# Driving a 8Ω speaker from a 3.3v DAC

I'm looking for some advice on driving an 8Ω 2W speaker near full power from a 3.3v DAC. I've put together a current amplifier using a rail-to-rail op amp and two BJTs, but it's not supplying as much current as I thought it might.

In an ideal scenario, I was hoping to be able to drive the speaker up to ~1.25W, but in reality I'm only getting about 0.3W when putting 3.3v into the positive input of the op amp.

3.3V / 8Ω = 412.5mA = 1.36W

Wondering if anyone has any suggestions for improving this circuit or any alternatives. The DAC is providing a 3.3v output and my available power supply is 3.3v so I would prefer any amplifier I use to have a unity gain.

Thank you!

• Is that schematic correct? Your inverting input should be connected to the output for unity gain, not ground. Apr 9, 2020 at 16:13
• Assuming your opamp is actually wired up correctly, your transistors are emitter-followers - you'll never get then to produce a rail-to-rail output. Best case is you'll get 0.6V to 2.7V (and that's probably optimistic). Then consider that that gives you 2.1Vpeak-to-peak, which equals 1.05Vpeak or about 0.74Vrms (assuming a sine wave). That results in less than 70mW to the speaker ... Apr 9, 2020 at 16:14
• Divide that 3.3 V by 2 due to AC coupling though that capacitor and divide that with sqrt(2) to get RMS. Still loud enough? Apr 9, 2020 at 16:16
• @brhans you are correct I diagramed the schematic incorrectly. The circuit is wired up properly. Apr 9, 2020 at 16:32