If it's a well respected make and model, there won't be much difference. If the difference is more than say 10 to 15 percent, I'd look at a different amp.
Like many things, you can't really simplify it down to such a blanket statement. Look at the
EIA definition from QSC that I referenced, "This easily gives numbers 10 to 20% higher than the
FTC ratings." Looking at a few 'well respected makes and models', the difference into 4 and 8
Ohm loads is smaller, typically well within the 10% to 15% noted. However, for the few amps that publish 2
Ohm EIA and
FTC ratings, the difference between the
EIA and
FTC ratings for 2
Ohm loads can easily be 20% or even 25%.
As far as the benefit of specifying
EIA versus
FTC, well if your competitor has a product that is advertised as 1,000W
FTC do you think that publishing that your amp is 1,100W
EIA instead of 900W
FTC will help or hurt sales? Do most people notice, much less understand, the
EIA versus
FTC ratings or do they simply see 1,100W versus 1,000W? And the
EIA rating is a recognized standard
rating method so it's not like you're cheating.
So on one
hand, the most important aspect is comparing apples to apples. But there is still a factor that comparing
EIA ratings does not necessarily tell you what the amps actually do with both channels driven and a broader spectrum signal, you could still have one that has a much larger reduction than the other in the actual application.
Oh, and to make it even more fun, keep in mind that the amps are rated driving a very nice, purely resistive load that is linear over frequency. Unfortunately, speakers are far from being a nice, purely resistive load that is linear over frequency. I believe that how an amp reacts to these other factors is greatly responsible for the differences one may hear between similarly rated amplifiers. They may be rated the same sensitivity,
gain,
etc. but their transfer function into particular actual
speaker loads may differ.
AES has been trying to develop new
speaker and
amplifier testing standards for years, ones that represent more real world conditions and the actual measurements that matter, e.g.
voltage and
current rather than the resulting
power. One hurdle is that the manufacturers understandably would prefer to avoid having to retest their existing products to new standards. Another is that some manufacturers aren't sure if they want a different testing method that may show flaws in some products. And many manufacturers are concerned about how people would react to the numbers likely to result from the tests proposed, it would affect all products equally but how would the average potential purchaser used to amps and speakers rated at hundreds and thousands of Watts react if they were instead rated at tens of Volts and double or single digit Amps. It's the usual dilemma of providing technically accurate and valid data versus data that is easy for the average person to interpret (and is more acceptable to marketing since it leaves more room for creative interpretation).
