rms vs. peak vs. program

[Red Planet]

Can you think of a repeatable experiment that proves this just wasn't a one off experience?

I have no need to, just relaying my experience.

Is perception reality or reality reality?

 

[Red Planet]

I had a Yamaha stereo head (about 200 watts per side) running into cabs that were rated 400 watts and all the speakers in the cabinet blew up within a couple of minutes, the first time I used it. Smoke came pouring out of the cab. I didn't even have it anywhere near practice volume, much less gig volume. So I was under-powering the cabs, right?

Turns out that something went very wrong in the amp and it fed some high voltage into the speakers, but still.....

The funny part is that when I called the store, the salesman told me to switch the speakers around. I told him I thought that would be a bad idea, but he insisted, so I did and ... Boom! Took the amp (and speakers) back in and he plugged it into some store speakers and Boom! + the lights went out. They fixed everything under warranty, but as I was leaving, I could hear the tech yelling at the salesman.

I dont keer who ye are at was funny.

 

[DukeLeJeune]

Guitar cabs don't have tweeters because any bassist or drummer with a sense of self preservation would poke a stick through the tweeter, before allowing the guitarist to plug in.

Ah yes, of course!

Thanks for the other information too; I appreciate your experienced perspectives.

 

[totallybacan]

Actually, i have a theory, but only a theory at that Regarding DukeLeJeune's question about guitar cabs.

If the distortion is caused by the amp because of clipping, then you have all the byproducts of the clipping such as the higher harmonics and whatnot. But once you mic it, the distortion heard in the PA's would then be caused by the sound and not the clipping. Same if you were to mic a bass cab that is bottoming out. The mic would pick up the "farting", but the PA would not itself be farting. It would just be reproducing the apparent sound. Unless the FOH has the subs all the way up, in which case there would be cone missiles everywhere.

 

[bongomania]

As far as a speaker is concerned, there is no difference between clipping and "the sound of clipping".

 

[Bob Lee (QSC)]

I have a question Bob.

You can take a smaller amp than the cab is rated for that lets say does not clip very friendly and blow drivers by trying to squeeze more out of less correct?

An amp rated for X watts can be made to put out >X watts by driving it into clipping. That could burn out a loudspeaker.

 

[Bob Lee (QSC)]

the first I'd check would be the DC offset in the output of the amp.

But since DC offset is quite audible, really the first sensible thing would be to check the hearing of the user.

DC offset is not audible by itself. It can be audible if it makes the loudspeaker hit Xmax or Xlim. DC offset dissipates power in the loudspeaker without also producing acoustical output.

Fortunately, amplifier DC offset larger than a fraction of a volt is very rare, and when it does occur it is usually accompanied by severe performance anomalies.

 

[Bob Lee (QSC)]

A couple of questions for Bill Fitzmaurice, or Bob Lee (QSC), or hasbeen, or agedhorse (if he happens by), or anyone else: My understanding is that two mechanisms are at play when an amplifier is driven into clipping. The first is a rise in the average power level, increasing the heat (wattage) that the voice coil(s) must deal with. The second is an upward shift, in the frequency domain, of the power envelope, due to the addition of higher harmonics from the flattened waveforms. In theory this upward frequency shift in the power envelope would result in a disproportionate amount of the additional power going to the tweeter.

My first question is, in practice is this upward frequency shift in the power envolope due to clipping a significant issue for a tweetered bass cab; and if so, any general guidelines on how much of a shift to expect? Like 1/2 octave, 1 octave, etc? Probably the answer starts out with "it depends...", but my grasp here is rudimetary enough that anything would be appreciated.

My second question is, as far the thermal effect on the speaker goes, what's the difference between distortion from amplifier clipping and distortion from a cranked fuzzbox? In an attempt to compare apples-to-apples here, let's assume the we have the fuzzbox on a bigger amp so we can compare similar average power levels without the addition of power amp clipping to the fuzzbox.

I have heard that guitar cabs don't have tweeters because the distortion will blow a tweeter, and yet I see PA systems seemingly reproducing that distortion with no problem. Is there some significant difference in the signal that a guitar cab is asked to reproduce vs what comes out of the PA system, or is it a matter of the PA system having big, beefy compression drives with ultrahigh efficiency and enormous thermal capacity?

Thanks!

Duke

Hi Duke,

During clipping the average power in the signal exceeds the amp's rated power. Over time, a signal with severe clipping will tend to have a compressed dynamic range, so that even the power in between the instances of clipping is pretty high. If you put the same signal through an amp at the same gain but with a higher power rating, it would put more power into the voice coil(s) and heat them even more, even if the amp doesn't clip.

There could be an upward shift in the power spectrum. It depends on the original signal; with a signal that has little or no harmonic content, clipping will add energy at harmonic frequencies. With a complex signal, though, severe clipping may obliterate much of the high-frequency content, resulting in the power spectrum shifting downward despite the addition of some harmonic energy.

The biggest difference between distortion from an amp during clipping and from a fuzzbox is the actual amount of power the amp is putting out. If an amp rated at X watts is driven to clipping, the output is >X watts + Y watts of harmonics. The harmonics of a fuzzbox appearing at the output of the same amp could be on a signal of any level, including <<<<X watts.

 

[DukeLeJeune]

Thank you very much, Bob.

 

[Pablo Icecreambar]

There is NO SUCH THING as underpowering a cab, what you are referring to is simply CLIPPING YOUR AMP (more specifically the power section)!

Also, it isnt DC, its a square edged waveform (hard clipping) with solid state amps.


Basically, as long as the cab is sounding ok, you should be fine, if it starts farting out or making other not too nice noises, be careful!

...AND Sweetwater has this to say about matching a cab with an amp:

Power Amp Buying Guide

***

 

[bongomania]

...AND that Sweetwater guide was not written by an engineer or designer of amplifiers or speakers. It should be taken as a quick reference guide written to help sell PA systems, not as an authoritative source for settling technical debates.

 

[figuredbass]

but then again, RMS doesn't mean anything unless you know how the mfg. derives that figure.

What are they feeding into the amp?
What do they deem are acceptable distortion figures?
At what frequency response?
What are the burst rating capabilities?

It is just as easy to get duped at one rating then at the other. RMS is easier then some.

+1