Loudspeaker power rating vs amp output power

[desertracer]

Hi,

When I grew up, my dad had friend who taught him selecting HIFI components. I remember one thing he mentioned about the power rating of speaker / amp was: Get a higher power output amp with a lower speaker power rating combination. In this case, you don't need to drive your amp all the way to its max level (distortion?) and the sound quality is better than the other way round.

My dad used to have an audio setup of Crown preamp, Luxman amp x 2 (biamp) and AR-9 3-ways speakers. I forgot the rating for the AR speakers, but I do remember the Luxman amps can put out 200W per channel.

That was 20 years ago. How is today's technology? I read some comments from this forum suggesting using a 80W AV amp to drive a 100-150W speakers. How is it compare to the 20 years old setup topology?

 

[MDS]

The power rating of a speaker is the maximum recommended power and is typically very conservative; ie most speakers can handle in excess of their rated power for short periods of time.

One advantage of having more power than you will use most of the time is increased headroom to handle sudden loud transients (like that explosion in the movie that catches you off guard) without clipping. A lower power amp may have to be driven harder to achieve the same output SPL and thus is working very hard whereas the higher power amp will just be cruising along at a jog. Distortion increases as you near the limits of the amp and hard clipping CAN damage tweeters.

So the rule of thumb you mention is basically valid. Speakers can be damaged by driving them with ALOT more power than their rating (the big amp case) or by driving them with a distorted, clipped signal that is well within or even below its rating (the small amp case). So in general its better to have the extra power on tap vs driving an under powered amp to its limits.

Now whether you actually need 200 wpc or can do with less is another topic and depends greatly on speaker sensitivity, the room, and how loud you want it to be.

 

[mickster1972]

Along these same lines, has anyone had practical experience purchasing a 200 watt amp for speakers rated at 100-150 watts? I am considering doing this since my room is somewhat large (9,000 cubic feet) and my current amplification, 110W/channel via my receiver, just isn't cutting it.

Since I know people will ask, the sensitivity of the speakers (from Infinity) runs about 89-90 dB. I have a 7.1 setup and am considering the Outlaw 770 amp, using my Onkyo receiver as a pre/pro.

I have asked the folks at Infinity if this is advised, and they said no, but their literature for the speakers basically says what this thread indicates. I would hate to blow up my speakers... thanks!

[By the way, I don't plan to turn the amp up anywhere near max volume, and the Onkyo has a volume limiting submenu that I plan to engage if I purchase the amp. I know it might sound like this would answer my question posed above, but I want to make sure there are no other considerations I am overlooking.]

 

[markw]

Your Dad was a wise man.

As long as you remember audio rule #1 you'll be fine.

Rule # 1. If it starts to sound funky, turn it down now. Whether it be the amp or the speakers, something is complaining and will show it's displeasure in an unplesant and possibly expensive manner.

 

[rschleicher]

As someone stated above, the maxiumum power ratings of speakers tend to be pretty conservative, and are usually based on a power level that would cause thermal problems with the tweeters.

The reason for wanting a lot of power capability out of an amp is (mostly) to be able to handle short-term transient power peaks without clipping, and most speakers can handle much higher wattage than their ratings for short periods.

As an example, check out some old reviews of the BP10 and BP8 speakers on www.definitivetech.com. These reviews were done 10 years or so ago, by Julian Hirsch of the old Stereo Review magazine. One of his speaker tests was to see how much of a single-cycle burst tone he could apply to a speaker before "bottoming out" the drivers. For these speakers he was able to apply around 1,000 watts (single cycle) at both 1,000 Hz, and 10,000 Hz, despite their "recommended amplifier" ratings being 20-300 W, and 20-200 W, respectively. Most speaker drivers have one or more resonant frequencies where they would bottom out with much less power, but the odds of there being so much power at those specific frequencies in real music is low.