When does my class Atoll IN200 A/B integrated switch from class A to class B?

So, guys, all I can tell you is this is what they told me. It has dual toroids but...I'm just a lowly humanities guy. If they're lying directly to a customer, that would be pretty wild. Of course, their native language is not English, but they're an international company. Whatever. Another mystery.

Yeah, there is no way that this amp is running 40w class A. To deliver 120w into 8 ohms, the amp probably has rail voltages of approximately +/- 46v. To deliver 40w RMS class A, you'd need a bias current of about 1.6A to allow the amp to deliver 3.2A peak while staying in class A (assuming a push-pull output stage which is the most efficient for class A).

Across the two rails, that's an idle dissipation of almost 300W for the two channels. To support that, you'd need heatsinks probably 20 times the size of the ones in that amp, and it would still get toasty - probably too hot to hold your hand on it.

A class A/B amp with heatsinks the size of the IN200 is probably biased just enough to prevent the output transistors from completely turning off near the zero crossing - maybe a bit higher. I'd be surprised if it would stay in class A beyond a few hundred milliwatts. 

If you want to get a flavor for what a class A amp sounds like, you should keep an eye out for a Pass Amp Camp Amp. These are sold in kit form in the DIY Audio store for about $330, but regularly show up on the used market in assembled form for $250 to $350. It only puts out about 8w, so you won't be able to play your current speakers very loud, but it should give you a good sense of what class A can provide.

It's also possible to bridge the amp and use two as monoblocks if you find you really like it and want a bit more power.

If you buy a used one, you should be able to sell it for what you bought it for after you've played with it for a while. 

Thanks for the reality check. And the suggestion. 

I wrote to Atoll and conveyed the skepticism posted here about the Class A operation of the Atoll. They replied:

From Atoll engineers:
"Hello, Thank for for this information.
This is one of the most current mistakes people make about our amplifiers; they compare the size of our heat-sink to a single one; this is nonsense regarding to thermal dissipation because when you use a single, big heat-sink you have to put in isolation components that increase (a lot) the thermal resistor. For that reason, and on that point they are correct -- in order to have the same dissipation you would require an almost 10 times bigger heat sink for each channel; but with our trick, the transistors are transferring about 100% of their heat to the heat-sink with no waste of energy. You'll find attached a measurement that shows the distortion when power is rising; as you can see (in the picture), the distortion is under 0,002% up to 50W on both channel. Best regards. ATOLL Team"

I don't think I can post pictures here, but anyone who'd like to see it can P.M. me.