This circuit corrects the problem somewhat and allows a larger voltage swing and probably more output power. In this circuit, the 1K load resistor is tied to the speaker so that as the output moves negative, the voltage on the 1K resistor is reduced, which aids in turning off the top NPN transistor. When the output moves positive, the charge on the 470uF capacitor aids in turning on the top NPN transistor. The original circuit in the radio used a 300 ohm resistor where the 2 diodes are shown but I changed the resistor to 2 diodes so the amp would operate on lower voltages with less distortion.

The transistors shown 2n3053 and 2n2905 are just parts I used for the other circuit above and could be smaller types. Most any small transistors can be used, but they should be capable of 100mA or more current. A 2N3904 or 2N3906 are probably a little small, but would work at low volume. The 2 diodes generate a fairly constant bias voltage as the battery drains and reduces crossover distortion. But you should take care to insure the idle current is around 10 to 20 milliamps with no signal and the output transistors do not get hot under load. The circuit should work with a regular 8 ohm speaker, but the output power may be somewhat less. To optimize the operation, select a resistor where the 100K is shown to set the output voltage at 1/2 the supply voltage (4.5 volts). This resistor might be anything from 50K to 700K depending on the gain of the transistor used where the 3904 is shown.

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Stereo circuit  and mono circuit are given in schematic .LA 4555 is basically a stereo amplifier with 2.3 watts into 4 ohms speakers at a total distortion of 10%. With a bridge circuit, it can be configured as mono amplifier delivering 4.6 watts. It has an input impedance of 30K and gain of 51 dB. It has an excellent voltage range of 3 to 13 volts. Both mono and stereo circuits are shown here.

Pin out is given in Figure

Input is given at Pin 8 in the mono circuit and output is taken at Pin 11 through a capacitor (C7) of 470 uF to a speaker of 4 ohms. In the case of stereo circuit, input is given at 5 and 8 pins and output is taken out at 2 and 11 pins respectively for left and right channels through the blocking capacitors C4 and C7.

A detailed description of the pin out will help now and in the future.

In the stereo circuit,

C5, C2 are feedback capacitors, zohich dictate the lower cut off frequency.

CI, C6 are bootstrap capacitors. If the capacitor value is reduced from the recommended 47uF, output

at flow frequencies falls.

Cll, CIO are oscillation blocking capacitors. Polyester film capacitors are preferable.

C7, C4 are output coupling capacitors. Lower cutoff frequency depends on their value and quality.

C3 is the ripple filter or decoupling capacitor.

C8, C9 are the power source capacitors.

R2, R2 are oscillation blocking resistors.

ICs dissipate heat as they dissipate more and more power at more and more voltages. The heat must be removed continuously such that IC operates at rated temperature. Failure to do so will result in thermal runaway. If the IC is well protected, output power will fall to a safe area. If not it will eventually fail and pack up. Copper foil area is made as large as possible in the vicinity ofIC to dissipate more heat.

Then heat sink, thermally conductive material such as copper or aluminum is mounted on IC to remove the heat as it develops. It must be of enough size. In case ofLA4555, the IC has fins which are soldered to the PCB and a small heat sink also can be soldered along with it. Solder copper heat sink as shown in the figure below. Aluminum cannot be easily soldered. Method of mounting heat sink is shown in Figure.

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• January 30, 2010 -- LM 386 Audio Amplifier (2)

LM 386 is just right for a door phone application. It is very useful application in the present urban households. One condenser microphone and a speaker are kept conveniently outside and another set with amplifier are kept inside. With a door phone installed at the main door, it will be easy to recognize the caller outside the door. Nevertheless, it is still a better method for shooing away those indomitable salesmen with bodybuilder or body toner, or some other plastic gadget.

Circuit is shown in Schematic. The amplifier along with the power supply is kept inside fully accessible by the insider. One microphone and speaker are kept outside the main door such that the caller can speak into the mike and listen to the speaker. When the caller knocks the door, insider can switch on the door phone and ask him to identify. Then the insider can switch on the outside mike to allow him to speak. This activates the inside speaker but switches off the inside mike. When the insider releases this switch, inside mike and outside speaker will be connected. This is simplex system.

There are two condenser microphones (MIC1, MIC2) biased by R3 (10k resistor). Bias is switched by SI such that any one mike will operate. Mikes are coupled to the amplifier by C2, C7. VR1 is the volume control. Similarly there are two speakers (SPK1, SPK2) which are switched by S2 from the amplifier. Both these switches are operated by the insider and are ganged. The switches are ganged so that when outside mike is on, inside speaker will be connected and vice versa. Rl preset is a gain control, which you can adjust and leave it at optimum level. You still have volume control on hand. Please note that all these controls should be inside in the hands of the householder.This amplifier is used with a power supply of 12V full wave. Regulated supply is not required. Use good quality capacitors as bad filtering would result in annoying hum

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LM386 is one of the earliest audio ICs but is still going strong. It works with a few external components and is indispensable for general hobby circuits where audio amplifier of low power is needed. It is mini audio power amplifier which would prove essential as a test amplifier for solving typical problems in the audio circuits of TVs, telephones, etc. It can be used as an output driver for certain tone and oscillator circuits.The following circuit is useful as a test mike but can be adopted for any of those requirements.

