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

Hacdrag said:

none of the very informative replies, such as that from @MrAl were a waste, as I learned a lot from them.

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Note that MrAl only calculated power at the output of the amplifier as seen by the resistor and speaker. You can measure the current into the lm386 to determine if the system needs to consume equivalent or less power for similar audible volume levels because of various concumption in the chip.

Hacdrag said:

I wasn't aware of this. All vendor documentation states:

• 500mW maximum power output @ 8 ohms, without any mention of 4 ohms.

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https://www.ti.com/lit/ds/symlink/lm386.pdf shows in figure 6.1 that the output voltage limits at about 3.5 V pk-pk with a 4 Ohm speaker.

ZipZapOuch said:

From the Texas Instruments datasheet...

View attachment 148504

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The exact page I got it from

Curves on the datasheet of the LM386 amplifier show that it works fairly well driving an 8 ohms speaker but instead of double the power into a 4 ohms speaker, the amplifier wastes much more battery power (by heating) when driving a 4 ohms speaker than double the heating of when driving an 8 ohms speaker. the heating is not enough to cause damage.

Also, the 9V battery voltage will drop as it is trying to get power out of the amplifier.

Yes, the 4 Ohm load will cause a higher current flow which drains the battery faster with many type of audio amplifiers. It's only the switching type amplifier (such as class D) that would not show this behavior, as they would have an inductor which allows for an actual power conversion similar to a buck circuit. The inductor stores energy part of the time and provides energy at other times which allows a (theoretically) ideal power conversion. If you put 1 watt in at whatever voltage is available, you get 1 watt out at whatever voltage is required. That's almost never the case with a regular amplifier of lower class which might have to take in 2 watts to produce just 1 watt output.
I think we are talking about the non-class D (and up) types in this thread so we would see this problem.