the ever-constant overpower/underpower debate

Posted: Fri Apr 11, 2008 1:25 pm

everyone knows the old "better to overpower than underpower" saying when it comes to passive speakers.

but WHY?

when your amp is way overrated for speakers , you use the trim knob to keep from blowing speakers, right? (for example, if your amp pushes 1000w and your speakers handle 250, then you'd keep the amp's trim knob pretty low).

but when your amp pushes less power than the speakers can handle, you dont want to clip, cant you just keep the trim knob pretty high. (for example, your amp pushes 300w and speakers handle 750, then you'd keep the trim just at the lower threshold of clipping, right??).

right? the only downfall i see is that it just wont be as loud. or is there an electronic thing i'm missing?

i want to hear from the pros on this one.

-dg

Posted: Fri Apr 11, 2008 1:29 pm

I wonder if Ken Kantor will jump in on this one.

Here's his reply from a previous post:

anxious wrote:

I've been on the speaker manufacturing side of things for years, so here is an answer from that perspective. It's a >really< complicated question, if you dig deep. Why?

1- There are very, very few speakers on earth that can stand even 100W of constant input power at a single frequency, (sine wave).

2- There are very few decent speakers on earth that would be damaged by a very short peak of 2,000W.

This means that it is tricky to state a speaker spec that gives the customer realistic guidelines for amps. If a manufacturer picks a low number sales will suffer, but also, many customers will not get near the full potential of the speakers. On the other hand, if a manufacturer states a higher number, a certain percentage of customers will be pissed off when they blow out their speakers.

Here's my advice:

If you are the kind of person who tends to push the volume limits, and the amp will be running flat out by the end of a long session, stick with a smaller amp. If you are the kind of person who pays attention to volume, playback distortion, etc, you can use a larger amp to get more out of the speakers on peak transients. Just don't run it flat out for hours.

-k

www.kenkantor.com

PS- you may encounter a very ingrained and persistant urban legend in the speaker world that speakers are actually safer with a larger amp, due to clipping. Having worked for the company that helped promote that idea in the 70's, I can tell you that it has since been proven false in 90% of circumstances. No need to get into that argument now, but there are very solid studies about this. Smaller amps are generally safer.

It's from:

http://recording.org/ftopict-41378-.html

Posted: Fri Apr 11, 2008 8:51 pm

rockstardave wrote:

everyone knows the old "better to overpower than underpower" saying when it comes to passive speakers.

but WHY?

when your amp is way overrated for speakers , you use the trim knob to keep from blowing speakers, right? (for example, if your amp pushes 1000w and your speakers handle 250, then you'd keep the amp's trim knob pretty low).

but when your amp pushes less power than the speakers can handle, you dont want to clip, cant you just keep the trim knob pretty high. (for example, your amp pushes 300w and speakers handle 750, then you'd keep the trim just at the lower threshold of clipping, right??).

right? the only downfall i see is that it just wont be as loud. or is there an electronic thing i'm missing?

i want to hear from the pros on this one.

-dg

Turning the knob on your amp does not turn up or down the amount of power you amp produces. It increases or decreases your sensitivity.

The things to consider. When matching power amps to speakers, you also must factor in the speaker's sensitivity, it's efficiency, cable awg, cable length, dseired headroom, etc.

By underpowering a speaker, you are not allowing for the power consumed by the driver's heat, wasted by the cable's length/awg, and you will have zero headroom. You are not likely to blow a driver by underpowering. Not with a state of the art modern amp. Back in the old days maybe. Speaker protection is pretty good now.

Posted: Sat Apr 12, 2008 8:12 am

sheet wrote:

By underpowering a speaker, you are not allowing for the power consumed by the driver's heat, wasted by the cable's length/awg, and you will have zero headroom.

now here's what i mean though... is that "underpowering a speaker", or is it "overstressing an amp"? which phrase is more appropriate?

Posted: Sat Apr 12, 2008 8:30 am

It is not underpowering a speaker. Think about the logic of that. How often in a music reproduction environment, that music is at a constant? It is not often. Music is dynamic. Are all of the drivers at their peak power capacity at all times No.

When people speak of this phenom, they do so with the assumptions of an amp out of headroom, clipping, distorting, with no driver protection. This comes from a generation of JBL/Crown engineers. It was a way to sell power and better driver science. Back then when the DC150 and 300 were current, it was a crap shoot either way, as both were known for shooting straight DC to drivers.

Consider guitar amps. Consider light bulbs. Case closed IMO. With modern drivers and modern amplification being what it is, the speaker is not harmed when you fail to provide it the maximum power before voice coil failure.

Posted: Sat Apr 12, 2008 9:08 am

sheet ok, thats great... thats exactly what i suspected.

is it bad for an amp, assuming that the amp is not constantly clipping?

