LM 386 Audio Amplifier
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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Pin 1 of the chip is not marked on the circuit.