Amp power, speaker ratings and feedback...?

[ariscorkos]

Can someone explain why a more powerful amplifier through a given set of speakers will feedback less?

[tlbonehead]

Originally posted by ariscorkos Can someone explain why a more powerful amplifier through a given set of speakers will feedback less?

Maybe if the amp is clipping,it will make a difference. A lot of times it won't help though.

[soundtecwannabe]

Who said that? High quality speakers can help reduce feedback, I don't think having more power through the same set will help.

[agedhorse]

 

Originally posted by ariscorkos Can someone explain why a more powerful amplifier through a given set of speakers will feedback less?

 

Won't help one bit. Another "urban myth"

[Tommy Tune]

You'll get more headroom, though. That is, if you don't overdrive them and smoke em'.

[Audioeast]

 

Originally posted by agedhorse Won't help one bit. Another "urban myth"

 

 

Wellllllll.......i think what you experience is that the "more powerfull amp" is most likley of better quality an offers less color to the speaker therefore less feedback?????.....or ya just a crackhead......

[agedhorse]

Originally posted by Audioeast Wellllllll.......i think what you experience is that the "more powerfull amp" is most likley of better quality an offers less color to the speaker therefore less feedback?????.....or ya just a crackhead......

Dan... ya pushing my button???

[Audioeast]

Originally posted by agedhorse Dan... ya pushing my button???

 

 

heheheh, haaahahhahha,, he ha cough cough....yep

[imperial]

Feedback is a matter of frequency not power amplifier output. Although increasing gain may activate feedback.

 

Re: power. Ideally your power amp should be providing twice the RMS rating of your speaker system. You can have more, if you know how to manage your system.

[agedhorse]

 

Originally posted by imperial Re: power. Ideally your power amp should be providing twice the RMS rating of your speaker system. You can have more, if you know how to manage your system.

 

I disagree with the 2x RMS rule of thumb... I recone an awful lot of speakers from prople who do this then have one of those "special accidents", or do not adequately high-pass subs and face big $ damage! Also, with the absurd power rating games being played nowadays, the RMS ratings by many speaker manufacturers are just plain false. Especially when you consider the incrimental damage being done to the drivers suspension components by being driven beyond their elastic limits. This severely limits their lifespan, and all this for an extra dB or two.

 

For example, the 2241 driver is rated at 600w RMS, but in my experience with many hundreds of these drivers, they should be powered around 800 watts per driver max (regardless of the marketing departmet's guidelines). Even JBL powers them under 600 watts RMS in their own powered boxes! Maybe because it's their warranty problem?

 

I'll stick to my 1.5xRMS max recommendation (with the exception to the experienced pro user with plenty of PA for the job) as the satisfaction (long run) will be higher.

[imperial]

Maybe those people need to learn how to drive. You can blow a speaker with too little power as well under those same conditions that you describe. Besides that, I would think that you would want people to have 10x the power so that you could make a crap load from recones.

 

The reason that JBL does not follow that rule is multi-fold. The price point, the fact that the drivers have custom designed amps for that driver's bandwidth, AC draw, and the difference in 2x the power or 1.5 x the power isn't enough to bicker over either way, since it takes a 100% increase in power to get a 3dBA difference is SPL. And there will be no audible difference between the two.

[Craig Vecchione]

 

Originally posted by imperial Maybe those people need to learn how to drive. You can blow a speaker with too little power as well under those same conditions that you describe. Besides that, I would think that you would want people to have 10x the power so that you could make a crap load from recones. The reason that JBL does not follow that rule is multi-fold. The price point, the fact that the drivers have custom designed amps for that driver's bandwidth, AC draw, and the difference in 2x the power or 1.5 x the power isn't enough to bicker over either way, since it takes a 100% increase in power to get a 3dBA difference is SPL. And there will be no audible difference between the two.

 

 

The difference is that the only way an underpowered speaker dies is if the amp is both underpowered and overdriven. At least an overdriven amp can give some warning by displaying a clip light. A 6000w amp into a 500w sub will only indicate trouble by smoking:D. Hence the recommendation that newbs and mid-level soundguys use 1.5 or less, use your limiters and watch your levels.

[imperial]

I don't understand what you were trying to say about "the amp being under driven." I think you meant speaker, right?

 

It is possible to blow a speaker by underpowering it with an amp that is square waving. That's right.

[Craig Vecchione]

 

Originally posted by imperial I don't understand what you were trying to say about "the amp being under driven." I think you meant speaker, right? It is possible to blow a speaker by underpowering it with an amp that is square waving. That's right.

 

 

[semantics]

 

My statement was, "if the amp is both underpowered and overdriven" and it refers to the amp being too small for the task at hand. While the term, "underpowered speaker" is used a lot, it's more important for those unfamiliar with the overdriving/clipping problem to know that it's not the lack of power that directly causes the failure; it's the clipping causd by overdriving a too-small amp. Pretty much every week on here or one of the other forums someone makes reference to 'blowing a speaker if the amp is too small', and has no clue why this can happen.

