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tony3d
11-29-2006, 06:47 AM
I'm am unclear how to match a power amp to a particular speaker to get the most power without endangering the speaker. I know it's much better to use a high powered amp than an under powered amp, but how do you know when you have exceeded safe reserves? Say your speaker is rated at 250 watts RMS power handling, they could take much more peaks, so would you be safe with say 450 watts to make sure you don't clip the peaks? Is there a formula to figure this all out?

kexodusc
11-29-2006, 07:17 AM
I'm am unclear how to match a power amp to a particular speaker to get the most power without endangering the speaker. I know it's much better to use a high powered amp than an under powered amp, but how do you know when you have exceeded safe reserves? Say your speaker is rated at 250 watts RMS power handling, they could take much more peaks, so would you be safe with say 450 watts to make sure you don't clip the peaks? Is there a formula to figure this all out?
You're not likely to find a simple formula to plug in some numbers and find out how much power you need.
Truth is, that 250 watt RMS power handling ratings is usually pretty useless. It most often refers to the electrical ability of the speaker system to handle power, and ignores the mechanical. Sometimes vice-versa. In most speakers, 250 watts of bass would drive the woofers beyond their excursion limits. You'll get really crappy sound, or worse, damage.
It's not a terrible approximation, just not a universally accurate one.

I advise buying power to satisfy your desired listening levels in terms of SPL. This involves knowing your room size, the sensitivity of the speakers, desired SPL and factoring in some amp headroom.
I have a pair of speakers that are 87 dB sensitive (1 watt plays 87 dB at 1 meter). At 1 watt of power to each channel, in that room (8 feet from each speaker, speakers a few feet out of courners), they play about 85 dB. That's pretty loud. Not rock concert volume, but not bad.

Back in college we measured the ASO concert hall from various seating positions and generally got average SPL in the mid 90 dB's. That's pretty loud, some much louder peaks, but not often.
To get a 95 dB average SPL on those speakers in that room, I'd need 10 watts per channel. That's about as high as I go for average volume in my rooms (peaks can be up to 110 dB though). I quite often listen below even 80 dB in that room (still fairly loud). So I don't need a huge amp.
If I want 12 dB of headroom (nice round number) in RMS, I'd probably want a 160 watt amp...but we don't need RMS power for dynamic peaks, or headroom (though it is nice to have, don't get too stingy)...most decent amps, even entry level receivers can sustain short dynamic peaks - that's where that max peak power can be useful...I have an old NAD integrated amp rated at 40 watts RMS that will push these speakers above 105 dB in my room during peaks without any noticeable distortion or clipping. A very cheap 80 watt/channel sony receiver too. That's rock concert level area, good enough for me. You want it louder, you'll need more power.
My 60 watt Adcom amps are comfortable sustaining near 100 dB average SPL on these speakers, much beyond that, the distortion lights flare up.
Change the efficiency of your speakers for the better, and you may require less power, any less efficient and you'll need more. Larger rooms generally call for more power too to achieve apparent loudness.
When buying an amp, it's nice to have a bit too much, and terrible to find you don't have enough.

Feanor
11-29-2006, 07:45 AM
I'm am unclear how to match a power amp to a particular speaker to get the most power without endangering the speaker. I know it's much better to use a high powered amp than an under powered amp, but how do you know when you have exceeded safe reserves? Say your speaker is rated at 250 watts RMS power handling, they could take much more peaks, so would you be safe with say 450 watts to make sure you don't clip the peaks? Is there a formula to figure this all out?

Not to detract from Kex' more in-depth explanation, maximum speaker ratings are irrelevant in home application. You would never, ever listen in your home to speakers putting out 250 watts RMS, or even 100 watts RMS -- that would be insanely loud.

I gather it's true that severe or continuous clipped signal is very bad for a speaker, but bear in mind that severely clipping will be quite audible and you will presumably reach for the volume control very quickly to get rid of it. In home practice, the symptom of an under powered speaker is a compressed, non-dynamic sound at higher volumes versus lower volumes, it's not the frank distortion you hear with severe clipping.

I would say be concerned to avoid a compressed sound at the highest volumes you normally listen to. This will depend on the qualities of the speaker as well as the amp, but will have only a loose correlation with the speaker's maximum power rating.

AudioBack
11-29-2006, 01:00 PM
I used to have a professional amplifier that drove my system years ago. It was 250 watts into 8 ohms and 425 into 4. My Monitor 9s are rated at 200 watts. I sent the amplifier into the red several times and never did any damamge to my speakers. It was extremely loud to boot. The sound quality of the amp was not great and thats why I switched back to my Onkyo at the time that didn't have nearly as much balls. I since then got a Denon that is 90 watts per channel, and have had no problems with amplifier clipping. The Denon is definately truely more powerfull than the 100 watt Onkyo was. I know that the school of though is to have an amplifier more powerfull than the speakers rating is. This is seemingly true maybe for music that has more predictable peaks in the sound, but for movies.....well let me just say that my buddy turned up his system while watching Jurrasic Park 3 at the beginning credits, and a big dinosar step blew his subwoofer cones right out of the cabinets. He had 2 Cerwin Vega subs rated at 500 watts RMS hooked in paralel to an amp that did 1260 watts RMS into 2 ohms. If he had perhaps 800 to 1000 watts, he probably would have had quite a bit of clipping, but would have saved his subs. With music, it's easy to tell when you're reaching a recievers's breaking point and can turn down the volume when distortion becomes present either by the speakers or amplifier. On movies its usually too late when an explosion sends your amps clipping or your speakers blowing. I would have to say its best to keep it at or under the speakers rating. Besides, a good amp that is 250 watts per channel is usually pretty exepensive. I used to listen to music alot back when I had the pro amp, so I didn't worry much about peaks in the sound. If you have a sub that can do all the rock bottom lows and then high pass your mains, then the danger of a amp too powerfull goes down considerably.

tony3d
11-29-2006, 01:41 PM
I'm curious, which pro amp did you have?