Pass Labs A/B amps vs Class A amp and Actual Power

Unsound, there are detailed measurements in the review by Stereophile and further explanation in the Soundstage review. The Stereophile measurements on the XA30.5 estimate a maximum 150 watts into 8 ohms, and 200 watts into 4 ohms after converting into AB. So, this little guy in 4 ohms provides the first 60 watts in class A, and the last 140 watts in class AB.

Peter, your description parallels what I hear. I believe many people underpower their speakers. The XA200.5 sounds incredible at high volumes. There is no strain whatsoever. It is relaxed, effortless, perfectly dilineated, and majestic. I think that recorded music has an intrinsic optimal volume, but now it sounds so good even when blatantly too loud. If I play Mahler 8 or Gotterdammerung for the neighborhood, the needles on the front will flinch slightly at 400 watts into 90db speakers at 4 ohms. The other thing that is remarkable is how good it sounds at very low volumes, even the bass. Whether at a whisper or a roar, the Pass maintains a firm grip on all the drivers.

Rtn1, I don't have access to those measurements. Am I correct in assuming that those measurements are suggesting 150 Watts per channel into 8 Ohms and 200 watts per channel into 4 Ohms? Do those measurements specify that the first 60 Watts into 4 Ohms are in Class A, or are you assuming that they are?
Again, though of a different design, Nelson Pass's Class A/AB Threshold amps provided the first 20% of out-put into 8 Ohms in Class A and the balance in Class AB, into 4 Ohms the Class A output dropped to 10% and the Class AB doubled it's output. In his Class A Threshold amps the provided 100% of their rated output in Class A into 8 Ohms, into 4 Ohms they provided 50% of their output in Class A as the balance of power output doubled into Class AB.

Is the "Class A" in an amp that switches to A/B at some point the same as pure Class A?

Brian, good question. There have been discussions here as to whether there is in fact such a thing as Class A/AB or for that matter even Class AB. Some here claim that there is either only Class A or Class B, but not combinations there in. It might be just that some amplifiers have part of their out-put having attributes of Class A. I don't have any where near the technical expertise to determine what is what. I will say that more often than not, those amplifiers that claim to have more Class A output, do sound better to me.

Unsound, the Stereophile review is archived on the Pass website. I think the new website was down this AM, but I found it on the old page. Perhaps your question is more technical than my knowledge goes. Nelson frequents several forums, seems to enjoy talking shop, and perhaps you should post the question or email him.

My understanding is that the original XA series was pure class A. The meter on the front did nothing, and there was no conversion to AB. When the amps ran out of juice, they were out of juice. Anyone who has compared the 'pure' class A XA series to the new XA.5 amps has remarked that the newer design outdoes the orignial XA in every way describable.