 

[/semantics]

[agedhorse]

 

Originally posted by imperial Maybe those people need to learn how to drive. You can blow a speaker with too little power as well under those same conditions that you describe. Besides that, I would think that you would want people to have 10x the power so that you could make a crap load from recones. The reason that JBL does not follow that rule is multi-fold. The price point, the fact that the drivers have custom designed amps for that driver's bandwidth, AC draw, and the difference in 2x the power or 1.5 x the power isn't enough to bicker over either way, since it takes a 100% increase in power to get a 3dBA difference is SPL. And there will be no audible difference between the two.

 

 

Sure, learning how to drive is part of what this forum is all about... and I feel that it's important for the pros to share their knowledge with those who are trying to learn. There are plenty of pros who lurk here and jump in when needed. I think that's a really valuable part of this forum.

 

Yes, you can certainly damage a speaker by overdriving an underpowered amp into a speaker. Technically, a clipped amp will deliver considerably more power to a speaker than when driven just to the point of clipping... this can be up to twice the rated RMS power in cases of severe overload. Also, the duty cycle (or crest factor) will be altered which will change the thermal load on the voice coil.

 

I agree about the reasons for JBL are multifold, but the CE-2000 module that they use in the M-PRO is not a custom designed amp, the amp's bandwidth is no different than any bandpass filter that might be selected by a knowledgable user (crossover), and I do agree that the extra dB or two is not worth the reliability issues because it will be barely audible anyway.

 

I don't recommend 10X power because most of the guys here would rather spend their hard earned money on something other than re-cones. If they are asking the questions, they deserve accurate answers. (IMO anyway)

[imperial]

I was talking about EON's.

[L. Ron Hoover]

 

Originally posted by agedhorse Yes, you can certainly damage a speaker by overdriving an underpowered amp into a speaker. Technically, a clipped amp will deliver considerably more power to a speaker than when driven just to the point of clipping... this can be up to twice the rated RMS power in cases of severe overload. Also, the duty cycle (or crest factor) will be altered which will change the thermal load on the voice coil.

 

 

 

Yup.

A square wave has 2x as much energy than a sine wave of the same amplitude. Of course we're not dealing with pure sine or square waves in the real world, but the simplified analogy works.

 

The crux of the matter is that VC damage in these situations is still a "too much power for the driver to dissipate" issue.

The waveform isn't the relavent measure. A perfectly clean 600W put into a 500W speaker will blow that speaker just as fast as 600W from clipping an 400W rated amp into it. Conversly, if you had a 10W amp, you could run it at 100% THD all day into a 500W speaker without worrying about damaging the driver.

[agedhorse]

 

Originally posted by L. Ron Hoover Yup. A square wave has 2x as much energy than a sine wave of the same amplitude. Of course we're not dealing with pure sine or square waves in the real world, but the simplified analogy works. The crux of the matter is that VC damage in these situations is still a "too much power for the driver to dissipate" issue. The waveform isn't the relavent measure. A perfectly clean 600W put into a 500W speaker will blow that speaker just as fast as 600W from clipping an 400W rated amp into it. Conversly, if you had a 10W amp, you could run it at 100% THD all day into a 500W speaker without worrying about damaging the driver.

 

Yes, this is a good analogy to what happens. And this doesn't even touch on the limitations caused by the purely mechanical issues!

[L. Ron Hoover]

 

Originally posted by agedhorse Yes, this is a good analogy to what happens. And this doesn't even touch on the limitations caused by the purely mechanical issues!

 

 

Nope,

Barely scratching the surface!

Amplifier-speaker interaction is a very complex topic and there's a lot of false information and conjecture floating around about it.

[tlbonehead]

Originally posted by agedhorse Yes, this is a good analogy to what happens. And this doesn't even touch on the limitations caused by the purely mechanical issues!

What gets me is when you can obviously hear the subs bouncing against their limits and the owner/operator just sits there listening with no idea that anything is wrong. How can you not hear that?

[agedhorse]

 

Originally posted by tlbonehead What gets me is when you can obviously hear the subs bouncing against their limits and the owner/operator just sits there listening with no idea that anything is wrong. How can you not hear that?

 

By that time, much of the damage to the suspension is already done. It may "work" but not like an undamaged driver. The TS parameters will be totally different since the mechanical spring constant will have changed (gotten softer).

[imperial]

What crap.

 

The amplifier power rating has abosolutely nothing to do with feedback.

 

Feedback: The phenomenon where the sound from a loudspeaker is picked up by the microphone feeding it, and re-amplified out the same loudspeaker only to return to the same microphone to be re-amplified again, and so on. Each time the signal becomes larger until the system runs away and rings or feeds back on itself producing the all-too-common scream or squeal found in sound systems. These buildups occur at particular frequencies called feedback frequencies.

 

It is an acoustic problem. Any system can produce feedback, regardless of power amp size. You guys mention changing amp size without considering speaker sensitivity. You also do not consider the fact that to get an audible difference of 3dB, you would have to double the power of the amp. So by deceasing an amp by 100W will not help.

 

It is the location of the mics, their sensitivity and lack of appropriate EQ that contribute to feedback.

[L. Ron Hoover]

 

Originally posted by imperial What crap.

