How do you match the power ratings?

[msilvestri]

How do you match the power ratings for an amplifier and speakers? From what I've read, this is different for guitar/bass amplifiers and PA systems.

Weber seems to say the power rating for a speaker and a guitar/bass amplifier should be the same. Also, the power rating of the guitar/bass amplifier should never exceed the power rating of the speaker.

Weber - Let's Talk Speakers

JBL says the power rating of a power amplifier should be about 2 times the power rating of the speakers.

JBL - Frequently Asked Questions

Is this correct? If so, why are guitar/bass amplifiers and PA systems different? Additionally, how would you match the power ratings for a power amp run through a bass cabinet?

This is my first post, so thank you for your patience.

-Mike

[Gibsonman63]

For instrument amplifiers, you want to be able to turn the volume up loud enough to creat musical distortion, which can be most of the power available from some amps. For PA, you want clean headroom and lots of it. A bigger amp recovers quicker from the transients.

[zero cool]

Here is the sticky on the subject....

You can take a 10 watt speaker, connect it to a 1000 watt amp and as long as you never physically over extend the speaker or run it into thermal limit...it will run all day.

however, if you take a 1000 watt speaker, connect it to a 10 watt amp and run the amp into hard clipping. you could burn out the 1000 watt speaker.

HEAT is a speakers enemy. Distortion creates heat in a speakers voice coil. heat melts glue, causes the coil to warp, enamel to bubble, coils to short etc etc etc. speaker goes POOF! this is bad mmkay.


SO...for a PA system where you KNOW it will get run hard. have occasional pops and loud plosives that can and will cause an amp to clip. you want enough power to be able to reach your desired volume level and NOT clip the amp. this means a lot more power then you think you will ever need.

and just a note about power...twice the power rating is not twice as loud. it is only 6db louder. sound is logarithmic and for something to "sound" twice as loud takes about 10 times as much power. so...if you have 100 watts and you want it to be twice as loud...you need 1000 watts!!! this is all detailed very well in any sound textbook. I recommend the Yamaha sound system handbook if you can find a copy.

So. lets say you have a pair of PA speakers rated at 200 watts RMS each. having an amp that is rated at 400 watts per channel INTO the speakers rated impedance would be ok as you have a bit of headroom...and be careful. manufactures play tricks with the power ratings... an amp that is rated at 400 watts into 4 ohms, is generally 200-250 watts at 8 ohms so you may or may not be getting your moneys worth if you don't read those specs carefully.

The trick is knowing where the speaker/systems limits are. if the clip lights are flashing...back it down. keep that amp OUT of clip. an occasional flash is ok. but if those clip lights are blaring away with every beat...you might be replacing parts soon!

Generally, you can't have enough power for the average bands pa. they are typically always underpowered for what a person really expects the system to do. if you think you need the 400 watt amp. save a few more pennies and buy the 800 watt amp.



A guitar speaker is a different subject. many are meant to be overdriven. a 50 watt amp can sound louder than a 100 watt amp as you can push it farther into distortion for the same volume level which "sounds" louder. but some people like the sound of a 25 watt speaker run hard on a 50 watt amp. when it comes to guitar amps...the rules pretty much go out the window...but for best bet. at least have a speaker rated the same as your amp....

[Chuck H]

You can take a 10 watt speaker, connect it to a 1000 watt amp and as long as you never physically over extend the speaker or run it into thermal limit...it will run all day.

+++

however, if you take a 1000 watt speaker, connect it to a 10 watt amp and run the amp into hard clipping. you could burn out the 1000 watt speaker.

Not in a thousand years.

HEAT is a speakers enemy. Distortion creates heat in a speakers voice coil. heat melts glue, causes the coil to warp, enamel to bubble, coils to short etc etc etc. speaker goes POOF! this is bad mmkay.

Heat IS the culprit in speaker failure. But running a distorted signal into a speaker doesn't create heat. Otherwise you couldn't run cranked amps into equally rated speakers. The speakers often outlive the amps. If, for example, a Marshall 100 watt head with the volume dimed (and therefore creating masses of distortion) were played into a 1000 watt speaker the speaker would simply recreate the distortion faithfully, as it should. This is supported by the fact that most cranked 100 watt Marshalls are played into 100 to 280 watt cabinets without speaker failure.

