I Have the Power…but How Much Do I Need?

Introduction
When dealing with live sound systems, a common question that I have both asked and been asked over the years is, “How much power amp do I need to sufficiently power my speakers?” Before I began to research this topic, I always assumed that you would want to buy a power amp with the same RMS power rating as the speaker that you were planning to power with it. For example, if you had a speaker with an input rating of 200 Watts RMS, you would want to power it with an amp that had an output rating of 200 Watts RMS per channel. However, it turns out that this assumption is not correct.

The Rule
After reading up on several Pro Audio forums, and checking the FAQ pages on the websites of several top power amp manufacturers, I kept seeing this same rule of thumb pop up:

When matching a power amp to your speaker, choose an amp that has an RMS power rating that is double the RMS power rating of the speaker/s that it will be hooked up to.

Double?!? That seems like overkill, right? That means if I have two speakers that are rated at 400 Watts RMS each, I would need an amp that is capable of producing 800 Watts per channel, or 1600 Watts!! It may sound strange, but that’s the rule.

Note that the rule assumes that you’re connecting a single channel of a power amp to a single speaker. If you are connecting multiple speakers to a single channel of a power amp, you’ll need to sum up the RMS power rating of the speakers, and get an amp that has double the RMS power output of the sum total. For example, if you are connecting 4 speakers to a stereo power amp (2 speakers connected in parallel to each channel), and each speaker has a 150-watt RMS power rating, you would need an amp that produces 1200 total watts of power:

((150 + 150) x 2) x 2 = 1200 Watts

Now, you may ask the question, won’t using an amp with that much power destroy my speakers? And the short answer is, probably not. Most professional loudspeakers are robust enough to safely handle more power than their RMS power rating. In fact, you’re more likely to destroy your speakers using an amp that doesn’t have enough power than by using one with too much power. The reason is simple: clipping.

Clipping
According to Wikipedia, “clipping is a form of waveform distortion that occurs when an amplifier is overdriven, which happens through attempts to increase the voltage or current beyond its maximum power capability. Driving an amplifier into clipping causes it to put out power in excess of its published ratings, which are customarily done with a ‘clean’ sine wave signal just at the onset of clipping.”

What that very eloquent description does NOT tell you is how much excess power can be produced. It’s more than enough to cause serious damage to your speakers due to over-excursion (causing the speaker to move beyond its intended mechanical limits) and/or overheating (increasing the internal temperature of the speaker/driver/tweeter until one or more of its components fail).

For this simple reason, it’s much better to use a power amp that has more than enough power, because it prevents you from being tempted to overdrive the amp to get sufficient volume out of your speakers, which can inevitably lead to clipping (which is waaaay too much power for your speakers!).

Conclusion
In conclusion, when matching a power amp to a speaker, remember this simple rule:

The output rating of the power amp should be DOUBLE the input rating of the speaker/s to which it will be connected.

Now, for those of you who thought you’ve been in danger of overpowering your speakers, you’re in good shape. But for those of you who purchased power amps by trying to match the power rating exactly to your speakers, good luck shopping for a new power amp!

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