Any rules of thumb for amp vs. sub RMS ratings?

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Any rules of thumb for amp vs. sub RMS ratings?

Postby Tacowner » Tue Nov 09, 1999 3:23 pm

This may be a dumb question, but....

Is there any general rule of thumb for matching an amplifier of a specific output to a sub of a specific RMS rating?

In my case, my amp states an output of 80W RMS and my subwoofer lists an RMS rating of just 150W, so roughly 1/2 the rated handling capacity of the sub.

SQ is my concern, not SPL. So, is powering a sub to just 25% of RMS too little, 50% just right, or maybe 75% a little high? (this is sounding strangely like little red riding hood) Image

Or does it all once again fall under, "user preference"?

Also, is there any easy way, read "for us idiots", to tell if I'm underpowering/overpowering a sub, or any speaker for that matter?

Just wondering....


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Postby DW » Tue Nov 09, 1999 5:06 pm

This is just my opinion so I may be wrong:

I don't think there is no rule of thumb as of the exact power ratio requirement between drivers and amps because Amps generally vary with performance.... as well as SUBS in that matter..
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Postby jlaine » Tue Nov 09, 1999 6:00 pm

With every sub there is a "threshold" of power where you will get accurate response and proper sound. As a general rule you don't want to be under this limit while in the SQ caterory, mainly because the sub cannot produce the sound clearly due to the insufficient input volume. The first rule is to make sure your amp passes this limit by a substantial figure. The minimum input for a sub varies by the design, due to cone weight, the power of the motor structure (BL), and the design of the enclosure that houses the speaker. "Matching" RMS values stated on the sub and the amp have become a common rule, but that can be misleading. Just because the value of the amp states that it can produce XXX watts RMS is nothing, the question you want to ask is how clean can it produce it at these levels. Now if you want to go to a "general" guideline, it is usually better to have an amp that puts out more than what the sub can handle, and have it tuned down to the limitations of the sub. You can drive an amp at clipping output for an eternity and do little if any damage, but if you "clip" your sub you will be lucky to use it again. If you have an amp that is clean and produces well over the limit of your subs, then you can tune the amp down to match your sub, but you can never tune down your sub to match your amp. If you have this you can listen for the first hints of distortion and lower the input of your amp so you know you are safe. That way you are getting the max from your subs, and your amp is not getting overworked. When the manufacturer gives you the RMS rating, they generally pad it generously. This is to save them on warranty work, and keep costs down. The rating they state is a value you can run constantly into the sub, hence you could push 150W into your sub, and it would never cook itself. (your motor will eventually wear out, but that is not usually covered by a warranty.) And besides, if you sat and ran a sub at its "max RMS value" and waited for the sub to wear out, you better have a good book and a few drinks. Now if you are worried about SQ, look into the frequency map for your sub, they usually come supplied with every sub. (LinearX maps are very common) Look at the frequency response in the area you are using your sub in. You should never really go beyond the -3 db range from where you are listening, simply because you will start to hear the valleys and curves in your subs response. Now if you want to know if you are under or overpowering a speaker, well, a trained ear is best if you don't have the equipment. Since your amp is rated less than what your sub can take, you will hear your amp distorting. Hearing distortion is easy enough, and in your case you don't have much to worry about when you do hear it, just turn your amp down until it is inaudible, then you will have it set. Running a sub at a % value of its max rating really hold little meaning, but you can have too little power! You should be able to run your sub at its max rating indefinitely and loose no SQ if the sub is rated well and designed well. So I guess in reality I took a page to say one thing- it is better to have a larger amp than larger subs if you want to push them to the limit.

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