Design And Construction Of A 250W Audio Amplifier

ABSTRACT

This work is titled the design and construction of power amplifier. A power amplifier is a device for increasing the audio power of a signal by use of an external energy source. It is design to meet up with the amplification demand in homes, offices, industries and especially in music industries. A power amplifier amplifies low-power audio signals (signals composed primarily of frequencies between 20 – 20 000 Hz, the human range of hearing) to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain, While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts.

This device is designed to deliver appreciable 250w power to the load. Audio Power amplifiers may be called upon to supply power ranging from a few watts in an audio amplifier to many hundreds or thousands of watts to a load (speaker). In audio amplifiers the load is usually the dynamic impedance presented to the amplifier by a loudspeaker, and the function is to maximize the power delivered to the load over a wide range of frequencies. The power amplifier in a radio transmitter operates over a relatively narrow band of frequencies with the load essentially constant impedance.

This amplifier when constructed can produce up to 250 watts (w) of audio signal.  It can be sued to amplify the sound of audio sound system like mini radio cassette player, compact disc player, turn table record player, car radio cassette player or any other sound system that requires increased audio.

This amplifier when produced in large or commercial quality, according to purpose will create job opportunity, provide indigenous electronics technology industry, and it will contribute to economic growth of the nation.

TABLE OF CONTENTS

TITLE PAGE

APPROVAL PAGE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

TABLE OF CONTENT

CHAPTER ONE

• INTRODUCTION
  • BACKGROUND OF THE PROJECT
  • PROBLEM STATEMENT
  • AIM OF THE PROJECT
  • OBJECTIVE OF THE PROJECT
  • APPLICATION OF THE PROJECT
  • PURPOSE OF THE PROJECT
  • SIGNIFICANCE OF THE PROJECT
  • LIMITATION OF THE PROJECT
  • RESEARCH QUESTION
  • PROJECT ORGANISATION

CHAPTER TWO

2.0      LITERATURE REVIEW

2.1      HISTORITICAL BACKGROUND OF AN AMPLIFIER

2.2     REVIEW OF CLASSIFICATION OF POWER AMPLIFIERS

2.3     CLASSIFICATION ACCORDING TO MODE OF OPERATION

2.4     REVIEW OF TRANSISTOR BIASING

CHAPTER THREE

3.0     CONSTRUCTION

3.1      BLOCK DIAGRAM OF AN AUDIO POWER AMPLIFIER

3.2      CIRCUIT DIAGRAM OF A 250W AMPLIFIER

3.3      CIRCUIT OPERATION

3.4     CIRCUIT DESCRIPTION

3.5     POWER SUPPLY CIRCUIT

3.6      PARTS LIST OF 500 WATT AMPLIFIER CIRCUIT

3.7      DESCRIPTION OF COMPONENTS USED

CHAPTER FOUR

RESULT ANALYSIS

4.0      CONSTRUCTION PROCEDURE AND TESTING

4.1      CASING AND PACKAGING

4.2      ASSEMBLING OF SECTIONS

4.3      TESTING OF SYSTEM OPERATION

4.4      COST ANALYSIS

CHAPTER FIVE

5.1      CONCLUSION

5.2      RECOMMENDATION

5.2      REFERENCES

CHAPTER ONE

• INTRODUCTION

1.1                                         BACKGROUND OF THE PROJECT

An audio amplifier is an electronic amplifier that amplifies low-power audio signals (signals composed primarily of frequencies between 20 – 20 000 Hz, the human range of hearing) to a level suitable for driving loudspeakers. It is the final electronic stage in a typical audio playback chain.

An audio 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, an audio 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.

An audio amplifier works on the basic principle of converting the DC power drawn from the power supply into an AC voltage signal delivered to the load. Although the amplification is high the efficiency of the conversion from the DC power supply input to the AC voltage signal output is usually poor.

The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification (this is particularly associated with record turntable signals), equalization, tone controls, mixing/effects, or audio sources like record players, CD players, and cassette players. Most audio power amplifiers require these low-level inputs to adhere to line levels.

While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens or hundreds of watts for a home system or thousands or tens of thousands of watts for a concert sound reinforcement system.

Not all amplifiers are the same and are therefore classified according to their circuit configurations and methods of operation. In “Electronics”, small signal amplifiers are commonly used devices as they have the ability to amplify a relatively small input signal, for example from a Sensor such as a photo-device, into a much larger output signal to drive a relay, lamp or loudspeaker for example.

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, which is the relationship between input signal and current flowing in the load.

• STATEMENT OF THE PROBLEM

• Addressing crowd used to be difficult, this lead to shouting on top of our voice which lead to loss of voice.
• Sometimes, we desired to play out little audio devices (such as cell phone, DVD, TV, etc) in a way that it can be heard by people from a distance.

In other to bring solution to this problem, an amplifier is used to amplifier small audio signal to a level it can be heard from a distance. An audio amplifier is one kind of electronic device, used to increase the amplitude of audio signals that permit it. This type of amplifier increases low-power audio signals to a level which is apt for making loudspeakers.

