This is probably a very trite question, but I want to make sure I'm not doing anything wrong.
So, got the above NAD amp hooked up to 2 Dali Lektor 6 speakers (according to the specs, they have a nominal impedance of 6 ohm). The NAD amp manual says the minimum impedance of the connected speakers should be 4 ohm or more (not sure whether that means 4 ohm per speaker, or 4 ohm in total?). So I should definitely be in the safe zone.
Nonetheless, after about 45-60 minutes operating time, the amp gets pretty warm (it doesn't get much warmer anymore after 45-60 mins, so in that sense it does reach a stable maximum). Warm as in: I can still keep my hand on the amp encasement without getting burnt and nowhere does it reach a point where I actually have to withdraw my hand, but it's still definitely much warmer than I'm used to from home compliances.
Question is: is this OK? And even if the heat doesn't harm my equipment, doesn't it still mean the amp's wasting a lot of energy (turning it into warmth I don't need)?
My NAD receiver gets very warm (about what you describe) just at idle. It gets slightly warmer when driving speakers to "normal" levels. I don't think what you describe is out of the ordinary for a NAD. Energy efficiency isn't a strong suit of Class A or A/B amplifiers.....maybe 20% efficiency.....the rest is lost, much of it as heat.
Also, some amps have more elaborate designs for cooling and/or heat destribution than others. If you're worried about the heat, make sure the amp has lots of space around it....particularly on top. You could even run a small fan around the amp to assist in cooling....but it would use even more energy, and make a little noise.
A/B amps are around 50% efficient at turning electricity into speaker power. The rest is heat. Some more efficient, some less. 'A' amps are awful. 20% or 25% tops. Class 'A' tubes? forget it.
Your NAD sounds OK. As Chris says, keep it ventilated with plenty of space above and Do Not stack.
In A/B amps the heavier the better, in general. Heatsinks cost money to buy and ship 'em around.
Yeah, I got my amp on top of my CD player, not the other way around (even though, strangely enough, most of the "official" pictures I see seem to show the CD player on top of the amp...).
I wouldn't stack any component on top of another. Keep them separate if at all possible. They'll sound better, and heat will dissipate more efficiently.
"The NAD amp manual says the minimum impedance of the connected speakers should be 4 ohm or more (not sure whether that means 4 ohm per speaker, or 4 ohm in total?)."
The warning is for each speaker in the stereo pair not the pair combined. Each channel's output devices sees only the speaker or speakers attached to that channel.
No matter what the warning from NAD states, as the impedance drops the amp must develop more current to deal with the load. This is simply a function of Ohm's Law. Since current represents the amount of work being accomplished (while voltage is only the potential for work to be done) the higher the current output into low impedance loads the harder the amp is working. The harder the amp works the hotter it gets as more energy is lost to the air. While the amp might not be shutting down or doing immediate damage to its components, a speaker with a higher and more benign impedance load would be easier on the amp and probably drop the heat a noticeable amount. In the end, since most consumer amplifiers are not very efficient and loose much of their energy to heat production you have to remember heat is the enemy of electronic circuits and excessive heat is a killer. Overall, I wouldn't say NAD has established a reputation for longevity in their modern line of components.