Posted: Sat Apr 12, 2008 11:02 am

One important thing to keep in mind, and this is very important where I work (lot's of bad grounds and ride power that leaks across to the amp rooms) - the lower you can set your sensitivity, the lower you're also putting the noise floor.

I purchase amps that will, in most instances, overpower drivers by 200%, simply for this reason.

Posted: Sat Apr 12, 2008 11:16 am

For more info I suggest the Crown amp forums.

Here's a good post in particular, though there are plenty more in-depth ones you can read...

http://www.crownaudio.com/forums/index.php?s=&showtopic=2329&view=findpost&p=8532

Posted: Sat Apr 12, 2008 4:32 pm

Is it bad for an amp to underpower the speaker? No in principle. Why would you want to? You want headroom to handle transients, etc.

Posted: Sat Apr 12, 2008 7:01 pm

Q1: The power rating of an amp determines the maximum amount of "oomph" it can give to the speaker when the signal gets up to clipping? So when the signal is less than clipping the oomph provided is (Max_Power * Signal / Max_Signal)according to the signal level at any instant in time?

Meaning that if the signal peaks around -20 and floats around -30dB on the mixer meter, the amp won't overdriver the speaker so long as the regular output roughly matches the Continuous Input of the speaker?
(Almost the case with our FoH)

And obviously if the amp is miles under the speaker rating, you don't get enough drive and the speaker isn't loud enough to drown out a fart.
(Almost the case with our monitors)

Posted: Sat Apr 12, 2008 9:07 pm

Codemonkey wrote:

Q1: The power rating of an amp determines the maximum amount of "oomph" it can give to the speaker when the signal gets up to clipping? So when the signal is less than clipping the oomph provided is (Max_Power * Signal / Max_Signal)according to the signal level at any instant in time?

Meaning that if the signal peaks around -20 and floats around -30dB on the mixer meter, the amp won't overdriver the speaker so long as the regular output roughly matches the Continuous Input of the speaker?
(Almost the case with our FoH)

And obviously if the amp is miles under the speaker rating, you don't get enough drive and the speaker isn't loud enough to drown out a fart.
(Almost the case with our monitors)

Power ratings = smoke in mirrors. Power is a commodity now. Manufactuers lie about their power ratings, sometimes stating things that have not been tested and approved by the governing authorities, UL, etc. Books have been written on this. I will not reinvent the wheel.

Power ratings give us the amp's power draw, efficiency, distortion and power output at various loads over time. Unfortunately, many manufacturers are providing ratings without listing some of these critical stats. There is rms power (which is not the correct term) and apparent power. Then there is the real power available, power factor:

http://www.rane.com/par-p.html#power_factor

Power amp specs will not tell us how much oomph a speaker will have. Speaker sensitivity tells us the dB-SPL at one watt/one meter. From that you work backwards, factoring in the desired headroom, make up power for that consumed by the cabling (distance, gauge, etc) and desired SPL of the system at a specific distance.

Also, if you have inferior summing amps and/or gain structure, you might not get the bang that you desire either.

Posted: Mon Apr 14, 2008 4:14 am

OK so basically it's all guesswork and sales talk...
Well thanks for that, at least now I know.

Posted: Mon Apr 14, 2008 7:45 am

so if i have an amp that pushes 75w, i can hook it up to a speaker that handles 250w ... without worrying?

Posted: Mon Apr 14, 2008 5:01 pm

rockstardave wrote:

so if i have an amp that pushes 75w, i can hook it up to a speaker that handles 250w ... without worrying?

Yes, but I dunno about "without worrying".

It depends on what you do. It also depend on the speakers' efficiency and how loud they get.

Huh? Shocked

If you can be disciplined enough to accept the level that a clean signal will provide to the amp and speakers, then you'll be OK. But you won't be using the full capabilities of that speaker.

But, if you keep trying to goose "just a little more" volume out of it by boosting the input signal to the amp, or nudging up the EQ settings, etc., then you'll start asking for trouble. Even though you'll likely not feed it too much power, you may start feeding it too much distortion. Distortion can fry speakers, especially high-frequency drivers. And, it's even possible to damage the amp, especially if you try to make it louder by pumping something in that is more than the input was designed for.

Where on one hand you can blow speakers by allowing too much power, on the other, it's possible to fry them with a nasty signal at a lower power level. A higher powered amp is not bad if caution is exercised.

You may not need to "worry", but only a fool would abandon all caution with an over-powered, under-powered, or even a perfectly matched amp/speaker setup.

You just need to be aware and accept the limitations and capabilities of whatever is in your setup. If it isn't doing what you want, you can't make it do it. You just need to find a setup that works better for you.

(Hmmm..how did I come to know about blowing things up?) Wink

Kapt.Krunch

Posted: Mon Apr 14, 2008 6:42 pm

Along with that, think about the gain structure - too small of an amp will cause you to run the preamps and mains beyond unity, too large an amp will cause the opposite (unless you properly set the sensitivity of the larger amp(s) that is...).

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