1.3                                                   AIM OF THE PROJECT

The aim of this work is to build an electronic device that amplifies low-power electronic audio signals such as the signal from radio receiver or electric guitar pickup to a level that is high enough for driving loudspeakers

1.4                                             OBJECTIVE OF THE PROJECT

The main objective of this work is to design and construct an audio amplifier that will deliver appreciable 250w power to the load. A 250w audio amplifiers may be called upon to supply power ranging from a few watts in an audio amplifier to 250w to a load (speaker).

1.5                                         APPLICATION OF THE PROJECT

Important applications include public address systems, theatrical and concert sound reinforcement systems, and domestic systems such as a stereo or home-theatre system. Instrument amplifiers including guitar amplifiers and electric keyboard amplifiers also use audio power amplifiers. In some cases, the power amplifier for an instrument is integrated into a single amplifier “head” which contains a preamplifier, tone controls, and electronic effects. In other cases, musicians may create a setup with separate rack mount preamplifiers, equalizers, and a power amplifier in a separate chassis.

1.6                                              PURPOSE OF THE PROJECT

Amplifier’s main purpose is to take the weak signal and make it strong to drive a speaker. An audio amplifier usually get necessary amplification energy of input signals from the AC supply. Ideally without distortion an amplifier can deliver signal at the output without distortion.

1.7                                         SIGNIFICANCE OF THE PROJECT

Today there are many categories of amplifiers used for multifarious purposes. In simple terms an amplifier picks up a weak signal and converts it into a strong one. It is widely used in several devices to boost electrical signals. Radios, televisions and telephones are a few examples to point out in this regard. There are innumerable other applications of amplifiers and it is not easy to enumerate all of them. Some of the common types of amplifiers are used in electronic gadgets and audio systems. These are also used in musical instruments and guitars.

1.8                                          APPLICATION OF THE PROJECT

Important applications include public address systems, theatrical and concert sound reinforcement systems, and domestic systems such as a stereo or home-theatre system. Instrument amplifiers including guitar amplifiers and electric keyboard amplifiers also use audio power amplifiers. This amplifier can be used to amplify audio signals from laptop speakers, portable speakers

1.9                                           LIMITATION OF THE PROJECT

Noise: This device tends to add some random noise to the signals passing through them, hence degrading the SNR (signal to noise ratio). This, in turn, limits the accuracy of any measurement.

Limited output voltage, current, and power levels. This amplifier cannot produce more than 250w.

1.9                                                  DEFINITION OF TERMS

Acoustics – the science or study of sound.

Alternating (AC) Current – currents that have a harmonic time dependence.

Ampere (A) – the unit of measurement for electrical current in coulombs per second.

Amplifier – an electrical circuit designed to increase the current or voltage of an applied signal.

Audio frequency – the acoustic spectrum of human hearing, generally regarded to be between 20 Hz and 20 kHz.

Capacitor – a charge storage device made up of two metallic plates separated by a dielectric, with equal but opposite charges. The AC impedance of a Capacitor is (1 / jWL) and acts as an open circuit in DC applications.

Circuit – a complete path that allows electrical current from one terminal of a voltage source to the other terminal.

Class A – transistor amp conducts for the entire cycle of input signal, conduction angle 360 deg. Runs hot, as the transistors in the power amp are on all the time, but has high sound quality.

Class B – positive and negative halves of the signal dealt with by different parts of the circuit, the output devices switching continually. Runs cooler, but the sound is not as pure.

Class AB – biasing the transistor amp at a non-zero DC current much smaller than the peek current of the signal source. Second transistor conducts during negative half cycle of waveform and the currents from the 2 transistors are combined at the load. A compromise between sound quality of Class A and efficiency of Class B. Most amp designs employ this method.

Current (I) – the flow of electrical charge measured in amperes.

Decibel (dB) – (1) a logarithmic scale used to denote a change in the relative strength of an electric signal or acoustic wave. It is a standard unit for expressing the ratio between power and power level. Using the logarithmic relationship for power PdB = 10*log[Pout/Pin] , a doubling of electrical power only yields an increase of +3 dB. Increasing the power tenfold will yield an increase of +10 dB and is a doubling of perceived loudness. The decibel is not an absolute measurement, but indicates the relationship or ratio between two signal levels. (2) SPL (sound pressure level) can be measured in dB. 0 dB represents the threashold of normal human hearing, 130 dB represents the threashold for pain, 140 dB causes irreparable hearing damage, and 150 dB can cause instant deafness, anything greater than about 192 dB can kill you.

Direct Current (DC) – current in only one direction.

Driver – a loudspeaker unit, consisting of the electromagnetic components of a speaker, typically a magnet and voice coil.

Enclosure – a box that contains the driver(s).

Equalizer – electronic device that acts as active filters used to boost or attenuate certain frequencies. Farad – the basic unit of capacitance. A capacitor has a value of one farad when it can store one coulomb of charge with one volt across it.

Filter – any electrical circuit or mechanical device that removes or attenuates energy at certain frequencies.

