Power Amplifier – Design and Construction of 60 Watts Power Amplifiers

POWER AMPLIFIER – DESIGN AND CONSTRUCTION OF 60 WATTS POWER 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.

2.0    METHOD OF COOLING THE SYSTEM

Great heat is being generated in the system due to individual behaviour of the components which made up the system for that great deal of work is done in design to get the system well vindicated and conduct away the heat in order to prolong the life of the system.

Components like transformers, transistors and IC’s in the circuit are bound to generate appreciable. The following methods are used to bring the temperatures of the system to normal.

2.1    PERFORATION OF THE PANEL
The panel is carefully designed to create enough air current into the internal structure of the system.

The panel is highly perforated to admit fresh air into the system this conducts away the heat being generated by the transformer due to transformer action.

2.2    POWER TRANSISTOR HEAT SINK
Power transistors equally generates internal heat which it not conducted away may load to its destruction.

The maximum power handled by a particular device and the temperature of the transistor junctions are related between the power dissipated by the device, causes an increase in temperature at the junction of the device.

Proper heat sinking technique will show operation of a device close to its maximum power rating power transistor are mounted on large Aluminum case to provide a large surface area from which the heat generated by the device may radiate into the air there by holding the case temperature to a much lower value than would result without the heat sink.

To determine the size of heat sink, some attention must be paid to the heat sinking. No heat sink is required with load impedance greater than 8 ohms, provided the case is well ventilated.

With impedance down to 4 ohms a heat sink of area at least 4 square inches is required.
Furthermore, for efficient performance, the D.C. power supply must be in line with A.C power output, to minimise the heat generated as loses and imbalance in the system.

2.3    EXTERNAL COOLING
Some high power amplifier system may require external cooling for effective performance and increased life span.

The cooling could be by Air Conditioning, or by using fan. This is in a case where the power handled by the system is so high and heat generated is therefore so large.

3.0    TROUBLE SHOOTING THE POWER AMPLIFIER
•    PRELIMINARY CHECKS
•    NO OUTPUT
•    REDUCED OUTPUT
•    DISTORTION AND NOISE
•    INTERMITTENCE

3.1    PRELIMINARY CHECKS
Good trouble shooting of the power amplifier involves:
•    VERIFICATION
•    INSPECTION
•    SECTIONALIZATION
•    LOCALIZATION
•    COMPONENT IDENTIFICATION AND REPLACEMENT

Verification involves confirming that such fault do exist. The nature of the fault should be identified.

Inspection must include preliminary checks of all control settings, proper connections and power supply operation.

The on-power light will indicate if the unit is plugged in and turned on.
Preliminary check is a visual inspection of interior of the equipment. Look for the followings Burned and discolored components; Broken wires and components, parts falling out of place, loose or partly seated connectors leaking components.

Obvious damage can be repaired at this point when the preliminary visual inspector is complete, a preliminary electrical checks should be made. The first part of the preliminary electrical check involves any signs of over heating. Your sense of smell may give important information. This must be done with extreme caution.

The next part of the electrical check is to verify the power supply voltage. Typical voltages expected at each test point of the circuit are shown below;

3.2    NO OUTPUT
The most obvious cause of no output is no input. It is worth the effort to check this early again, the output device may be defective.

If there is nothing wrong with the speaker the power supply, on the input signal, then there is a brake in the signal chain. A break at any point in the chain will usually cause — no output symptom. The efficient way to troubleshoot is to isolate the problem to one stage.
Signal tracing is another way to isolate the detective stages.

The important thing to remember is signal tracing is the gain and frequency response of the instrument being used. Once the fault has been located to a particular stage then determine with part has failed.

3.3    REDUCED OUTPUT

Low output from this amplifier can be caused by low input to the amplifier. The signal source is weak for some reason. A microphone may deteriorate with time and rough treatment.  To check, try a new signal source or substitute a signal generation.

Another possible causes for low output is reduced performance in the output device. A loud speaker defect or a poor connection may prevent normal loudness.
Again, for correct output, the impedance of the speaker used must match the output impedance requirement of the amplifier. It must also be rated to safety dissipate the output power of the amplifier.

