4 ohms and 8 ohms

Power delivered to speakers is equal to output voltage squared divided by speakers impedance. It means that power at 4 Ohm will be 2x power at 8 Ohms (if amplifier has the same output voltage). In reality amplifier's voltage drops a bit with lower ohms and power is not exactly proportional. At 2 Ohms it will be close to quadruple power if amplifiers current limit protection won't kick in. Loudspeakers have impedance changing with the frequency and often impedance of 8 Ohm loudspeaker (shown at 1kHz) varies from 4-16 Ohms with frequency. Don't pay too much attention to power - remember scale is not linear (to make it 2x louder you need 10x power). Higher impedance speakers are easier to drive and damp.

Forget the ratings for a second. An amplifier presents a voltage to the speaker. A speaker presents a resistance (ohms) to the amplifier. This pairing causes a current to flow from the amplifier to the speaker. The current is directly dependent on both the voltage from the amp and the speaker load. This is what defines the power - how much current flows into the speaker load. Or, correctly, how much current the speaker load draws from the available voltage. But defining the power presents a small problem.

One problem is the speaker does not present a steady load resistance because the speaker is not like a light bulb or a space heater, which is pure resistance. A speaker has, in addition to resistors, capacitors and inductors (coils of wire). The speaker driver itself is an inductor. So the speaker, as a whole, is an "inductive load".

Another problem is that the music signal is not a steady voltage. It's ac, which means the voltage's frequency varies. As the frequency varies, the load varies (because of the inductive nature of the load) and the power varies as well. So it is very difficult to define the amplifier power. However, if the speaker were a pure resistance of 8 ohms, the power would be constant over the entire 20hz to 20 khz range.

That's why amplifiers are specified for WPC not only for a given load, but a given frequency as well. For example "200 WPC at 1khz into an 8-ohm load, 400 WPC at 4-ohms".

The way to look at it is that the signal from the source is the "switch" that determines power.

Dear all:
Thanks a lot for all your information.

Reference "M1sst1 & Eldartford", for tube amp, what happen if wrongly plug 4 ohms speaker into 8 ohms output (not enough current)? or 8 ohms speakers into 4 ohms output (too much of current)?
Which ohms to use if the speakers rated at 6 ohms?

Reference "Kijanki", what would you suggest WPC for 4 ohms speakers with 84 or 85 dB?

Reference "Gs5556", looks like to add an a/c conditioner will bring up more steady current and produce more steady "WPC", isn't it?
It brings up another question. Does "hi-end" power cord really help to improve the "sound of music"? or just kept the current more steady into amp?

///// Guess I ask too many questions /////

Maybe someone can suggest a website or books that is easy to read and is easy to understand about my "questions / problems"?

Thanks a lot
Have a wonderful day

Cibr...

If an 8 ohm speaker is connected to the 4 ohm tap of the amp, the voltage delivered will be less than the amp could deliver at the 8 ohm tap, so the available power is reduced.

If a 4 ohm speaker is connected to the 8 ohm tap the current delivered will be insufficient for full rated power. (This is a bit like driving your car up a steep hill in high gear).

Either way, there will be no damage. Some folk claim that their 8 ohm speakers sound better on the 4 ohm tap. If you don't need full rated power this might be OK. However, the load presented to the output tubes via the transformer will not be what the designer intended. It will be too low (High impedance...confusing isn't it). I know that tube amps "don't like" operating into an open circuit (nothing connected) so perhaps this light loading may have adverse effects.