SO...for a PA system where you KNOW it will get run hard. have occasional pops and loud plosives that can and will cause an amp to clip. you want enough power to be able to reach your desired volume level and NOT clip the amp. this means a lot more power then you think you will ever need.

This is also a misnomer. You state that the amp is running hard enough to create pops and loud "plosives" (this IS clipping) so you want enough power to reach your desired volume and NOT clip the amp?!? Are you trying to say that it's best to run underrated speakers with a PA because they will be at their max output before the amp is? That's fine, but I disagree. Apart from that any amps power rating accompanies a spec'd distortion figure. The only wat to reach that output is with appropriately rated speakers... Did I miss something?

and just a note about power...twice the power rating is not twice as loud. it is only 6db louder.

Twice the power is 3dB louder. But your right that twice the power doesn't equal twice as loud. Considering the broad range of efficiencies in speakers it's entirely possible to get louder with a speaker upgrade than by doubling your power.

So. lets say you have a pair of PA speakers rated at 200 watts RMS each. having an amp that is rated at 400 watts per channel INTO the speakers rated impedance would be ok as you have a bit of headroom.

It absolutely would not be OK. You could easily blow the speakers to smitherines trying to get 400 watts per channel out of them. THEIR 200 WATT SPEAKERS!!!!!!

The trick is knowing where the speaker/systems limits are. if the clip lights are flashing...back it down. keep that amp OUT of clip. an occasional flash is ok. but if those clip lights are blaring away with every beat...you might be replacing parts soon!

Excuse me, but what the hell do the clip lights on the amp have to do with the speaker system limits??? But for the amps sake no, you don't want to see the clip indicators activating.

Generally, you can't have enough power for the average bands pa. they are typically always underpowered for what a person really expects the system to do. if you think you need the 400 watt amp. save a few more pennies and buy the 800 watt amp.

Lets see... So if I wanted an 800 watt amp I should save for a 1600 watt amp instead, is this your logic? What? No, more power won't hurt if you need clean reproduction. But there is a reason for smaller sized PA's. Not every act or venue needs unlimited power for clean reproduction. And I think most musicians with some experience do pretty well at knowing what they need. Otherwise the guy at te store can most likely advise. By your logic of bigger is always best and everyone is under powered I should be lugging my 1500 watt PA to coffee shop gigs.

A guitar speaker is a different subject. many are meant to be overdriven. a 50 watt amp can sound louder than a 100 watt amp as you can push it farther into distortion for the same volume level which "sounds" louder.

What??? A 50 watt amp can sound quieter than a 100 watt amp too if you turn it down. But it won't sound louder than a 100 watt amp overdriven. What the hell are we comparing here. Distortion does sound louder than clean. But the 100 watt amp is still louder. Because you can distort it too.

but some people like the sound of a 25 watt speaker run hard on a 50 watt amp. when it comes to guitar amps...the rules pretty much go out the window

No they don't. You'll blow up the speakers. I don't know anyone of intelligence that is running their amp into a speaker rated at half the amps wattage and cranking it. That's just asking for trouble. The rules still apply to guitar amps because watts are watts and limits are limits.

..but for best bet. at least have a speaker rated the same as your amp....

And you thought this was the "safe" closing statement... Ha... For a combo type amp with an open back the speaker is more free to excrude. Since most guitar amps have bursts and peaks well over the amps rated wattage when turned up to distorting it's best to use a speaker at 1.5 to 2 times the amps watt rating in open back combo's. For closed back speaker systems the that are somewhat protected against over excrusion matching watts between speaker and amp is fine.

Dude???

Chuck

[zero cool]

i just spent and hour replying to this and its gone now..i am not retyping that. chuck you misread much if what i was trying to say. I suggest you take a look at the JBL faq here Frequently Asked Questions for the short answer.

P.S you were correct 3db it was late. i was thinking twice voltage...

