LM386 Mini Audio Amplifier
A line-level input, a few capacitors, and an 8 Ω speaker — your first stand-alone power amp.
Difficulty: Breadboard. Estimated build time: about 35 minutes. Estimated parts cost: about US$2.50. 8-line bill of materials. Compare supplier offers when available. A pocket-sized audio power amplifier built around a single LM386N-1 in DIP-8.
Pin out the LM386
With the notch facing up: pin 1 = GAIN, pin 2 = inverting input (tie to GND), pin 3 = non-inverting input (signal in), pin 4 = GND, pin 5 = OUTPUT, pin 6 = VS (supply), pin 7 = BYPASS, pin 8 = GAIN. Place the chip across the gap of the breadboard. The two GAIN pins (1 and 8) set the closed-loop gain: leave them open for 20×, or bridge them with a 10 µF electrolytic for 200×.
Power and bypass
Connect a 9 V supply (or 4×AA pack) to pin 6 and GND to pin 4. Drop a 100 nF ceramic from pin 6 to GND right at the chip, and a 220 µF electrolytic across the supply rails at the speaker end of the board. Without that bulk cap the speaker will draw current spikes that drag the rail and you will hear motorboating — a low-frequency oscillation that ruins the audio.
Input network
Wire the 3.5 mm jack: sleeve to GND, tip through a 10 kΩ linear potentiometer to act as the volume control. The wiper of the pot goes through a 10 µF electrolytic coupling capacitor (positive lead facing pin 3) into pin 3. Tie pin 2 directly to GND. The coupling cap blocks the source's DC offset and the bias network inside the LM386 sets pin 3 at half supply automatically.
Set the gain and Zobel network
Bridge pins 1 and 8 with a 10 µF electrolytic for 200× gain. From pin 5 (output) place a Zobel network to GND: a 10 Ω resistor in series with a 47 nF ceramic. This snubber prevents the speaker's inductive impedance from causing high-frequency oscillation. Skip it and you might hear a faint hiss or, worse, ultrasonic ringing that warms the chip.
Output coupling and test
Connect a 220 µF electrolytic from pin 5 to one terminal of the 8 Ω speaker; the other terminal goes to GND. Apply power, play music into the jack, and slowly raise the pot. You should hear clear amplified audio. If it sounds harsh, lower the pot — the 200× setting clips easily with a strong source. If it hums even with no input, the input jack ground is not connected to circuit GND. The chip will warm noticeably at full volume; that is normal (the LM386 dissipates a few hundred milliwatts) but it is a reminder that linear amplifiers throw away the difference between supply and output as heat.