Class A, AB

And in class A mode, Plinius amps run hotter than you want to touch for very long, even while outputting no music. In class A/B, they are almost stone cold even while operating at amazing capacity.

Gyphon makes several amps that operate pure class A and consequently are very big, very hot , have huge power supplies, and only put 150-175 watts into 8ohm loads due to inherent class A inefficiency. Their power supplies do allow them to double down into difficult loads ( 300w 4ohms, 600w 2ohms ). There is a switch which allows for AB bias operation for "non critical" listening which saves electricity and heat. I never use it. I always run class A. Maich, the reason pure class A operation is rare in ss amps (even with a switch for AB)is for the above reasons and the expense of the power supplies, required capacitance, and heat sinks. It's nearly impossible to build one thats operates in pure class A with significant output that's less than 200 lbs or so and they get very very hot and use lots and lots of electricity. 95% of the market would rule them out on these grounds alone not to mention the $/watt ratio they represent to less picky consumers. Most people would not justify the sonic benefits they(class A) confer.IMHO they are great for difficult loads (4 ohm and under) or for high efficiency speakers. I personaly am hooked on these big ugly beasts. - Jim

Here's my question about the Plinius, and others, but I'll use the Plinius SA100 for example.

Rated at 100W stereo into 8 ohms. More into 4 ohms and all the way up to something like 650W into 4 ohms as a bridged monoblock. It would be silly to think all of that would be class A. How much, I not really concerned about but it does indicate that the load plays a role. It still draws the same 500W from the wall, in class A bias, which was about 40% efficiency for that 2 X 100 watts. I haven't bothered to put an ammeter in there to see if the draw jumps up when I get it loud. That would have to be VERY, VERY loud.

The origional Pass XA series (unique feedback/stabilization) and most tube amps are different. They don't vary output to the load, as much.

Aldavis:
I am the opposite orientation on the A vs. AB choice -- after some intial experimentation and excitement about the sonic benefits of the class A mode, now I almost never go that route -- not so much concerned about the electrical use as the heat factor -- that is troublesome in a way that is not so easily overcome. Also I will say that in the case of Plinius at least, they are phenomenally close in AB, so I guess that makes me a "non-critical" listener for the most part.

Ngjockey:
According to the SA100 manual (page 6) "The current from the mains/line supply by the amplifier in BIAS A is now approximately 1,000 watts." So looks like the current draw actually is up by quite some margin to use it this way, also should be constant (see below).

Elizabeth:
Page 21 same manual:
"A class A circuit topology is one in which the total current the amplifier is capable of delivering, is kept flowing in the circuit regardless of the demand....
In a Class A circuit, current draw should be constant therefore there is an absence of the power supply modulation common in Class AB design amplifiers."

This under the heading of WHY IS CLASS A BETTER? So again, it sounds to me like these amps are switchable from Class A to AB.