Class A, AB

From a Plinius manual:

"The Plinius [model] power amplifier operates in class A and is therefore capable of generating a moderately high temperature..."

Later in the same manual referring to the toggle switch I referred to:

"...A/B position reduces the bias on the output stage to operate in class AB. This is a bias configuration used by the majority of High End amplifier designers for their products."

Sounds to me like it operates in class A as well as A/B.

Example to explain:
Class A: If amp peak current is 10A then with no signal all 10A flows thru the + rail transistors also the - rail transistors and none thru the speakers. At half power 5A flows thru the speakers and 5A flows thru 1 of the transistor rails while 10A flows thru the other (depending on polarity of signal).
Class A-B: If peak current is 10A, then with no signal maybe only 1A (amount can vary according to how heavily biased it is) flows thru both transistor rails and none thru the speakers. Above 1A of speaker current one transistor rail turns off and the other carries the full speaker current (which rail depends on polarity of signal)

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.