Volt Amps

Unfortunately there isn't a simple formula for the actual RMS power achieved by an amp.

The actual RMS output power of an amp is measured using sine waves into pure resistive loads and depends on the precise details of the amps electronic output structures and power supplies.

If the electronics were perfect (i.e. no inductance and no resistance) then the values could be worked out theoretically, however in the real world they have to be measured at specific load values.

Unfortunately completely determining a match between speakers and amps is a lot more than just looking at the RMS power rating.  Even then with all the measurements you could ever take you'd still not be certain of a good match without listening.

Yes we do (me included) and no I won't, as Huge says, it's not that simple. Some cheap amps can deliver power at a distortion level so high you would be better off connecting filament lightbulbs to the amp and reading sheet music via the 'fuzz' generated rather than listening!

A good rule of thumb for a well designed real-world power amp is for the power to increase by a factor of 1.5 or a bit more as the load drops by a factor of 2.

Lets say you have 3 types of metering equipment.

1 to measures Watt, Volts and Amps.

You connect them all to that resistor in the correct way.

You amplifier makes the voltage. Lets say 1V (to keep it easy).

The resistor being what it is, will demand 1A of current out off it (giving it a resistance of 1 Ohm).

When connecting speakers it all changes.

Speakers are an electrical motor so they do not behave like a resistor 

Applying the same 1Volt you will still measure 1 volt.

But the meters for power and and current well tell a whole different tale.

It is even possible to measure 0W and 2A.

So even when measuring 0W if you do the math it is also 2VA.

I also disagree about calling it transient power. Because it is something different.

Transient power is the ability of the amplifier to change it's voltage as quick as the inputvoltage of the source.

Thanks All, I couldn't fill in that blank, and forgot about distortion in amps completely.  I have a very basic understanding of amps but it does have gaps. I thought International Standards could apply to offer definitive values, I was wrong.

Krell produce power amps that deliver twice the power as the load impedance halves. Not sure how they behave as the power factor moves away from 1. Whether you like their sound is a different matter of course.

The Naim 500 seems to get close to the 1.5 rule outlined above. I presume the Statement manages to get closer to the factor of 2.

The slew rate seems to be a more important factor, ie how quickly the amp can deliver a massive demand for power. Naim have always seemed to concentrate on this aspect of design.

Yes a very, very, very well designed amp will be able to get close to the magic 2! I've never looked for the statement figures but they must be 'adequate' to say the least.

And don't forget, looking at the other side of the equation:

The higher the load impedance, the closer to the magic '2' any given amp will get.

Hi-fi top trump cards, the perfect stocking filler for Xmas! Brands and criteria ideas?