Circuit Tutorials

 

500mWatt Audio Amplifier

The LM386 chip is a low power audio amplifier specifically designed for low voltage battery operation. The circuit will operate well with any supply voltage between 6V and 12V dc (The Maximum voltage the circuit can stand is 15V). An input signal voltage of only 100mV RMS is all that is required to produce a maximum continuous output of 500mW (using a 9v supply voltage into an 8-Ohm speaker). The circuit is so simple and straight forward that it is perfect for the beginner / hobbyist who is just starting out in electronics.

While this is not a high power amplifier by any stretch of the imagination, the circuit can be used as a small general purpose amplifier for a variety of purposes.

It is robust enough for experimental use as well as for test-bench equipment where an audible output is required.The kit contains all the circuitry needed to become a battery powered signal tracer: Just add a probe and a crocodile clip. It could be used as a small radio / tape / cd player booster, or for servo motor purposes.

Together with an audio pre-amplier kit from Professor ETK, it can be used as a miniature microphone or a low cost headphone amplifier.

Circuit operation 

Referring to the schematic circuit diagram on above, the operation of the amplifier can be followed as follows:

The signal is applied across VR1 which acts as a voltage divider. This potentiometer acts as a volume control and can vary the input signal from full to zero- amplitude.C1 acts as a decoupling capacitor, removing any DC present in the input signal. The gain of the amplifier can vary between 20 times and 100 times depending on the impedance between Pin 1 and Pin 8. This gain is increased or decreased by the amount of positive feedback conducted by VR2, R1 and an electrolytic capacitor C2. If VR2 is in its minimum position, then the gain will be about 150 times. If VR1 is set to maximum resistance, the gain will be about 50 times.

If C2 is simply removed, there will be no positive feedback at all, and the gain will drop to about 20 times.

The maximum output power will, however, not only be dependent on the gain but also on the internal dissapationtemperature, which in turn is governed by the speaker impedance and the supply voltage. The greatest continuous power output is optained with a (V supply and an * Ohm speaker.

The distortion level is acceptable if the chip is operated within its specified limits. If the amplifier is forced to develop more than 500mW, however, by too high an input signal or too low speaker impedance, the distortion level will increase dramatically.

Referring to the internal schematic, it can be seen that the inverting and non-inverting inputs are ground referrenced, while the output is biased to one half of the suply voltage- so no dual power supply is required.Since the LM386 operates on a single supply, there will be an average DC voltage of half the Power Supply (3-6V) on the output pin 5. Because of this, C4 acts as the main speaker de-coupling capacitor; it removes the entire DC component from the signal that is fed to the speaker coil.

C3 is required to dampen and stabilize the input stage of the amplifier, while R2 and C7 provide frequency compensation.The power supply itself is de-coupled and buffered by C5 (high frequencies)and C6 (low frequencies).

Construction:

The circuit is extremely easy to put together, just place the components as shown on the component overlay below. 

Solder each component onto the board in the correct place as shown below.

 

Component Overlay

 Ensure that the electrolytic capacitors (C2, C3, C4 & C6) are inserted to the correct polarity (positve lead to +Ve and negative to –Ve as shown on the overlay).

Solder the IC holder to the PC Board observing the correct orientation so that the IC pins will be inserted correctly later on.

Alignment:

When the assembly has been completed and checked, connect up your input signal and adjust VR2 to give the loudest signal possible without distortion. If there is a lot of didtortion at high volumes, the amplifier is probably being over driven. You should then reduce the input signal or attenuate it by putting aresistor in series with the signal lead in.

VR2 should now never have to be adjusted again, and you will be able to use VR1 as a volume control. If you need a panel mounted volume control, you can replace VR1 with a shaft potentiometer. If you have a little more bass than is needed, the value of C2 can be decreased.

The input sensitivity of this little amplifier module is 100mV (R.M.S) into an impedance of 47K, which makes it sensitive enough to be driven by any standard pre-amplifier or by the “Line” output level of any standard musical  instrument or cd/MP3 player.

Although it is not suitable as a microphone amplifier, it’s sensitivity is ideal to amplify the output of most signals and noise generator circuits. The module is particularly suitable as a workshop audio signal tracer since it can operate using a 9V PM3 battery together with a home-made probe for the signal input from the circuit under test and a crocodile clip for the OV input lead.

 

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