
reduces to zero. Charger can be left connected indefinitely without
harming the battery.
If the battery voltage drops to 13.8V, the charger changes from any mode
to Constant Current mode and restart charging. The charging cycle will go
through Stage 2 to Stage 4.
For this project, the multi-stage, constant Voltage/Constant Current
charger was adopted based on the criteria explained above. Charging Regulator Circuit
Fig. 3.4: Charging Regulator Circuit Diagram
The full charger feedback control circuit can be seen in Figure 3.4 above.
This circuit implements a three stage charger algorithm: constant current
state, constant voltage full charge state, and constant voltage float
charges state. This circuit requires an input voltage of at least 14.7 volts to
output the 12.4V for charging because of the 2V drop across the regulator.
The The LM 741 comparator is used to provide feedback of the current
that the battery is drawing from the circuit: as the battery charges, the
current drawn decreases. The current sensing resistor is used to convert
that current into voltage, which can be used to compare to a reference
within the circuit. This will be the logic needed for the state switching
mechanism. The full charge state will provide 12.4.V on the battery and
float charge will provide 13.8V.The battery will try to draw maximum
current, in this case: (12.4V-10.5V)/.1Ohm= 1.9A (assuming the battery is
completely dead) The current limiting of the voltage regulator will force the
current to 3A. The charger will continuously pump this 3A until the battery
current falls below the limit of 500 mA. This will bring the voltage of the
battery above the reference point, therefore causing the comparator to turn
on the transistor switch, pulling the output voltage to the float charging
Preamplifier Stage
Fig. 3.5: Circuit Diagram of the Pre-Amplifier/Summing Circuit
The summing network of the pre-amplifier stage, is used to combine several audio
inputs into a common output signal. Here the uA741 amplifier is configured in
summing configuration using C1, C2, C3 and R1, R2, R3, with R4 as a feedback
all connected to the inverting input of the Op-amp. The summing network allows
multiple input signals to be added simultaneously to the op-amp for amplification
without any interfering with each other. The Preamplifier stage amplifies the
resulting multiple audio signal before passing it to the tone control stage for
proper separation of the desired frequency, where it will then distribute the signal
in equal proportion to the power amplifier stage.
Tone Control Stage
Fig. 3.6: Schematics of the Tone Control Stage
The Tone Control circuits are often found in most high fidelity amplifiers. All
tone control circuits produce “boost” in one region of the audio spectrum by
reducing response at all others. This arrangement uses gain reducing negative
feedback to achieve the desired result.
Power Amplifier and Output Stage
Fig. 3.4: Shows Circuit Diagram for Power Amplifier
The power amplifier / output stage comprises of two power amplifier
integrated circuit type TDA 2030 and a dual push-pull complementary pair of TIP
35 and TIP 36 power transistors. Only a few external components are required by
this single Power Amplifier integrated circuit in other to deliver the required
output. The above mention chip contains one low distortion amplifiers that will be
used to drive the stereo speakers in bridge tied Load (BTL) configuration, with
rail to rail swing on the outputs and is inherently stable with fixed gain.
Complete Circuit Diagram
Fig. 3.7: Amplifier Complete Circuit Diagram
Circuit Analysis and Principles of Operation
As seen in the circuit diagram (Fig 3.7), the amplifier is quite
straightforward. With an input preamp stage followed by a tone control stage and
finally a power amplifier stage. Source input allows the user to input signals from
any one of three sources as provided by the preamplifier stage. These include
Microphone, CD, MP3 and TUNER, but any of these sources can be used for line
level audio signals.
From the input, the signal passes through the summing network. This filters
out any RF (radio frequency) signals or noise that may be present with the
incoming audio signals, to prevent them from causing trouble. After this, the
signals are applied directly to volume control potentiometer VR1a. From the
wiper of VR1a, the signals pass through a 100nF coupling capacitor to the
inverting input of the op-amp in the tone control stage. Which further amplifies
the input signal. The output signal from the preamp is then fed into the tone
control section. The Tone Control circuit is found, with variations, in most high
fidelity amplifiers, all tone control circuits produce “boost” in one region of the
audio spectrum by reducing response at all others. This arrangement uses gain
reducing negative feedback to achieve the desired result. Potentiometers VR2 and
VR3, and capacitors C1, C2 and C3, form a frequency selective network that
controls the feedback from the output of the op-amp. Interaction between Bass
control VR2 and Treble control VR3 is limited by resistors R2 and R3.When this
is augmented by the tone control circuit, the result is more bass than most
reasonably sized speakers and amplifiers can handle when delivering a good level
of sound. Bass control VR2 can, of course, be turned back to avoid overloading,
but a better arrangement is to make provision for switching in the extreme bass
boost when the amplifier is working at low volume.