Supply voltage can range from 4 to 12 V. Gain is internally set to 20 but it can be increased to any value up to 200 by a manipulation of external capacitor and resistor between Pin 1 and 8. Quiescent current drain is only 24 milliwatts at 6V operation. LM 386 N-4 can operate up to 16V and can deliver 1W of power into 32 ohm speakers. With these features LM386 is a superb startup amplifier generally used as an output device for a voice IC or radio receiver. You should make two such amplifiers to make it stereo. External line in can be connected at C4 after switching out the mike.

LM 386 has an internally fixed gain of 20 as in the case of the present circuit. It can be increased to 200 by bypassing IK resistor by switch SI. With resistor in place the gain is 50 and without both capacitor (C2) and resistor (R3), (i.e., by keeping both pins 1 and 8 open) the gain will be 20. These components actually bypass an internal resistor of 1.2 K across pins 1 and 8. .047uF capacitor (C5) and 10K ohms resistor between pin 5 and pin 1 will boost the bass frequencies and are optional. Pin out of the IC is given in Figure.

Circuit is shown in Schematic 38. Electret mike is powered by 10K resistor (R4) and is coupled to the amplifier by .47 uF capacitor (C4). R1 is volume control. Input is given at Pin 3 in the circuit and output is taken at Pin 5 through a capacitor (C1) of 470 uF to a speaker of 8 ohms. There is an additional Bass boost circuit given along side the main circuit which can be connected to boost low frequencies.

The circuit may be built on a Veroboard provided the precautions suggested in the earlier circuit are taken care of. Hence precaution must also be taken to avoid motor boating due to bad power supply filtering at the ICs.

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• January 31, 2010 -- LA4555 Audio Amplifier (0)
• January 28, 2010 -- A Collar Mike (2)

Preamplifiers are used to amplify low level signals such as those from mikes, tape heads before they are fed into power amplifiers. Power amplifiers are generally less sensitive. Frequency response also can be suitably trimmed and modified at preamp stage. LA 3161 is one of those widely used in tape decks and amplifiers as a stereo preamplifier.

Block Diagram is shown in Figure. LA 3161 has two low noise preamplifiers with good ripple rejection on chip catering to stereo applications. External part count is low and Single In line (SIL))(Figure 43) package makes mounting easy. Wltile the operating voltage is 9V, the IC can tolerate voltages up to 18V. Typical input resistance is WOK and output resistance is 10K with an open loop gain of78dB. Block diagram of the IC is given below. Input is given at Pin 1 and 8, output is taken at Pin 3 and 6, and negative Feedback is given at Pin 2 and 7. Power is at Pin 5 and Pin 4 is the ground terminal. There is an internal voltage regulator.

Ready made PCBs, even populated PCBs are available using this very useful IC. You can still build one, with a Veroboard provided proper care is taken about the ground returns. It simply means that one should not connect ground terminals of output and input at the same place. This will create serious oscillations and normal hobbyist will be left in the woods. Please read general instructions for working with amplifiers in the end.

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Microphone is added here for the very use it has, and by the ease of making one. It can be used as a good quality microphone. Here is a simple circuit that makes the condenser mikes work, which can also be used as a collar mike. Condenser mikes are rugged, have excellent frequency response, and they develop fairly high signal output. They can certainly be implemented to test later circuits. Make this and I am sure you will make many for many of your friends.

These ubiquitous devices are seen everywhere, in portable tape recorders to cell phones. These are small, sensitive, and rugged and have an excellent frequency response, and high signal to noise ratio. They deliver higher level of signal into preamplifier. It has an internal field effect transistor which needs power at less than half a milliampere. Condenser mike has two terminals; one terminal can be seen as connected to the body of the mike. The other terminal is the HOT terminal. It doubles up as an output terminal and power +ve terminal. A single pen cell can power it.

Circuit is shown in Schematic 35. Condenser mike is biased with a 3.3K resistor to power the internal FET. Output is also taken from this terminal to the Mike (MIC) input of an amplifier. There must be a capacitor at the input of the preamplifier to block the DC voltage from condenser mike. Preamplifiers generally have this capacitor at their input. Negative terminal of the battery is connected to the ground terminal of the mike. It is also connected to the ground of the amplifier through the shield of the connecting mike cable.

Connect 3.3 K resistor to the positive side of single cell battery holder with the other end of the resistor to the HOT terminal through a mike cable. Negative side of the battery holder is connected to the ground terminal of Mike: Solder a shield wire of required length to the

terminals of the mike. Central core wire goes to the hot or output terminal and the external shield goes to the ground terminal. The other end of the shield wire is connected to the appropriate connector suitable to the power amplifier. Use good quality shield wire and appropriate input jack pin suitable for the amplifier. Make two such mikes for stereo applications

Preamplifiers are used to amplify low level signals such as those from mikes, tape heads before they are fed into power amplifiers. Power amplifiers are generally less sensitive. Frequency response also can be suitably trimmed and modified at preamp stage. LA 3161 is one of those widely used in tape decks and amplifiers as a stereo preamplifier.

Block Diagram is shown in Figure 42. LA 3161 has two low noise preamplifiers with good ripple rejection on chip catering to stereo applications. External part count is low and Single In line (SIL))(Figure 43) package makes mounting easy. While the operating voltage is 9V, the IC can tolerate voltages up to 18V. Typical input resistance is 100K and output resistance is 10K with an open loop gain of78dB. Block diagram of the IC is given below. Input is given at Pin 1 and 8, output is taken at Pin 3 and 6, and negative Feedback is given at Pin 2 and 7. Power is at Pin 5 and Pin 4 is the ground terminal. There is an internal voltage regulator.

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• January 30, 2010 -- LM 386 Audio Amplifier (2)
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