[Jazz P Bass]

Yeah, you have to watch the speaker literature, too.
Case in point.
Fender Blue Alnico 10". Everyone states this is a 25 watt speaker.
Hah.
Into a closed back cabinet, yes it is.
An open back cabinet derates the speaker to 15 watts.

[pdf64]

The problem as I see it with that JBL document is that in small PA systems with live bands, there isn't usually a guy on the sound desk, and during set up the gain levels tend to cranked to almost feedback, then backed off a little. It's then all too common for a mic to get moved, maybe knocked over and the system feedbacks back for several seconds it takes for the problem to get sorted. So for 5 maybe 10 seconds, the speakers have been exposed to double their power rating. To make it worse, the power rating will be for music equivilant signal like pink noise, rather than one frequency, so it will be taken by 1 driver rather than split between 2 or 3.
So for the small systems most of us might use, my view is that it would be better to have the speaker power rating well over the amp power rating.

[Gibsonman63]

The problem as I see it with that JBL document is that in small PA systems with live bands, there isn't usually a guy on the sound desk, and during set up the gain levels tend to cranked to almost feedback, then backed off a little. It's then all too common for a mic to get moved, maybe knocked over and the system feedbacks back for several seconds it takes for the problem to get sorted. So for 5 maybe 10 seconds, the speakers have been exposed to double their power rating. To make it worse, the power rating will be for music equivilant signal like pink noise, rather than one frequency, so it will be taken by 1 driver rather than split between 2 or 3.
So for the small systems most of us might use, my view is that it would be better to have the speaker power rating well over the amp power rating.

For my system, it really depends on whether you are talking about mains or monitors. My monitor speakers are generally running pretty hot, but I have enough amp to have power to spare. I kill the feedback with by EQing and I keep the interpretive dancers away from my mic by setting one leg of the tripod stand between my pedal board and my monitor. We try to keep the stage volume and monitors about the same from gig to gig.

The mains are a different animal. You want enough power and speaker to cover your largest gig with power to spare. Feedback in the mains shouldn't occur as long as they are pointing out, your mics are behind the throw of the speaker and you aren't playing at ridiculous volumes in a really small room.

[selectortone]

..... some people like the sound of a 25 watt speaker run hard on a 50 watt amp. when it comes to guitar amps...the rules pretty much go out the window...but for best bet. at least have a speaker rated the same as your amp....

Running a 50 watt guitar amp hard into a 25 watt rated guitar speaker for any length of time will almost certainly result in:

1. serious damage to the speaker

and

2. serious damage to the amp shortly thereafter.

[J M Fahey]

1) I do not want to be hard on *anybody*, specially someone who obviously tried to help on good faith.
2) However, I must say that quoting Internet pages is *not* a guarantee of truth.
Having a revered name signing them is no guarantee either, per se.
3) As much as I respect the late Ted Weber and find his work impressive, I find him a "practical guy" (which is very good) but not that strong on the theory side.
His explanation on "Alnico compression" is a good description of Alnico de-magnetizing; any speaker where that happens would practically become deaf-mute in seconds.
So, his theoretical explanations should be taken with a grain of salt.
The speakers themselves sound good, of course.
4) The JBL paper is simply misquoted and absolutely unrelated to our work.
It refers *specifically* (and they clearly say so) to Home speakers, used at, say, the Living room, and listening not to live sound but "canned" one (CD, LP, Tape, FM, whatever).
No matter if you have a 5 , 50 or 500 Watt system there, you will *not* use it over an *average* power of 5 Watts, with 50 Watt *very brief* peaks (we are talking milli-seconds here) every now and then.
Any louder and the Police will arrive in no time.
In that particular case , the 5 W one will clip all time and probably kill the tiny tweeters (that's what the JBL paper says), the 50W one will not, and the 500W one will probably sound "more transparent", noticeable only on Deutsche Grammophon or Telarc (or similarly exotic recording companies) direct-to-disc Symphonic Music (or unprocessed direct-to-CD , with just a couple high quality mics and preamps straight into the CD burner).
But on live PA, either at Yankee Stadium or Joe's Bar you'd better have strong robust overrated speakers or the show will be cancelled before starting, just at the sound test (or on the opening bar).
Besides, all Pro PA systems make good use of limiters and avoid clipping like the plague.