The circuit diagram for the amplifier module reveals just the TDA 2030
power amplifier (IC1) and a handful of support components. The closed loop gain
of the amplifier is set to 23 by the 22kΩ and 1kΩ resistors on the inverting input
(pin 2), following the standard non-inverting amplifier feedback rules (i.e., voltage
gain = 22k/1k + 1 = 23).The 22µF capacitor in series with the 1kW resistor sets
the lower end of the amplifier’s frequency response. Another factor in the low-end
response is the high-pass filter in the input signal path, formed by the 2.2µF
coupling capacitor and 22kΩ resistor. Overall, the result is a rapid frequency
response roll-off below about10Hz. Following this, a 1kΩ series resistor and a
330pF capacitor form a low-pass filter, eliminating problems with high-frequency
noise pickup on the input leads. Non-polarized electrolytic capacitors (marked
NP) are used in these positions because the voltages present are too small to
polarize conventional capacitor.
Wiring of the Amplifier
Heavy-duty (7.5A) multi strand cables were used for all DC power and
speaker connections. The +25V, -25V and 0Vwires to the amplifier module were
twisted together to minimize radiated noise. On the mains (230V AC) side, only
mains-rated (250VAC) cable was used in the connection of the amplifier. The
mains earth was connected to a metal chassis using the appropriate screws. All
earth wires were Return to this point to eliminate potential earth loops.
Calculations for the Tone Control circuit
The Tone control circuit is a frequency dependent feedback arrangement,
and provides boost and cut for high and low frequencies. The formulae for the
active tone control circuits are fairly simple, but can be quite inaccurate under
some circumstances. Formulae for the bass control lower turnover frequency
(Fb0) and +/-15dB frequency (Fb1) are
Fb0 =
1 / (2 * π * C * VR2a) ... where C is the cap across variable
resistor VR2a, and VR2a is the variable Resistor value
Fb1 =
1 / (2 * π * C * Rs) ...
where C is the cap, and Rs is the series
resistance to the pot
These give
1 / (2*π * 10nF * 100k) =
1 / (2 * π * 10nF * 22k) =
For The treble, since there is an interaction with the bass control, due to the
bass feed resistance from the bass variable Resistor VR2 wiper. In theory the
frequency is determined by;
Ft = 1 / (2 * π * C * Rb) ... where C is the treble cap (4.7nF see circuit
diagram above) and Rb is the bass feed resistor from the variable resistor VR2a
Substituting the Values Ft = 1 / (2 * π * 4.7nF * 22K) =
Calculation for the Amplifier Power Output
Actual (real) power may be calculated by using the power formula
Preal = V² / R (Voltage squared, divided by impedance of the speaker in ohms)
Preal = 12² / 8 = 144 / 8 = 18Watts
As a design example consider an unregulated bench supply for our projects.
Here we will go for a voltage of about 12 - 13V at a maximum output current (IL)
of 500mA (0.5A). Maximum ripple will be 2.5% and load regulation is 5%.
Now the rms secondary voltage for the power transformer T1 must have the
desired output Vout PLUS the voltage drops across D2 and D4 (2 x 0.7V) divided
by 1.414.
This means that Vsec = [13V + 1.4V] / 1.414 which equals about 10.2V.
Depending on the VA rating of the transformer, the secondary voltage will vary
considerably in accordance with the applied load. The secondary voltage on a
transformer advertised as say 20VA will be much greater if the secondary is only
lightly loaded.