The specification for the amplifier rate it at 60 watt of continuous sine wave power output. For the amplifier to meet its specification and does not suffer from low output.

3.4    DISTORTION AND NOISE
Distortion and noise in the power amplifier means that the output signal contains different information other than input signal.

Noise can produce a variety of symptoms. Some noise problems that may be found in the audio amplifier;

– CONSTANT FRYING OR HISSING NOISE
– POPPING OR SCRATCHING SOUND
– HUM
– MOTOR BEATING

Noise problem can often be traced to the power supply. In troubleshooting for this symptom, use an oscilloscope to check the various supply lines in the equipment.
The most common noise problem is HUM.  Hum refers to the introduction of a 60 H2 interference from full wave supplies. Hum can also get into the amplifier because of broken ground connection. Another cause of Hum is poor grounding or circuit boards. Check the fastener to make sure they are secure.

Again, shielded cables is used in high gain amplifier in areas where the A.C. line frequency can induce signals into the circuit.

A constant frying or hissing noise usually indicates a defective integrated circuit. Signal tracing is effective in finding out where the noise is originated. Resistor can also become somewhat noise. The problem is generally limited to early stages in the signal chain.

It is worth mentioning that the noise may be coming from the signal source itself. It may be necessary to substitute another source or disconnect the signal. Motor beating is a problem that usually indicate an open filter capacitor, an open by pass capacitor is a defect in a feed back circuit of the amplifier.

The power amplifier device is intermittent when it will work only. Some of the time. The source of this kind of problem may be very difficult to locate. However, the two basic way to find the cause of intermittent problem are

i)    By running the equipment until the problem appears and then use ordinary troubleshooting practice to isolate it.
ii)    By using various procedures to force the problem to show up. Some of these are:
1.    Heat various parts of the circuit.
2.    Cool various parts of the circuit.
3.    Change the supply voltage.
4.    Vibrate various parts of the circuit.
The actual techniques used will depend on the symptom and how much you service it.

THE CONSOLE

The console is constructed from a combination of galvanized steel sheet used for the main body, that is the base and the cover. An aluminum sheet used for the front panel. The dimension used for the construction was dictated by the size of the transformer used. The length of the Printed Circuit Board (PCB) and dimension of heat sink with some added allowance for both ease of accessibility during maintenance.

CONCLUSION
The aim of setting out of this project which is the design and construction of a 60 watts power amplifier has been achieved. The system is of high fidelity, and will function properly when operated within, the limits of safety at normal room temperature and pressure. It is designed to the almost performance and will compete favourably with any of the imported ones of the same rating.

Fellow Nigerians, who have needs for amplifier have something reliable to go for and can contact us at the Department. Since all the components are available in our local markets, bulk production of this equipment will be cheap.

RECOMMENDATION
With the present situation as obtained in this institution whereby lecturer have to stain their voice in order to make themselves to be heard loud and clear y the students, and students overcrowding the front seat in order to hear properly, this power amplifier is recommended as a public address system for use in such lecture halls for communication between lecturers and students.

It is also recommended for use in conference halls of this institute and conference halls of Government state house and other places.

In order to prolong the life of the equipments it is recommended that the equipment much be handled with care to avoid undue pressure as a result of falls or packing. When not to be used for a long time store in a cool dry place.

COMPONENT LIST AND VALUES
RESISTOR                    CAPACITORS
0.33 Ω 5w                        2200uf 50v
330 Ω 5w                        100uf 25v
270 Ω                        1uf 50v
2.2k                            220ud 10v
470
1k                        PAPER CONDENCER
8.2k                            151pf
15k                            223pf
47k                             10pf
10    104pf

VARIABLE RESISTOR            TRANSISTORS
50v                            MJ 15003
TIP 41
DIODES                         TIP 41
IN4004                        TIP42
TRANSFORMER                     C 1815
D 600
B 631

POWER AMPLIFIER – DESIGN AND CONSTRUCTION OF 60 WATTS POWER AMPLIFIERS