Speakers with different nonimal impedance

[Mirage29]

Is it true that it is bad for a power amp to have speakers with a different nonimal impedance hooked up to them? I have a co worker who recenlty purcahed some martin logan speakers that are 4 ohms. Everybody knows that 4 ohms is rough on a receiver power amp. But his rear speakers are mirage nanosat which are 8 ohms. How bad is it for him to have 8 ohms and 4 ohms speakers hooked up to same power amp?

[HAMP]

It's not good for receivers, but should be fine with amps.

[Mirage29]

Originally Posted by HAMP

It's not good for receivers, but should be fine with amps.

I have heard many people say that it's bad for the power amp that is built into the receiver to have speakers with different Impedance hooked up to it.
Does anybody know why it so bad? What exaclty does it cause by doing this?

[ZoetMB]

By virtue of Ohm's Law (the current (I)drawn by a circuit is equal to the voltage (E) divided by the resistance (R), or in this case, the impedance (Z)), the lower the impedance, the higher the current draw. Power=Voltage (E) * Current (I), so you need more power for the low impedance speakers than for the higher impedance speakers.

This shouldn't be a problem if the receiver has enough power. The reason why people say it's "okay for amps, but not for receivers" is only because receivers tend to have lower power ratings than separate amps and in the case of separate amps, because they are separate the amps have their own power supply. But with the same power rating and as long as the receiver amps aren't being over-driven, it should make little or no difference. It's really no different than if all the speakers were 8 ohms, but the ones in question were always set to play much louder.

If you're using a receiver that doesn't have a lot of power and your friend tends to play at loud volumes, then it would be better if all the speakers were 8 Ohms.