If we accept the 2.5% ripple as adequate for our purposes then at 13V this
becomes 13 x 0.025 = 0.325 Vrms. The peak to peak value is 2.828 times this
value. Vripple = 0.325V X 2.828 = 0.92 V and this value is required to calculate the
value of C1. Also required for this calculation is the time interval for charging
pulses. If you are on a 60Hz system it 1/ (2 x 60) = 0.008333 which is 8.33
milliseconds. For a 50Hz system it is 0.01 sec or 10 milliseconds.
The formula for C1 is:
C = IL X T/Vripple x 106
IL is the load current
T is the time in seconds
Vripple is the ripple voltage
4529 or 4700uF
Note that the tolerance of the type of capacitor used here is very loose. The
important thing to be aware of is the voltage rating which should be at least 13V X
1.414 or 18.33. In this case since there ins no 18.8V capacitor the preferred value
to use would be at least a standard 25V or higher (absolutely not 16V).
In the calculation above 0.5A was taken out of the Vsec of 10V. So The VA
required is 10 X 0.5A = 5VA. This is a small PCB mount transformer available in
the local electronic spar part dealer outlet. This would be an absolute minimum
and if you anticipated drawing the maximum current all the time then go to a
higher VA rating.
Calculating for the Rectifier Bridge
With our rectifier diodes or bridge they should have a PIV rating of 2.828
times the Vsec or at least 29V. This rating doesn't exist. In this case a high standard
or even higher specification was used. The current rating of the diodes was at least
twice the load current maximum i.e. 2 X 0.5A or 1A. So a type 1N4004, 1N4006
or 1N4008 types was chosen for the design. These are rated 1 Amp at 400PIV,
600PIV and 1000PIV respectively.
List of Components Used in Construction of P.A System
Resistors (Ω)
Capacitors (F)
560 * 2
470µ 35V * 2
1N5404 * 4
BC549C * 3
2.5A * 2
1K * 2
100µ 16v * 4
ZD1W 15V * 2
10K * 2
220K * 2
10µ *4
100K * 3
100 * 2
2.2n * 2
4.7K * 3
1M * 2
220µ 35V * 2
22K * 2
2.2n 16V
22µ 16V
220n 100V
The construction and implementation was done using step to step approach
in other to achieve a functional hardware. Some of these steps are listed and
explained below:
Designing the schematic and layout diagram
Tools and Equipment Used
Mounting and soldering of the components
Construction of Casing
Designing of the Schematic and Layout Diagram
As a design criterion, the circuit was first developed using Microsim Design
& Simulation Software. This program helps in the simulation of the project before
it is actually implemented. The simulation makes it easier to ascertain and do
corrections and decision before final implementation.
The design of the schematic and printed circuit board was another important
process, even more so when high voltages and frequencies are present. The
program used in creating both the PCBs and schematic diagram for the project is
Dip Trace which can be downloaded free from (www.diptrace.com). After
designing the schematic the next step was to convert it into a printed circuit Board
(PCB). The PCB was printed on a transparent film used in PCB production.
Tools and Equipment
The following tools were used during the construction of the electronic
Soldering Iron
Soldering lead
Disordering pump
Side cutter
Digital multi- meter
Sand paper
Electrician knife.
Mounting and soldering of the components on the printed circuit Board
The PCB was cleaned to ensure a dry surface before soldering. The
components were carefully mounted on the board, I used minimum amount of
heat needed to make a good joint as too much heat may cause damage to the
components, however little heat will result in a dry joint (a solder joint that may
not conduct electricity though it looks secure.)
I carefully fitted and soldered the resistors, capacitors, diodes, and
transistors etc., all this were mounted appropriately according to their ratings and
specifications in the circuit.
In fitting the power amplifier I took into consideration the mounting of the
heat sink, screw holes were drilled on the heat sink to enable me firmly fix the
heat sink to the power amplifier.
Then I connected the loudspeaker wires and the power supply wires after
soldering all components.
The terminals of the components were cut short to ensure a neat finish.
The PCB was carefully checked for mistaken or bridged tracks. Then I
carefully mounted all the completed PCBs into the casing that was designed with
screw holes to enable me firmly fit them in. the casing was built in such a way as
to allow easy access to components.
