Mucho gras Al.Hi Jim,
Would you mind clearing up this thought of your’s?
Any amplifier having low output impedance can be rated to deliver twice as much power into 4 ohms as into 8 ohms, if the 8 ohm rating provides a large enough margin relative to actual capability
Margin? Section? Area? Range? Who picks them or it? The amp maker, Yes? No>?
Yes, I’m referring to ratings that are claimed by the designer/manufacturer.
I’ll explain further with a hypothetical example: During the design process the designer targets a capability of 120 watts into 8 ohms and 180 watts into 4 ohms, based in part on a belief that he can accomplish that with a design that will fall into a price range he wants to compete in. When he tests his initial prototype of the design he finds that it can meet but not significantly exceed those numbers, on a continuous basis (as opposed to just being able to meet them for say a few minutes without self-protection mechanisms being triggered), with reasonably low distortion, when supplied with an AC input of 120.0 volts.
He will then presumably want to allow some margin in his published numbers, relative to those numbers, to account for sample-to-sample differences that may occur in production, and to take into account reasonable differences that can be expected in line voltage, ambient temperature, and other variables. So let’s say that he decides to introduce a margin of approximately 16% (about one sixth) into each of the published specs, resulting in published ratings of 100 watts into 8 ohms and 150 watts into 4 ohms.
However **some** designers in that situation may instead choose to publish ratings of 75 watts into 8 ohms and 150 watts into 4 ohms, which as George has pointed out may help sell amplifiers because it can lead potential customers to consider the amp as being able to "double down." While from a marketing standpoint the downside of that kind of "specmanship" would be mitigated in the minds of many potential customers by the fact that 75 watts is only 1.25 db less than 100 watts!
The only sensible thing to do, If both are the same or quite similar topology, is to weigh the two final amp choices, and buy the heavier one.Weight is actually a criterion that can indeed be helpful to take into account in choosing an amplifier, IMO, as there does tend to be a correlation between weight and quality (albeit a loose correlation of course), among amplifiers having similar topologies and that provide roughly similar power capabilities. Unfortunately, though, weight also tends to be significantly correlated with price, given those similarities.
Best regards,
-- Al