What's the right size power amp for your speakers

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What's the right size power amp for your speakers

Postby TheBartman47 » Tue Jun 07, 2005 11:42 pm

I came accross this excellent article written by Marty McCann of Peavey.

The accepted standard for rating and testing loudspeakers was written by the Audio Engineering Society as Standard: AES2-1984 (r2003)

The AES came up with the standard that uses Continuous, Program, and Peak Power ratings. The reproduction of recorded music is considered Program material. When playing typical program material through a sound system, if the Peaks are just below where the power amp would reach full power, then the actual average level is about 1/8 power or -9 dB below that.

In other words the system has about +10 dB of headroom. It has been determined that with Live" music performances, if the system can be maintained to have an average of +10 dB of Headroom, then it should sound clean or undistorted to most listener's.

Now actually in the recording studio they do everything they can to maintain +20 dB of headroom above the average level, because when clipping occurs in the recording chain it is more noticeable.

Just to establish some boundaries, when a complex un-compressed musical event occurs, such as the simultaneous strike, of rim shot on a snare, at precisely the same time that the Bass Drum pedal hits the skin, and exactly in time with a cymbal crash, a very complex waveform is created.

The initial Impulse has a very quick and sharp rise time that makes up the attack of the leading edge or Peak of the waveform, the signal the quickly falls, but then levels off to an average short but sustained Crest level, before finally decaying. Often it doesn't even decay to complete silence before the next note or percussive strike.

Now in uncompressed Live Music the actual level from the extreme Peak to where it falls to average can be anywhere from +12 to +20 dB above the Crest.

So in my mind the AES standard is a guideline that doesn't quite encompass the most extreme possibilities of the young peoples music of today. So how can we use this standard as a guide so that we may determine what is indeed safe power for a given loudspeaker?

The first thing that needs to be understood and accepted is that in the hands of the inexperienced, stuff gets abused. Power Amps can be clipped. But what about DDT and other forms of "Soft Limiter's" in Power Amps? These unique circuits do indeed prevent the power amplifier from clipping unto itself. However if the signal arriving at the input of the power amp is itself clipped, the power amp can produce clipping at a power rating that is as much as twice it's full power rating for a clean sinewave.

If a Power Amplifier is driven into clipping it can produce twice its rated power. This is a mathematical reality. A Sinewave produces and average that is 0.707 times the Peak Voltage. If the Peak available Voltage available is 100 Volts, then the most that the since wave can produce before clipping is 70.7 Volts. If we put 70.7 Volt RMS into an 8 Ohm loudspeaker load we would have; (VxV/R = W) 70.7 x 70.7 = 4998.49 4998.49 / 8 = 625 Watts. If the Sinewave is clipped we would have a continuous squareware of 100 Volts; 100 x 100 = 10000. 10000 / 8 = 1250 Watts, or twice the power of 625 Watts at 70.7 volts.

Now to keep the Math simple lets say that a loudspeaker can handle 500 Watts of Continuous Power, 1000 Watts of Program Power, and 2000 Watts of Peak Power. Each increment is a doubling of power or a +3 dB increase. We tell you that if you are new to the business of providing sound, you should be conservative in your choice of available power. We say that you should have at least a power amp that is rated equal to the loudspeakers rated Continuous power level, but not more than twice that or what is called the Program power rating. Now if the inexperienced operator should introduce clipping and the amplifier is rated at 500 Watts, then it will produce a 1000 Watt distorted signal that is still right at the program level. Hopefully they will notice the distortion and do something about it, like turning the system down. Even if they don’t reduce the level, it will sound bad but shouldn’t blow up immediately. Unfortunately (for me) I have heard some DJ’s operate their system like this all nightlong. Of course I didn’t stay all night, but a few hours later I drove by it was still highly distorted and still working.

Now let’s say that another system operator has a 1000 Watt power amplifier for this very same 500 Watt Continuously rated loudspeaker. If this amplifier is driven into clipping it will produce 2000 Watts of distorted signal. Now if the operator has more than a clue that this distortion could result in loudspeaker damage and even possibly shorten the fun that evening, most likely he or she would turn it down. The system could survive a little of this, but just not long term.

However it this operator were a Trust-Fund Hippie who had more money than sense, and he or she choose to put a 2000 Watt amplifier on this 500 Watt Continuously rated loudspeaker, then as long as the average program power level did not go beyond 1000 Watts, they would be in high cotton. However if the 2000 Watt amp is allowed to clip even briefly, it will produce a 4000 Watt clipped signal, and he or she will need to take out their checkbook.

If you do know what you’re doing you can choose to use more than the program rating, we just don’t recommend this for most folks. Generally the listener, as in typical audience is not a critical listener, and they will not complain if the system is fairly well distorted. And quite frankly most musicians and many sound people don’t know the sound of a system under stress. Those that do, hear the bad stuff way before it becomes a risk, which is way below gross distortion.

I routinely use a GPS-900 power amplifier in Bridge Mode on our Peavey 44XT driver. I use it above 1.6 kHz with a 4th Order Linkwitz/Riley filter circuit and CD horn EQ, where it is rated at 100 Watts continuous, 200 Watts Program and 400 Watts Peak. The GPS-900 will produce 660 Watts of Continuous Power, now if I should clip it, that would be an immediate 1320 Watts of voice coil failure. However my average power is more like 20 to 40 Watts with +16 to + 12 dB of Headroom. There are not many whom I would let have free reign of this system.

How much power do you need? If you are new at this, you need as much as the Continuous rating of the loudspeaker system, and maybe a little more, up to the Program rating for many. If you feel you know what you are doing and know that you can recognize when to throttle back, then consider an amp that matches the peak rating. Don’t blame the manufacturer however if you have driver failure, if you go beyond the Peak power rating of the transducer.

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Postby dokai » Wed Jun 08, 2005 11:46 am

Excellent article! Thanks!
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Postby Hiram Garcia » Thu Feb 09, 2006 4:28 pm

I usually try to over power my speakers,if my speakers are 500 watts I try to run them at about 600 watts. It is better to over power by a little than under power. by under powering you can burn your speakers,I hope this helps. :shock:
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Postby CJ Greiner » Thu Feb 09, 2006 8:37 pm

That's a great article. It provides some nice insight into how to choose matching amps vs. speakers.

Thanks for posting it!
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Postby djmc » Fri Feb 10, 2006 1:04 pm

good article.

but it omits the difference of having highly efficient speakers.

simply put, a very efficient system (103 db) will sound great with a 250 watt (RMS) amplifier.

a similar sized 2-way system with lower efficiency (95 db), will need more wattage (perhaps 400 watts or more) to sound as loud as the more efficient speakers.
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Postby TheBartman47 » Fri Feb 10, 2006 2:01 pm

Actually, there is no difference in selecting an amp for a speaker based on its efficiency. Total power handling rating is what will determine amp size. Yes, a more efficient speaker needs less power to be as loud as a less efficient one, but that makes no difference in power amp selection.
If you have a 103dB efficient speaker rated 250 Watts program, and a 97dB efficient speaker rated 1,000 watts program, then select a 250 Watt amp for the first and a 1,000 watt amp for the second. End result is they will be equal volume (double the power for every 3dB difference), but the sensitivity rating still had nothing to do with your amp selection.
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