# Adjusting the voltage for a small speaker

I have a 4Ω 3W speaker and I plan to use it with MAX98357A amplifier. I need to make sure the speaker receives optimal voltage. When I test the voltage output from the amp playing nothing, I get 2.5V. I assume this is how zero level looks like, and when sound starts playing, it would oscillate between 0 and 5 volts. I don't have oscilloscope to confirm this, though.

From the P=V²/R I can calculate the voltage: V=sqrt(P*R)=3.46 volts. I am not sure whether that is safe with my amp that gives 2.5V at rest. I am looking for advice whether I should adjust the voltage coming from the amp.

• Both output pins on the MAX98357A are at 2.5V with silent input. Ignore that 2.5V DC - it doesn't affect your speaker at all.
– JRE
Commented Nov 15, 2018 at 11:52
• You adjust the audio voltage comming out of the amp by adjusting the volume control - higher voltage gives higher volume. Commented Nov 16, 2018 at 16:51

From the P=V²/R I can calculate the voltage: V=sqrt(P*R)=3.46 volts

That's equivalent to 9.8 volts p-p and, if you are prepared to accept a distortion level of around 10% then that is what the MAX98357A amplifier can deliver into a 4 ohm load. If you are only prepared to accept 1% distortion then 2.5 watts is what the data sheet says you can expect.

However, if you are using it for music then be aware that a sine wave test doesn't really tell you much because the dynamic range (peak to RMS) for music is greater than 10 dB (compressed music) and usually much more for speech and other forms of music. For a sine wave the dynamic range is 3 dB (peak is 3 dB above RMS level).

I get 2.5V. I assume this is how zero level looks like

Correct (for a 5 volt supply and MAX98357A bridge amplifier).

When there is no music playing you should have 0 volts at the speaker. This is achieved with a decoupling capacitor in series with the speaker. If the amplitude of the signal is 5V, the RMS voltage is 5V/sqrt(2)=5V/1.41 = 3.55V. The power delivered to the speaker is ((3.55V)^2)/R=12.6/3=4.2, which is just above the power rating of the speaker. You can try lowering the supply voltage of the IC for that.

• The MAX98357A has floating outputs. The speaker goes between the two output pins, not output and ground. No capacitor needed. The two output pins will both output 2.5V with a silent input.
– JRE
Commented Nov 15, 2018 at 11:49