 

 

What are you referring to exactly?

At any rate, there's no need to be insulting about it. That's not very constructive.

[agedhorse]

 

Originally posted by imperial What crap. The amplifier power rating has abosolutely nothing to do with feedback. Feedback: The phenomenon where the sound from a loudspeaker is picked up by the microphone feeding it, and re-amplified out the same loudspeaker only to return to the same microphone to be re-amplified again, and so on. Each time the signal becomes larger until the system runs away and rings or feeds back on itself producing the all-too-common scream or squeal found in sound systems. These buildups occur at particular frequencies called feedback frequencies. It is an acoustic problem. Any system can produce feedback, regardless of power amp size. You guys mention changing amp size without considering speaker sensitivity. You also do not consider the fact that to get an audible difference of 3dB, you would have to double the power of the amp. So by deceasing an amp by 100W will not help. It is the location of the mics, their sensitivity and lack of appropriate EQ that contribute to feedback.

 

 

Generally, the higher the amplifier's rated power, the higher the amplifier's voltage gain (Av transfer function) in order to achieve this. Otherwise, it would take a +12dBu input signal to drive the 1000 watt amp to the clip point referencing to the 100 watt amp's +4dBu sensitivity at clip. Therefore, it would be possible that when substituting a 1000 watt amp in place of a 100 watt amp, and not changing anything else in the system, the system would feedback at a lower fader setting on the console because there is more overall loop gain within the entire system. By normalizing the amplifier's voltage gain rather than output power, I think little difference would be noticed with respect to feedback.

[imperial]

 

Originally posted by Audiopile Although I agree that the power rating of the amp in-itself has little to do feedback sensitivity of a PA system, feedback is more than an empirical acoustic problem... although acoustics is a prominent consideration. I'm under the impression that feedback (or lack there-of at high levels of gain) is complex and multifaceted which almost qualifies as art. Oftentimes, what initiates a feedback situation is not the resulting feedback. Example: walking across the floor of a wooden stage, subjecting the microphone to frequencies of maybe 10's of Hertz will trigger a feedback of maybe 5KHz in the system. The reason being: the low frequency is a natural subharmonic of the feedback sensitive frequency that's on the verge of oscillation. Once the Needed Acoustical Gain of a system approaches (or exceeds) the Potential Acoustical Gain (before feedback) of a system, you're always banging your head against the feedback envelope and any little trigger can set it off. This might be something as simple and obvious as inadvertently mis-positioning a microphone to a location that changes the PAG (like walking out in-front of a speaker stack); however, it could be something as intangible as an inadequately powered amplifier distorting when pushed hard which produces unperceived rogue (or fugitive) sounds that will trigger feedback oscillation. So, in a nutshell... sure... I accept the possibility that an underpowered amplifier could contribute to feedback where a higher powered amplifier would not at the same SPL (everything else being equal), and under a different set of circumstances, the higher powered amplifier might offer just the right feedback sensitive signal to the system that the lower powered amp wouldn't. Same goes for the speakers, the mics, the cabling (yes, the cabling can contribute), the stands, the stage, the humidity, the temperature, the lighting, and on, and on, and on. Yes, I said lighting too. Yes, I've had a situation where hitting the bump button on just one light triggered feedback oscillation, where-as, the same exact light brought up with a fader didn't trigger feedback oscillation... go figure? So I guess I could say bump buttons on lighting boards sometimes cause feedback more-so than faders.

 

 

Feedback, in the context of sound reinforcement, not guitar or amplifier designs, has only to do with acoustics. Nothing else. That's why in the Rane.com audio reference (compiled from 100's of sources from the industry) it is called Acoustic Feedback.

 

Two matching speaker systems with 98dB sensitivity (1 W @ 1 M), playing 98dB, use 1 W. If one speaker is power by a Crown Macrotech5000, and the other a Peavey CS800S, both are playing equally, but only producing 1 W. They will both feedback as easily if you stick a mic in the horn of the speaker. There is no way that you can limit feedback by headroom. Now. If that system were not limited to the 1W, then heck yeah, that corwn is going to rip the speaker out of the cab, because it can produce the same feedback loop at an increased amplitude.

 

True, atmospheric conditions have an effect on the air. It is the effect of how molecules travel, especially a sudden change in them that can cause a PA to feedback. Again, because feedback is what I said it is. When the PA goes from being tuned to match the air, surroundings, capacity of the crowd, and then those factors change, suddenly what was boost, like bass, needs to be boosted no longer. Suddenly the highs that were just not punching through part your hair.

 

As far as notes/frequencies being close to oscillation, that is not so. Oscillation is circuit induced. An oscillator is a circuit that continuously alternates between two (or more) states. The period between alternations defines the frequency of oscillation.

 

Go to a repair center and ask them to let you see some pure waves and complex waves on a scope. You will see that your theory is inccorrect.

 

About your bump button on a light board. You did something that caused the audio to react to DC or something. That is too complex to say that I a lighting console created feedback. As for your other comments about stage rumble. Tell your engineer to use a high pass filter. granted it won't fix everything, but it helps. Then go to the EQ and clean up the mud that does not need to be reproduced.