Axiom Home Page Forums General Discussion Home Theater How much power do I need - m60 measurements

Gena: thanks for input about the 703 setup. I have read the manual several times and just did not pick up that capability.

It never hurts to have a second or third set of eyes/ brains.

speaker: remember that the bottom line issue is not absolute dB or power. The bottom line is how much peak power do you require to accurately reproduce the music at your desired listening level (SPL) in your home theater.

I just don't think that there is a universal answer for this question.

Chooser, the basic 7 channel, 120 watt, 20-20KHz, 0.04% THD, 8 ohms rating of your 2400 can be relied upon as accurate since it's governed by the FTC power regulations and a manufacturer would be foolish to violate the regs with bad numbers. Under the regs the rating for a multi-channel unit has to be taken with at least two channels driven at the full rated power for at least five consecutive minutes. If nothing is specifically said, the rating is to be understood as being with two channels driven; manufacturers have the option to test with more than two channels driven(e.g. "all", although this isn't a realistic scenario for home, rather than laboratory, use)and will state that as part of the spec. However, the 500 watt power consumption figure has no connection with the maximum power rating. Unless the power consumption rating is specifically described as e.g. "maximum", it's an average figure derived during a UL required test for overheating with all channels driven at 1/8th power(considered to be roughly the average power consumed over the course of playing an entire CD). Although class AB amplifiers are roughly 50% efficient at full power, efficiency falls steeply at lower power levels and is roughly 20% at 1/8th power. 1/8th of 840 watts(7x120)is 105 watts, and at about 20% efficiency will require about 500 watts of consumption. So, there's no inconsistency between the seven 120 watt channels and the 500 watt consumption figure, although we read comments to the contrary by those uninformed on the matter

A little on power peaks. The IHF Dynamic Power rating is derived by using 20 millisecond tone bursts repeated every 500 milliseconds. Although this varies somewhat with the instrument involved, 20 milliseconds was adopted by the IHF as being a typical number.

Note again that if you'd like to know the requirements for power, that the two sources from Voelker and Linkwitz which were given earlier are very useful, since the entire sound pressure level from combined direct and reverberent sound has to be taken into account to get a realistic number. Since over half of the sound reaching a listener in a typical seating position comes from reverberent, rather than direct, sound, considering only the 6dB decrease of the direct sound for each doubling of distance doesn't describe the total situation.

John,
Thanks for the wealth of information. I really enjoy reading it.

This really has been an excellent thread!!!

Of course there are those who have measured receiver amplifier section output and found a substantial variance between actual output and rated output.

http://www.geocities.com/Area51/Hollow/3401/ratevsac.htm

2x6, note that the linked site, whether done intentionally or not, is misleading and has been a cause of unwarranted suspicion. The manufacturer's ratings are in the majority of cases with the FTC reg required two-channels driven, yet the site doesn't show that, but rather a five-channels driven number. The tests(including Stereo Review/S&V) have confirmed the accuracy of the ratings and any "variance" has been to the upside.

Your post moved me to google ratesvac to see if there were any posts which suggested that the site's tests raised "unwarranted suspicions," or are regarded by the audio community as misleading. I only went through 3 pages, but I did not see any discussions which would support your contention. To the contrary, several respected audio sites, and discussions refer to ratesvac test results as authoritative.

Your suggestion that it is misleading to test multichannel receivers for output with all channels driven confuses me. Seems to me that it is misleading for manufacturers to rate a 7 channel receiver at 100 watts per channel (or 7x100 watts) when the receiver is not able to produce even half that output with all channels driven.

I had a Sony STDB1070 receiver which Sony rated at 6x100 watts. Although I liked the receiver, and thought its processing section was pretty darn good for the money, I always thought its power output was seriously overrated. Sure enough, ratesvac tested the 1070 and found that it was putting out only 31 watts per channel. Now, it is difficult for me to understand how you can say that Sony's rating of 6x100watts (that was what the sticker on the receiver said) is proper, while ratesvac publication of the true 'all channels driven' output of 31 wpc is misleading. Perhaps you can explain that to me.

Read the law, counsellor.

Thanks for the explanation. Very helpful. Seems to me that if the law permits manufacturers to mislead the public, there's a law that should be changed. Also seems to me that such smarmy regulations which permit manufacturers to mislead the audio consumer should be criticized rather than embraced by folks who are interested in audio and in truth in advertising. But, hey, there are lawyers who live for the loopholes. I guess you think that's how it should be.