Originally Posted by oztech
Where some AVR's fail is the more channels driven the advertised power drops off and thats why bench tests are important
Bench tests have their place and God knows I've bench tested enough amps for three or more people over the past 55+ years. However bench tests just generate numbers and interpreting those numbers is not trivial. There are even some non-obvious factors related to bench test numbers: LOTS!
There is a gigantic asymmetry between bench tests and actual use when it comes to "All Channels Driven" (ACD) measurements and actual use.
Actual use is with music or drama, not sine waves. Sine waves stress amplifiers several times more than music does. A key parameter is called Crest Factor (CF) which is the ratio between peak and average power. For sine waves CF is 3 dB. For music CF is always at least 6 dB for the worst case, and if you get into the real world of listening to music its up to 20 dB or more. Highly compressed music has a CF in the range of 10 dB.
What this means is that if I design an AVR to to ACD tests on the test bench I would have to put in from twice to 10 times as much power transformer and heat sinks as well as other parts as would be required for an amplifier that was designed to amplify music.
also some fall short when the impedance drops below 8 ohms
I know of no mainstream (Denon, Marantz, onkyo, Pioneer, etc.) AVRs that have any difficulty driving loudspeaker loads that go down below 4 ohms. The average impedance of speakers is maybe 150% of their rated impedance. So bench tests with 8 ohm resistors put that much more stress on the amp than real world 8 ohm speakers.
also the current marketing seems to want to advertise WPC at 1% distortion rather than.01.
I've seen that happen in the detailed portions of spec sheets but I again know of no mainstream examples. In fact all good solid state amplifiers can be reasonably be expected have less than 0.05% THD just below clipping. I've seen this be true even when the amp was speced at 10% THD!
Manufacturers fudge power ratings to push the amp into a little bit of clipping to produce most of the specs we see. The power goes up a little. BFD.