[Wilder Amplification]

In terms of PA systems, power amps are like a power supply. The trick is to have more power available than what you need and never run out. Most people think of amps in terms of them PUSHING power. They all say "Yeah I'm pushing 'XXX (whatever the max rated output power of the amp is) watts' when in reality this is backwards. The correct way of thinking of it is that the amp's output power rating is the MAXIMUM POWER IN RESERVE...not the "maximum power you're pushing".

Where most people make the biggest mistake is they look at the program power rating of the speaker, then get an amp that is rated to meet or falls just below the program power rating. However, the one thing they fail to realize is that an amp's power rating is a MAXIMUM CLEAN POWER rating. In other words, you have to run the amp at maximum to get that power.

Now...do we ever have to drive our cars with the pedal floored to go 40mph? Not even close! Just the same, you should never have to run your amplifiers maxed out to get the volume you need. If you're having to milk every last bit of power out of the amp you're underpowered and risk blowing drivers when you clip your amps.

So the trick is to use power amps that feature a maximum power rating that is well and above what the speaker's program power rating is. This way once the amp is putting out the program power, it's nowhere near maxing out and is well away from the onset of clipping. This makes things sound very clean, tight and punchy and your speakers will thank you from keeping DC out of the voice coil.

Another common mistake...maxing out the knobs on the power amps thinking that "you won't get all of the power output of the amp if you don't max them out". Far from the truth!

When firefighters fight a fire, do they open up the bail on the end of the hose all the way and control it at the hydrant? Absolutely not! This is essentially what you're doing when you dime your power amp knobs then run the trim controls sparingly (again another backwards way of thinking).

First off...if you're even needing to push the amps to full output to get the volume you need you're underpowered. Second off...those knobs are "pads" for the inputs. Most power amps only require a 1-2Vrms signal at the input to swing them to maximum output with the knobs maxed out. However, most pro mixers max out at 8-10Vrms output. With the power amp knobs maxed out, your amps will be pushed into clipping long before the mixer ever gets close to clipping and you end up with poor s/n ratio and wasted headroom. By setting the power amp knobs so that the mixer and the amp reach clip at exactly the same time, this forces you to have to use more gain at the mixer's front end "i.e. the 'trim' control", which is a good thing (always best to add gain as far upstream as you can for best noise performance).

[J M Fahey]

Sorry to disagree.

So the trick is to use power amps that feature a maximum power rating that is well and above what the speaker's program power rating is.

speaker's program power rating is usually stated as twice maximum continuous power.
You even go further , suggesting :

to use power amps that feature a maximum power rating that is well and above what the speaker's program power rating is.

Let's introduce some numbers:
A typical PA box rated at 500W "RMS" continuous power is quoted as supporting 1000W "program rating". (Just browse some brochures)
You suggest well above that, so let's say: we may be talking 1200/1500W? ... for a 500W rated PA Box: no way José.
You also suggest lowering the Power amp volume control , so the mixer works as a sort of very crude, farty, horrible sounding clipper? No way.
The way to go is either use speakers that stand the maximum power output and then some, or, the real way, used by every PA company around:
use robust speakers (as stated above) plus sensibly set limiters.
That, if speaking about a simple, school or small Pub type setup, because in any larger venue the system will be bi or tri amplified, with limiters in every band.

wasted headroom

??
I thought we were just trying to get that, a nice clean system with at least some headroom.

[Chuck H]

To add... If you can never push an amp into clipping for fear of blowing up your speakers there will most definitely be wasted headroom.

