Hmmm that so about 4 ohm? Why particularly? Seems a silly range to design for, especially nominal ratings. Even many consumerl amps aren't all that great below 4 ohm. Buying an avr with low impedance speakers in mind seems sillier....failure problem.
We have been over this before. This is the issue, to really produce the output required in that all important 80 to 400 Hz range requires a drop in impedance unless the speaker is active. That is just the way it is. There is no way round it.
So here is the problem. If you drop impedance, then you will have higher current and more heating in the output device. And don't forget that the heat increase is not linear, but goes up by the square of the current. So why not increase the impedance? That means increasing the voltage and so the impedance then becomes higher above the half space/full space transition. That means the voltage has to increase. And again it is all against us. The voltage again has to increase by its square. Now modern output devices especially, have thinner semiconductor layers. This improves performance. However it makes them more vulnerable to the voltage punching holes through the semiconductor layers resulting in output device failure. It is far easier to deal with the heat problem, than the voltage failure problem. So actually 4 ohms below the transition frequency does turn out to be optimal and not eight. So the sweet spot is in that four to six ohm range for a speakers impedance. Now of course the speakers impedance is varying with frequency. That means a speaker manufacturer can quote any impedance he feels like. 90% are fiction, may be more. The only impedance that matters is the impedance across the power band. For most speakers that is four ohms and not eight. Unfortunately for most speakers the impedance rating is totally dishonest, or their are no third party measurements available. You will do not wrong however assuming any speaker with more than one bass, or bass/mid driver is actually a four ohm speaker no matter what 'nominal' impedance is claimed. The latter being totally meaningless.
So this really stacks the deck against the poor receiver. One power supply trying to drive multiple power amps in a small case. So current is limited, the output devices are too small and vulnerable the heating from high current and also vulnerable to excess failure from higher voltage.
The receiver really is a very bad concept from and engineering point of view.
Peter Walker of Quad who was a great mentor to me understood this instinctively. He was absolutely obsessed that his products have long life and very, very low failure rates. Despite his marketers begging for a receiver in the line up, he always refused, and that was when there were only two channels!
Probably 90% of his production is still in use. Old Quad units going back 50 years or more are easy to find in eBay. They continue to command prices above their original purchase price. I note in these difficult times the prices for those old units are actually increasing as people run for safety so to speak.
Now do you wonder why there are so many receiver failures and that they are not as group known for longevity?
There is an escape route, that is going away from class AB and going class D. However making them robust and reliable is not inexpensive. You also have the potential for RF radiation to high gain circuits and increased noise. But really getting the power amps out of the "receiver" really is the best solution.
Now if you understand this you come to understand why a receiver is a really, really bad concept. As you add channels it gets worse.
