The Arcam in question certainly has me quite curios on how they achieve a class A operation output of 20W until it switches over. My own Pass X250.5 used to power the Revel's hit about 15wpc before going A/B, and are far heftier and run decently warm from the get go. The Arcam appears very light for running that many watts in class A, which often requires plenty of heat dissipation.
There's no reason that the Class-G ARCAM cannot output 20W/ch in pure class-A & then switch over to class-AB. It's more a question of how much output current delivery does the ARCAM have compared to your Pass X250.5. The ARCAM looks like a traditional class-AB design. From the size of the chassis & the heat sinks size it appears that the power transformer used is modest so one cannot expect high current delivery like the Pass X250.5 & that is one difference.
The other difference is that the class-G ARCAM probably runs nominally at a lower DC power rail. Unlike the the Pass X250.5 that runs at its max DC power rail voltage thereby dissipating more heat while idling. This the efficiency that ARCAM is talking about in their literature on their A39 product page. They call in the higher power rail only when the program material is playing & when the program material requires it. So, if the ARCAM is running off a lower supply rail then the heat dissipation will be much less than the Pass X250.5.
I once demo'd some ATC speakers using ATC amplification and they sounded wonderful. Then the dealer replaced the ATC preamp with one from another brand (sim audio I believe) and the sound was completely wrecked. I then realized how much the electronics can affect a very transparent speaker.
Earlxtr
good observation but the wrong conclusion.
IMO, that ATC speaker is a bad design since it is so sensitive to the electronics. A well-designed speaker should not be & if it is then there's something going on with its impedance. In such a case the manuf can make some electronics that can exactly compensate for that varying impedance such that the overall sound from the amp+speaker is satisfactory but if you remove the compensating electronics, as you did, the varying impedance is back & destroys the music playback. So, the user gets limited by what electronics one can use. This should not be the case.
This ATC speaker seems like a high-Q design - just a little off the peak & you fall down sharply into the depths of mediocre playback. Bad news!