[Mark Hammer]

1) Power ratings are a funny thing. I've seen sets of little $10 plastic 3" computer speakers described as being "180W PMP". Yeah, right. That's not to say that one should dismiss any and all speaker power ratings, but they do require more qualification than a single or simple number, and a little more thoughtfulness when pairing them up with an amp. Certainly, I have never seen any speakers rated in terms of how long they could sustain a continuous high frequency at this wattage, that one, or the other one. Speakers are generally rated in terms of how much wattage they can tolerate before going "poof" like a fast-blow fuse. (See #4 below)

2) Speakers vary considerably in efficiency, and so do cabinets. The output power levels required to achieve a desired loudness level with speaker A in cabinet Q might be a mere fraction of what speaker B requires in cabinet P. So, an amp could be rated at 100W, and the speaker rated at 30W max, but you get all the volume you happen to need with 10W. Of course, it could also be that while the desired loudness/SPL is achieved with very modest power levels, the sort of speaker tone you want won't happen until higher wattage is fed to the speakers. At that point, a) you place those speakers in jeopardy, and b) you're obliged to play at volumes you or your audience don't like. In such instances, the smart move is to identify a less efficient speaker. Whether that speaker has the same, a higher, or a lower wattage rating is another thing entirely.

3) It works the other way, too. Years ago, a room-mate had what appeared to be a kickass stereo system...on paper...in comparison to my budget/homebrew system. Mine sounded better, though. Why? His Altec speakers were so efficient that it only took a mere 100-200mw from his power amp to produce the levels he wanted, and at that power, both the amp and speakers were running "non-linearly", with more distortion and spectral corruption than he would have had at 2W output. My speakers, by comparison, were less efficient, and even though he had 100W a side and I had a mere 20, MY amp and speakers were running nice and linear, as the designer intended.

4) Speakers vary in their ability to manage heat buildup. Typically, the narrower the gap between the voice coil and the magnet structure, the more efficiently the amp current is translated into speaker motion and the more efficient the speaker. But small gaps also increase the risk of heat buildup by friction, so unless the designer/manufacturer has implemented a means to siphon heat away from the voice coil (fins, ferro-fluid, or a near flawless "spider" structure that aligns the coil so perfectly, it NEVER touches anything at any volume level or frequency), there IS a risk of average power levels well below the maximum handling capacity of the speaker to cause damage of various types and possible failure. I'm not so sure that the "fatal 10W" level noted by zero cool above is entirely realistic, but neither is it a sure thing that the speaker is bulletproof against anything up to the power rating.

The analogy to consider is that of making a campfire, boyscout style. It is the speed of the stick-rubbing which causes a buildup of heat that is faster than the loss of heat. Each rub generates X calories, and the faster you generate them, per unit of time, the quicker the heat buildup, and the easier it is to reach critical burn temperature. So, while the odd extreme excursion due to a transient might pass enough current through the coil to burn it out like a fuse, or cause the wire connecting to the coil to fracture, momentarily exceeding the power rating (e.g., for 100msec here and there) is generally safe. In contrast, if I am running a steady stream of heavily distorted content into the speakers fairly continuously, whatever rubbing the voice coil does at that high speed (which is what sizzling treble IS, right?) provides little opportunity for heat buildup to drain off, and my voice coil CAN be damaged in some manner.

5) Voice coils and cones are two different things. Most contemporary speakers will have cones that can hold up against vigorous shaking and movement quite easily. Older paper cones with paper surrounds, however, can be a little frailer, and fracture more easily, even though the voice-coil is fine. The power rating is based on a new speaker, and the properties of the coil, so the power rating of a vintage speaker is not necessarily an iron-clad assurance of what the speaker can handle, and whether it is a suitable match for the amp.

[Wilder Amplification]

Sorry to disagree.

speaker's program power rating is usually stated as twice maximum continuous power.
You even go further , suggesting :
Let's introduce some numbers:
A typical PA box rated at 500W "RMS" continuous power is quoted as supporting 1000W "program rating". (Just browse some brochures)
You suggest well above that, so let's say: we may be talking 1200/1500W? ... for a 500W rated PA Box: no way José.
You also suggest lowering the Power amp volume control , so the mixer works as a sort of very crude, farty, horrible sounding clipper? No way.
The way to go is either use speakers that stand the maximum power output and then some, or, the real way, used by every PA company around:
use robust speakers (as stated above) plus sensibly set limiters.
That, if speaking about a simple, school or small Pub type setup, because in any larger venue the system will be bi or tri amplified, with limiters in every band.
??
I thought we were just trying to get that, a nice clean system with at least some headroom.

If you seriously think that's how it all really works, you sir...need to go back to sound engineering school. Not even the text books teach it the way you've explained it.