LM386 Audio Amplifier Circuit: Mini Speaker Amp with Gain 20 to 200
The LM386 is a low-voltage audio power amplifier in an 8-pin package. It runs from a 9 V battery, needs only a handful of parts and drives an 8 ohm speaker at a few hundred milliwatts, enough for a desk speaker, an intercom, a radio receiver or as the sound stage of other projects. In this build you will make a mono amplifier for a phone or MP3 player, choose its gain, and add the parts that keep it stable and quiet.
Components Required
| Part | Value / Type | Quantity |
|---|---|---|
| Amplifier IC U1 | LM386N-1, N-3 or N-4 with 8-pin socket | 1 |
| VR1 (volume) | 10 kilo-ohm log potentiometer | 1 |
| R1, R2 (stereo to mono mix) | 10 kilo-ohm | 2 |
| C1 (input coupling) | 100 nanofarad film | 1 |
| C2 (gain, pins 1-8) | 10 microfarad electrolytic | 1 |
| R3 (optional, in series with C2) | 1.2 kilo-ohm for gain of about 50 | 1 |
| C3 (bypass, pin 7) | 10 microfarad electrolytic | 1 |
| C4 (output coupling) | 220 to 470 microfarad, 16 V | 1 |
| R4, C5 (Zobel) | 10 ohm and 47 nanofarad | 1 each |
| C6, C7 (supply decoupling) | 100 microfarad and 100 nanofarad | 1 each |
| Speaker | 8 ohm, 0.5 W or more | 1 |
| Input | 3.5 mm stereo jack and shielded cable | 1 |
| Power | 9 V battery or 6 to 12 V DC adapter | 1 |
Circuit Explained Stage by Stage
Stage 1: Input and volume
A phone gives a stereo output. R1 and R2 mix left and right into a single mono signal without shorting the two channels together. The mix goes to the top of VR1, the volume control, whose wiper feeds the amplifier through C1. The LM386 inputs are referenced to ground internally, so the non-inverting input (pin 3) takes the signal and the inverting input (pin 2) is grounded.
Stage 2: Gain setting
Internal resistors fix the gain at 20 (26 dB) when pins 1 and 8 are left open. A 10 microfarad capacitor between pins 1 and 8 bypasses an internal 1.35 kilo-ohm resistor and raises the gain to 200 (46 dB). Putting a resistor in series with that capacitor gives values in between; the datasheet shows 1.2 kilo-ohm in series with 10 microfarad giving a gain of 50. For a phone, gain 20 or 50 is usually enough; gain 200 is for weak sources like a microphone or crystal radio, and it amplifies noise too.
Stage 3: Output and stability
The output (pin 5) sits at about half the supply voltage, so C4 blocks this DC from the speaker. The larger C4 is, the better the bass reaches the speaker; with 8 ohm, 220 microfarad sets a low cut-off near 90 Hz. R4 and C5 form a Zobel network from pin 5 to ground that keeps the amplifier stable into the inductive speaker load and prevents high-frequency oscillation.
Low-frequency cut-off at output: f = 1 / (2 x pi x R_speaker x C4) 220 uF, 8 ohm: f = 1 / (2 x 3.14 x 8 x 0.00022) = about 90 Hz 470 uF, 8 ohm: about 42 Hz
Stage 4: Supply decoupling
C6 and C7 across the supply right at pin 6 supply the current peaks of loud passages. C3 on pin 7, the bypass pin, improves supply ripple rejection. Battery-powered amplifiers that motorboat (a low putt-putt sound) almost always lack these capacitors.
| LM386 pin | Function | Connection |
|---|---|---|
| 1 | Gain | C2 (and optional R3) to pin 8 |
| 2 | Inverting input | GND |
| 3 | Non-inverting input | C1 from volume wiper |
| 4 | GND | Ground |
| 5 | Output | C4 positive; C4 negative to speaker; Zobel to GND |
| 6 | Vs | Supply, with C6 and C7 to GND |
| 7 | Bypass | C3 to GND |
| 8 | Gain | C2 negative side |
How It Works
The LM386 is a complete small amplifier: a differential input stage, voltage gain stage and a class AB output stage biased at half the supply. With a 9 V supply, the output can swing only a few volts each side of 4.5 V, which limits power into 8 ohm to a few hundred milliwatts. The datasheet lists typical output power by suffix and supply, for example around 325 mW for the N-1 at 6 V into 8 ohm and around 700 mW for the N-3 at 9 V into 8 ohm. That is loud enough for a room but not for a party.
Assembly and Testing
- Use a socket and wire pins 4 and 6 first, with C6 and C7 directly beside the chip.
- Connect pin 2 to ground, C3 on pin 7, and the output network on pin 5.
- Start with gain 20 (pins 1 and 8 open). Wire the input jack, mixing resistors and volume pot with shielded cable, connecting the shield to ground at the amplifier end only.
- Power up with volume at minimum. You should hear at most a faint hiss. Measure pin 5: it should be about half the supply.
- Play music from the phone at about half volume and slowly raise the amplifier volume.
- Try adding C2 for gain 200 to hear the difference in loudness and noise.
Troubleshooting
- Loud hum: the input cable shield is not grounded or the input wires are long and unshielded.
- Motorboating or squealing: add or enlarge supply decoupling and check the Zobel network is fitted.
- Distortion at moderate volume: gain is too high for the source. Reduce the phone volume or remove C2.
- Chip gets hot with no signal: the circuit is oscillating at an inaudible frequency. Shorten the wiring and check the Zobel.
- Weak bass: increase C4 or use a speaker in a closed box.
Summary
With a few capacitors and a volume pot, the LM386 becomes a useful battery-powered speaker amplifier. Gain is set between 20 and 200 by the network across pins 1 and 8, the output capacitor blocks DC and sets the bass cut-off, and the Zobel network and decoupling keep the amplifier stable.