LM358 Audio Amplifier
Single-supply non-inverting op-amp drives a small speaker from a 3.5 mm jack.
Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$1.75. 11-line bill of materials. Compare supplier offers when available. A single-supply audio amplifier using one half of an LM358 dual op-amp in a non-inverting configuration.
Power and bias
Connect the LM358P across the breadboard divider. Power it from a 9 V supply (pins 8 and 4). Build the virtual-ground bias network: two equal 10 kΩ resistors from Vcc to GND, with the midpoint going to the non-inverting input (pin 3) via a 100 kΩ resistor. Bypass the midpoint to GND with a 10 µF capacitor to stiffen the bias.
Input coupling
Connect the 3.5 mm jack ground to the circuit GND. Run the left (or right) channel through a 1 µF ceramic or film capacitor to the non-inverting input node. This blocks DC from the source and passes audio frequencies. Do not use a polarized electrolytic here—the voltage across the cap reverses with signal.
Set the gain
Wire the feedback network: a 1 kΩ resistor from the output (pin 1) to the inverting input (pin 2), and a 22 kΩ resistor from pin 2 to GND. This gives a closed-loop gain of 1 + 22/1 = 23×. Keep the inverting input bypass path short to minimise oscillation risk.
Output coupling and speaker
Connect a 100 µF electrolytic capacitor from pin 1 (output) to one terminal of the 8 Ω speaker. The other speaker terminal goes to GND. Mind polarity: the pin 1 side is the positive terminal. Without this capacitor, the half-supply DC bias at the output will flow through the speaker voice coil continuously.
Test and troubleshoot
Apply power and play audio into the jack. You should hear amplified audio from the speaker. If you hear only hum, check that the input jack ground is connected. If there is motorboating (low-frequency oscillation), add a 100 nF ceramic capacitor directly across the supply pins of the LM358. The LM358 saturates near the positive rail, so do not exceed the supply voltage range and keep input signals below 100 mV peak for clean reproduction.