Construction of Casing
The casing is very important to the longevity of the design and construction
of the project. This great consideration was made as regards to the material used
for its construction, these considerations are:
Based on the above consideration, Plastic Perspex casing was used. The
Perspex piece was cut into appropriate size. The pieces were then glued together
using special glue called “UHU ALLPLAST” to form rectangular box. All
connection were done using standard cable to the complete project work.
After all, a wooden cabinet was designed to house both the amplifier and
loudspeakers. The cabinet was carefully crafted to foster easy carriage of the
Public Address system and also to protect electronic components.
The following precautions were observed to ensure success of the design
and constructions of the Project.
The components were handled with care in other to prevent damage due to
electrostatic discharges.
During soldering, the soldering iron was not placed on the components so
as to protect the semiconductor from damage due to overheating.
IC sockets were used to avoid heating the ICs while soldering.
Heat sinks were used were appropriate to conduct heat from the ICS
Testing and Result
The resistors, capacitors and diodes were tested using a multi-meter before
they were mounted on the PCB.
The complete system was subjected to various test using a multi-meter and
The point-to-point connection test was carried out i.e. continuity and short
circuit test, starting from the pre-amp to the control section and to the power amp
Problems Encountered
The problem encountered in the implementation of this project work were;
The epileptic power supply by the Power Holding Company (PHCN) which
caused a major setback in the implementation of this construction.
Poor access to materials needed for the project.
Lack of workshop facilities for students to work in.
During the design of this project, so many things were learnt which includes: Knowing
the process of simulation, How to apply design principles to achieve desired result, How to
interpret circuit, How to make printed circuit board and mounting of components on it, How to
trouble shoot and rectify problem, Soldering techniques etc. in the course of this project, quite a
number of problems were encountered but were overcome.
With increasing sophistication of electronic systems, system designers are better
acquainted with various dynamic aspects of signal amplification technology. However working
in my capacity as an amateur designer I have been able to acquire knowledge of basic physical
behaviors of electronic devices. This project has given me insight into some of the most
intricate parts of electronics. I look forward with great enthusiasm to one day becoming a force
to reckon with in the world of electronic design.
This thesis should serve as an aid to any subsequent project work on Amplifiers.
Application of this work is highly recommended to anyone who desires to carry out sound
management research. This project can still be improved upon. These include the following:
Future designs should include Dual Channel to boost the performance of the amplifier.
It is recommended that mains-rated cable for all connections to the socket, the relay
contacts be used. And all wiring be secured with cable ties.
Additional protection systems such as DC detector should be incorporated in the future
design in other to prevent and protect the amplifier from getting damage due to fault.
This project has been challenging, interesting and most of all a valuable experience in
the domain of operational Amplifier.
5.3 Maintenance
The maintenance of this system is expected to be carried out by a qualified personnel.
The process may be either by trouble shooting, servicing or repairs.
During maintenance, the circuit is to be followed step by step from the input to the
output terminal.
Alexander, C. K. & Sadiku, M. N. O. (2000). Fundamentals of Electric Circuits, McGraw-Hill
Bruce, R. (1997). Beginner’s Guide to Tube Audio Design.
Charles, A. H. (1985). Handbook of Components for Electronics, McGraw-Hill Inc. (Second
Edition). (Page 11-15).
Irvine, M. G. (1994). Power Supplies, Switching Regulations, Inverters and Converters, Tab
Books, 2nd Edition.
Lorimer & Lechner (1995). The Webster’s Dictionary of the English Language, International
Edition, Lexicon Publications Incorporated, New York, USA.
Morgan, J. (1999). Valve Amplifiers.
Operational Amplifier and Linear Integrated Circuits (Fourth Edition). By F. Coughlin
Frederick F. Driscoll.
Stan Gibilisco: The Illustrated Dictionary of Electronics, (Eighth Edition).
Internet Links:
www.diptrace.com schematic capture and printed circuit board design software.
www.eleccircuit.com LM1875 Datasheet -20 watts audio amplifiers.
www.basatek.com Audio Amplifiers Circuits.
www.techniks.com Printed circuit board transfer film techniks Inc. Ringos NJ.
www.HiFiVision.com Types of Amplifiers - Class A Class B Class AB Class.
www.electronic-tutorials.com Introduction to the Amplifier an Amplifier Tutorial.
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