Frequency – the number of waves (or cycles) arriving at or passing a point in one second, expressed in hertz (Hz).

Frequency Response – the frequency range to which a system, or any part of it, can respond.

Full-range – a speaker designed to reproduce all or most of the sound spectrum within human hearing (20Hz – 20KHz).

Ground – refers to a point of zero voltage or potential.

Harmonic – the multiple frequencies of a given sound, created by the interaction of signal waveforms.

Hertz (Hz) – a measurement of the frequency of sound vibration. One hertz is equal to one cycle per second. The hertz is named for H.R. Hertz, a German physicist.

High-pass Filter – an electric circuit that passes high frequencies but blocks low ones by acting as a large impedance to those frequencies. See Band-pass and Low-pass filters.

Hum – audio noise that has a steady low frequency pitch.

Imaging – it is the speakers ability to localize different instruments playing simultaneously. See Soundstage.

Impedance – dependent on frequency, it is the AC equivalent of resistance in a DC circuit.

Inductance (L) – the capability of a coil to store energy in a magnetic field surrounding it. It produces an impedance to an AC current (jwL) and acts as a short circuit to DC. Inductors are commonly used in audio as low pass crossovers. See Le.

Input – connection from signal source.

Maximum power rating – a value which means almost nothing, but is used nonetheless by manufacturers to entice the unsuspecting into purchasing their product based solely on the big number. Technically, it is the maximum wattage that an audio component can deliver/handle as a brief burst during a musical peak. Most reputable manufacturers will provide both an RMS and Max power rating. Typically, the given value for the maximum power rating is twice to three times that of RMS.

Microfarads (mF) – a measurement of capacitance (XC*10^-6).

MOFSET – Metal Oxide Semiconductor Field Effect Transistors. Used in most modern, quality car audio amplifiers in the power supply (and sometimes in the output stage). MOSFET’s run cooler than normal bipolar transistors, and have a faster switching speeds and higher slew rates.

Ohm – a unit of electrical resistance or impedance.

Ohm’s Law – a basic law of electric circuits. It states that: the current [I] in amperes in a circuit is equal to the voltage [V] in volts divided by the resistance [R] in ohms; thus, I = V/R.

Oversampling – used in DAC systems. Increases signal frequency, making it easier for conversion circuitry and ancillary systems to filter out unwanted signals.

Out of Phase – when your speakers are mounted in reverse polarity, i.e., one speaker is wired +/+ and -/- from the amp and the other is wired +/- and -/+. Bass response will be very thin due to cancellation.

Output – the sound level produced by a loudspeaker.

Peak – the maximum amplitude of a voltage or current.

Phase – Refers to the timing relationship of two or more signals or soundwaves. It’s especially important to be sure that your stereo speakers are playing “in phase.” This means that the drivers (cones and domes) of your right and left speakers are moving in and out at the same time. If your speakers are “out of phase” you’ll hear significantly less bass, and instead of producing a strong center image, the sound tends to stay localized at the speakers.

Phase Coherence – the relationship and timing of sounds that come from different drivers (subs, mids, tweets) mounted in different locations in the vehicle.

Phase Distortion – a type of audible distortion caused by time delay between various parts of the signal; can be caused by equalizers.

Polarity – the orientation of magnetic or electric fields. The polarity of the incoming audio signal determines the direction of movement of the speaker cone. Must be observed when wiring speakers, so that they are “in phase”. See Out of Phase.

Power (P) – the time rate at which work is done or the rate at which energy is used. Basic equations for Electrical Power are: P = V^2/R or P = I^2*R.

Resistance (Re) – in electrical or electronic circuits, a characteristic of a material that opposes the flow of electrons. The higher the gauge of wire, the less cross sectional area contributing to DC series resistance (DCR).

RMS – an acronym for “root mean square.” Used in audio to help rate the continuous power output of an amplifier or input capability of speakers. This is the preferred method for comparing anything in audio applications.

Sensitivity – The sound pressure level directly in front of the speaker (on axis) at a given distance (usually 1 meter) produced by a given amount of power (usually 1 watt).

Signal-to-noise (S/N) – the ratio, expressed in dB, between the signal and noise.

Volt (E) – a unit of measurement used to measure how much “pressure” is used to force electricity through a circuit.

Watt – a unit of electrical power. A watt of electrical power is the use of one joule of energy per second. Watts of electrical power equals volts times amperes.

1.10                                      PROJECT WORK ORGANISATION

The various stages involved in the development of this project have been properly put into five chapters to enhance comprehensive and concise reading. In this project thesis, the project is organized sequentially as follows:

Chapter one of this work is on the introduction to 250w audio amplifier. In this chapter, the background, significance, objective limitation and problem of 300w audio amplifier were discussed.

Chapter two is on literature review of a 250w audio amplifier. In this chapter, all the literature pertaining to this work was reviewed.

Chapter three is on design methodology. In this chapter all the method involved during the design and construction were discussed.

Chapter four is on testing analysis. All testing that result accurate functionality was analyzed.

Chapter five is on conclusion, recommendation and references.