What do you mean by de rate?
That's a short question, but I am going to provide a long answer as I want to further clarify the point I made in post#108 and 114. By de-rate, I meant lower the rating, just like in the example I used how any AVR could have been rated 4 Ohm if the manufacturers wish to, simply by "halving" the 8 Ohm rated. Same thing with the continuous rating.
Let me use two numerical examples for your specific AVR, just to make things clearer for you:
AVR-5308 CI
Specs: 150 WPC, at 0.05% THD, 20-20,000 Hz, 8 Ohms
Using the power formula and Ohm's law you will find that at rated output, that is 150 W, 8 Ohm resistor load:
P = V^2/R, V = square root P*R = square root (150*8) = square root (1200) = 34.64 V
V = I*R, so I = V/R = 34.64/8 = 4.33 A
In order for Denon to rate this amp 150 W into 8 ohms, it has to be able to sustain 4.33 A minimum, into an 8 ohm resistor load.
Now if you want to find out what it
could have been rated, for a 4 Ohm load, consider the following facts:
1) We know the amp can output 34.64 Vrms (by the way, all V in the example I am using shall be in RMS).
2) We know the amp can output at least 4.33 A.
So that means the amp can deliver 4.33 A into a 4 Ohm load at voltage up to 34.64 V.
You can see that if the same 34.64 V is applied to a 4 Ohm load, the current would be:
I = 34.64/4 = 8.66 A, that is "double" that of the current into an 8 Ohm load, using the power formula again:
P = V^/R, or = I^*R, either way you will end up with (34.64^2/4, or 8.66^2*4) = 300 W
If the Denon had a large enough power supply, it would "double down" from 150 W, 8 Ohm to 300 W, 4 Ohm.
The problem is, most manufacturers don't put large enough power supply in their AVRs, or even high end power amplifiers so that they can "double down" every time you lower the load resistance by half, such as going from 8 ohm to 4 ohm, or 4 ohm to 2 ohm etc.
In reality, you can find that in many cases, a mid range AVR from the major brands, as well as power amps, will give you more output into 4 Ohm than into 8 Ohms, but not by a factor of 2 (double), but rather, 1.2 to 1.5X.
That's why you will find the on a bench test, your Denon will likely be able to do 220 to 250 W into 4 Ohm "continuously" and would remain stable, in a both channel driven test.
"continuously" does not mean literally continuously, it typically mean the test signal is a "continuous" sine wave, not a short duration pulse, and for an extended period that could be 5, 10, 15 minutes or longer but definitely not hours, though some manufacturers, the likes of Krell, McIntosh, Boulder etc., might in fact mean literally continuous.
Back to my original point about "derating" and "half down..":
Any mid range and higher major brand AVRs could have provided the users a 4 Ohm rated output if they were willing to specify a lower number, i.e. "de-rated", from say 100 W 8 ohm to 50 W 4 Ohms.
Back to your Denon, to summarize:
- It could have been "officially" rated 75 W into 4 Ohms, that is de-rated from 150 W to 75 W, because then the the output voltage would be lower by half, in order to keep the current the same, thereby avoiding "overheating" due to the otherwise doubling of the current flow in the 4 Ohm load.
- It is certainly capable of much higher output in 4 Ohm than into 8 Ohm, if measured on the bench, in 2 channel (or even 3) driven condition because most mid range AVRs have power supply large enough to sustain higher current for a minute or even a few minutes, before the protection system shut the unit down. The AVR-5308CI is a flag ship class AVR so it certainly can do it for a few minutes, but no, they could not have provided such a rating, either out of fear that users would misinterpret such a rating, and/or due to the standards (FTC, or others) they applied to their products.
One more point, we have been talking about RMS voltage and current, and average power use in bench tests using sine wave. Program material such as music would be a different story as music contents usually have much higher crest factor than sine waves. That's another topic for another day.
