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.
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 = 1.414 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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