True that a 4 Ohm amp can power 8 ohm speakers at half wattage?

[Ausghostdog]

Hi,
So I have been reading in a few places that an amp that states that it can power 4 ohm speaker at 80w per channel, can also power 8 ohm speakers at 40w per channel is this true even if the model manual does not list this compatibility?

 

[RayDunzl]

Hi,
So I have been reading in a few places that an amp that states that it can power 4 ohm speaker at 80w per channel, can also power 8 ohm speakers at 40w per channel is this true even if the model manual does not list this compatibility?

An amplifier creates a voltage across the speaker leads.

The speaker draws current according to its impedance (resistance).

You can calculate the expected values:

http://www.sengpielaudio.com/calculator-ohm.htm

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Your statement is probably true, as you state it.

If, however, you expect a 40W into 8 ohm amplifier to send 80W to a 4 ohm speaker, you might be disappointed.

 

[Ausghostdog]

An amplifier creates a voltage across the speaker leads.

The speaker draws current according to its impedance (resistance).

You can calculate the expected values:

http://www.sengpielaudio.com/calculator-ohm.htm

Thanks, so the amp I currently have has an output of 24v 6.67 amps according to your calculator, at 8 Ohms at 4w it will use 2 amps, at 17 volts so more than enough to power them

 

[Dzhaughn]

If you look at the impedence curves in the speaker measurements, you'll see the impedence is for most frequencies much larger than the speaker's impedence rating. The difficulty is with low impedence.

As impedence drops to zero, we approach a short circuit across the speaker terminals, which is so bad your amplifier blows a fuse to save itself and the house it lives in. Large impedence resembles running the amp with no speaker attached, which is just fine.