Questions about connecting 8 ohm amp to a 4 ohm speaker.

I'm going to be connecting a low-voltage 8 ohm amp module to a 4 ohm loudspeaker. Using the same amp and a different speaker, I've done this before without issues. I'm not expecting high-fidelity, just having fun with a project. A vendor suggested putting a 4 ohm resistor between the amp and the speaker which I haven't done in the past.

I have a question regarding the pure mathematics of the setups with and without the resistor. Will I get more or less power out of the amp and more or less volume out of the speaker with the resistor in series? The amp will be powered by a 9-volt battery. Will a 4 ohm resistor in series drain the battery faster?

Hacdrag said:

Will I get more or less power out of the amp and more or less volume out of the speaker with the resistor in series?

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You will likely get less power from the amp with the resistor, but more dissipation in the amp without the resistor at higher volume levels.

Hacdrag said:

Will a 4 ohm resistor in series drain the battery faster?

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At the same volume level, no, if it's a linear power amp.
If it's a Class D switching amp, then yes.

Hacdrag said:

Will a 4 ohm resistor in series drain the battery faster?

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If you keep the volume knob in the same position in both cases, then the battery will drain slower with the resistor in place.

If you turn up the volume knob when the resistor is in place to make both options play at the same audible noise level, you will likely drain the battery faster with the resistor.

ZipZapOuch said:

If you turn up the volume knob when the resistor is in place to make both options play at the same audible noise level, you will likely drain the battery faster with the resistor.

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Since the current through the speaker is the same in both scenarios, I don't see how there can be much difference in the battery drain with a linear amp.

crutschow said:

Since the current through the speaker is the same in both scenarios, I don't see how there can be much difference in the battery drain with a linear amp.

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It's a class AB amp, if this makes a difference to the answer.