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DESIGN AND CONSTRUCTION OF A 60 WATTS POWER AMPLIFIER
ABSTRACT
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
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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