INTRODUCTION
An audio AMP is basically a system which is used as a means of increasing the level of sound or audio signals for maximum use.
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.
TABLE OF CONTENTS
CHAPTER ONE
1.1 Power Supply
Chapter two
2.1 Perforation of the Panel
Chapter three
3.1 Preliminary Checks
Chapter four
4.1 Conclusion
CHAPTER ONE
LARGE SIGNAL 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
A pre-amplifier is used to meet one or move to the following requirements.
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 = 1.414 x vrms
= 1.414 x 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.
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