Unfortunately, this is one of those questions that seems simple, but is actually rather complicated.
The heat is actually generated inside certain components, then conducted to their heat sinks (or their casings), and finally to the air.
What counts - as far as the electronics are concerned - is the temperature at the actual silicon junctions inside the components.
And the ability of heat sinks to transfer heat to the air (and so get rid of it) depends on all sorts of things, including mainly the temperature of the air, and how fast the air is moving.
That second factor is especially important with amps that don't use cooling fans.
Because of the way convection cooling works, all of the "calculations" are based on the inside temperature of the amp running a certain number of degrees "above ambient".
With any device, the inside will
ALWAYS be hotter than the air outside; and, without fans, the difference will be greater.
AND that difference will depend on there being unrestricted air flow so air can get into and out of the cabinet.
AND you can make the same sort of calculations about a cabinet, or an "equipment closet", or a whole room.
Unfortunately, calculating "limits" on all this stuff can get very complicated.....
Some good safe guidelines are:
1)
You need to have "unrestricted airflow" around the equipment.
This means that you can't block the vents, and you need at least an inch or two of space on the sides with vents.
It also means that the air has to have someplace to go... it has to be able to move into the vents
FROM somewhere and out of the vents
TO somewhere...
This is true at
EVERY level; you can't block the vents on the equipment; and you can't seal your entire rack; and the closet the rack is in must have some sort of air flow.
2)
Assuming you've met that requirement, none of the equipment itself (the hottest spot on the top plate) should ever get too hot to touch with your fingers.
If you can put your hand there, and hold it for half a minute without screaming, then you're probably OK (that's about 135 degrees F).
So, if you have two amps in a rack, squeeze your hand in between them; if it hurts to do so, then more cooling would be in order.(And, since it takes a while for the heat generated by power amps to reach the outside of their cases, you should play those as loudly as you ever do, then keep checking the temperature.
It should never exceed that safe temperature limit AT ANY POINT IN TIME - and you should check it from about one minute to about a half hour after you turn the volume up. )3)
Excluding safety and "damage" per se, with most electronic equipment, the cooler you keep it, the longer it will live (and, usually, minimizing the changes between hot and cold - "temperature cycling" - also helps there).
(This is not quite an absolute, but many electronic components have some sort of "temperature derating" which says that, however long they usually live before they fail, that life span gets shorter at higher temperatures.)
Incidentally, even with tube equipment, where the tubes themselves run at relatively high temperatures, many other internal components have this same sort of relationship between temperature and service life.
4)
Always remember that heat takes a while to "work it's way from the inside to the outside", and that the amount of heat made by components like power amps depends on how much power they're producing. A Class A/B amp has some minimum heat that it produces all the time, based on things like bias current. Added to that is the "waste heat" generated by the output it is producing. With most Class A/B amps, including ours, the efficiency runs between 50% and 70%. That means that, if your XPA-2 is putting out an average power of 300 watts, it's making about as much waste heat as a 200 watt electric heater... and that heat will eventually have to be "gotten rid of". Therefore, after you play your amp very loud for a period of time, it will heat up.... and you must make sure you have enough cooling to get rid of
THE MOST HEAT THE AMP WILL EVER PRODUCE. (This is why things like thermostatic fans, which can rev up to remove a lot more heat if they sense it's necessary, are usually a good idea in enclosed areas.)
No info on the maximum safe operating temperature? My cabinet is staying under 95 degrees with an ambient temperature in the room of 80 degrees but I still want to know the temperature to start worrying about cooling and what the thermal shutdown temperature is on Emotiva Gear.
