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

I have an Atoll IN200 integrated amp (earlier version of this: https://www.atoll-electronique.com/en/products/integrated/integrated-in200sig/)

Having read a lot of praise about class A power, I’m wondering how to tell from specs when my integrated likely needs to switch from class A to class B?

The reason I’m curious is that I’m still wondering how my speakers might sound with a pure class A amplifier (such as Pass, etc.) but perhaps the integrated I have is already demonstrating much of that if there is a long enough delivery of class A power from it already? My speakers are 8 ohms, 87 db sensitivity, standmounts.

I apologize if this question is too simple; I hope I’ve posed it clearly.

P.S. This is not about dissatisfaction with my integrated or other gear (I have tubes, too). It's really just part of a longer term attempt to understand how different power sources change the character of a system, particularly the speakers.
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Showing 3 responses by jaytor

jaytor's avatar
jaytor
742 posts
 
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. 
jaytor's avatar
jaytor
742 posts
 
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. 
jaytor's avatar
jaytor
742 posts
 
I think they are jerking you around. Eliminating the electrical insulating material between the transistor and heatsink will reduce the thermal impedance, allowing a bit more of the transistors heat to be transferred to the heatsink, but this doesn't change the laws of thermodynamics. The heat still has to go somewhere. 

This would allow the heatsinks to run a little hotter without risk of damaging the transistors - say 60 or 65 degrees C. But even so, heatsinks this small would not allow nearly enough heat to be dissipated for 40W class A operation. And your case would be so hot that you wouldn't be able to touch it for more than a second or two.

Take a look at the Pass Int-60. This is rated at only 60w/ch into 8 ohm (although that's pretty conservative) and has rail voltages considerably less than your amp, which means the transistors need to dissipate less power at the same idle current. The Int-60 will do class A up to 30w/ch. Now take a look at the size of the heatsinks on this amp. 
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