A public address system consists basically of microphone, Amplifier and speaker(s) to facilitate the communication to intelligible speech to groups of people or an audience. The main purpose of providing sound reinforcement indoors with sufficient high – fidelity being the coverage of large groups with unobstrusive sound system. Directional horns are often employed in outdoor systems to concentrate sound energy efficiently over the areas served. Intelligibility may be the performance criteria for an indoor system such as an air waiting room. On the other hand, high fidelity reproduction is required for sound reinforcement system for instance, a conference hall, or a class room.

Most power amplifiers of a public address system have provisions for one or more microphones and one or other sound sources such as FM, AM radio tuner output, tape recorder and record player generally tagged auxiliary sound sources. Level traders or volume contracts are provided to adjust the volume of each of those sources independently.

In this project, the concentration of a high fidelity (H1 – F1) power amplifier for the in public address system.

This amplifier is designed as a unit utilizing three input sources. Operating controls are provided for each input source with the possibility of mixing the three input sources depending on the setting of the operating or gain controls.

This flexibility of mixer operation is without sudden increase or decrease in volume.

SPECIFICATIONS

THE POWER AMPLIFIER

TRANSISTORS: ………………………………..

POWER OUTPUT: ………………………………

SUPPLY VOLTAGE: ……………………………

TABLE OF CONTENTS

Title Page

Approval Page

Dedication

Acknowledgement

Introduction

Specifications

CHAPTER ONE

1.0 Power Amplifiers

1.1 Power Supply

1.2 The Preamplifier

1.3 Impedance Matching, Signal and Equalization

Chapter two

2.0 Methods of Cooling the System

2.1 Perforation of the Panel

2.2 Power Transistor Heat Sink

2.3 External Cooling

Chapter three

3.0 Trouble Shooting of the Power AMP

3.1 Preliminary Checks

3.2 Likely Faults

3.3 No Output, Reduced Output, Distortion & Noise Intermittence

Chapter four

4.0 The Console

4.1 Conclusion

4.2 Recommendation

4.3 Circuit Layout

4.4 Components Lists & Values

4.5 Bibliography

CHAPTER ONE

LARGE SIGNAL AMPLIFIERS

1.1 POWER AMPLIFIERS

All amplifiers are power amplifiers. However those operating in the early stages of the signal processing system deal with small signals. These early stages are designed to give good voltage gain. Since voltage gain is the most important functions of these amplifiers, they are called voltage amplifiers. The set up of a simple audio amplifier large – signal stage and the speaker. The microphone produces a very small signal, in the millivolt range. The small – signal stage amplifiers this audio signal and it becomes larger. The larger signal. It is called a Power Amplifier.

A Power Amplifier is designed for good power gain. It must handle large voltage and current swings. These high voltages and currents, the power is high.

It is very important to have good efficiencies in power amplifier. An efficient power amplifier delivers the signal power for the D.C. power. It takes from the supply since the job of the power amplifier is to change DC power into signal power

1.2 THE PREAMPLIFIER

A pre-amplifier is used to meet one or move to the following requirements.

1. Impedance Matching: For optimum performance, different sources of signals have different output impedance. It is not feasible to alter the input impedance of the power amplifier to suit the requirement of the signal source. Generally, a power amplifier has a low to medium input impedance.

2. Signal Amplification: Raw signal from the source generally lies in the region of 0.15MV to 100MV whereas signal strength required at the input of the power amplifier for rated output is of the order of 350MV to 1V. A preamplifier is used to boost the signal sufficiently to drive the power amplifier.

3. Equalization: While recording on tapes and discs, certain frequencies are emphasized while others are attenuated. During play back, the amplifier must reverse this deliberately introduced nonlinerity. To achieved this, the amplifier is required to provide different gains at different frequencies. This is termed Equalization.

4. Additional Facilities: Very often a preamplifier is expected to provide such additional facilities as tone controls and various types to filters to modify the response to make up for deficiencies of the listening area and to cater for the personal taste of the listener.

1.3 THE POWER SUPPLY

Electronics circuits needs energy to work. In most cases, this energy is provided by a circuit called power supply. A power supply failure will affect all the other circuits. The supply is a key part of any electronics system.

Power supplies use rectifier diodes to convert to direct current. They may also use zener diodes as voltage regulators.

This chapter also covers the features influencing the design of the power supply for this power amplifier circuits.

The preamplifier requires positive and negative po9wer supply. Therefore, the dual power at 3.5A 12vdc and the pre amplifier taking circuit in milli amplifier, the transformer with average D.C. current of 3.5A is chosen to be able to deliver this current tot he load, a rectifier diode IN400 (4 for Bridge rectifier) are used. With 2400 across the transformer’s primary 12vac is developed across the secondary.

The impedance that will thus be presented across the power supply by the circuit is: za = VDC IDC = 120 / 3.5 = 3.43 ohms

A full wave rectifier with center; tapped transformer is used in order to achieve a dual polarity power supply for the amplifier unit. The 4 diodes are connected in such a way that during the positive half cycle, D₂ and D₄ conduct. On the negative alternation, the polarity across t her secondary is reversed. The election leaves the centre tap, flow through D₁ and D₃. The load current is the same for both alteration. Since the direction never changes the load current is directly current.

Again, the full wave rectifier enables the unit to deliver high power to the amplifier in a given time. It also enables the output wave voltage to be effectively filtered with average sized capacitor.

Smoothing is carried out with capacitors C₁ and C₂, connected between the position of VCC and ground.

The D.C. output voltage of a filter power supply end to be higher than the output of a non filtered supply. With vims voltage of 12v, after the switch is closed, the capacitor changes to the peak value of the wave form.

VP = 1.414x vrms

= 1.414x 12 = 160

This represented a significant change in output voltages. However, as the supply is loaded, the output voltage will drop the required 12V.

Therefore, capacitor C₁ and C₂ are chosen with values of 330uf / 16v respectively. These capacitors are connected with positive of C₁ to VCC and positive off C2 to ground, so as to achieve the dual polarity power supply in addition the secondary function of filtering

Va. C= 0.9 x vrms

= 0.9 x 12 = 10.8 = 11v

Transformer is rated at 240v / 12v gain,

1p = 12 x 3.5 / 240 = 0.175 = 0.200A

The rating of the fuse required is thus

200MA / 240 V.