Just blew a cab for the first time

[Old Garage-Bander]

In theory, cabs should be able to withstand bursts of power double that of their RMS rating.

The flip side is that amps are usually capable of also putting out approximately double their RMS rated output in bursts. Those smarter than me can weigh in, but I believe that whether we're talking amps or cabs, the formal calculation for burst or "max" power is double RMS (or is it RMS is half of burst...)

I'm not going to go into the difference between thermal and mechanical limitations here, but suffice to say that from an RMS standpoint, as long as the values aren't manipulated or misrepresented they're at least somewhat useful as far as thermal ratings being aligned is concerned.



Unless I'm mistaken, it's not like an amp starts clipping and all of a sudden immediately produces double the watts. Amps are given their RMS rating at a certain, usually still "clean" level of distortion (THD). Go above that, and you get more power with more distortion - audibly more as you get higher and higher.

Depending on how you run your amp, you might only get those bursts on transients, and so they're pretty fleeting, which is why they might not be very noticeable, but they can still tax a speaker.

Let's suppose you are running amp at a continuous value of RMS.
Then let's say you periodically insert a burst at twice the RMS level.
Over the period of time, the peak value becomes part of, added to the RMS value, thus adding to the RMS value applied to the speaker. Does that make some kind of sense?

To maintain the RMS value while periodically adding peaks, shouldn't you also have valleys equal to the peaks.

So are some folks hitting their speakers harder than they think?

Are they fooled by where the knobs point?

Do they mistakenly think that turned half way up is half power?

Or is this all OK, because they are generally not maintaining a constant RMS value as in my example, but the RMS is actually comprised of the aggregate of all the instantaneous peaks and valleys?

 

[MDBass]

That amp happens to be a 400W amp when used with that cab. And I was assured that the combo worked fine by several members of this forum, as long as I didn't do anything crazy with the settings. I don't see how my settings could have been extreme enough to pop the voice coil (but I'm sure someone here will say they were).

~3 o'clock on both the master and bass knobs IS a pretty extreme setting.

An amp rated for 800 watts at 4 ohms will also put out around 500 watts at 8ohms.

 

[MDBass]

Obviously that is probably what happened...but at 3/4 bass (didn't even sound bassy FWIW) and less than 3/4 master, it seems remarkably flimsy.

That line of thinking is not how bass amps actually work.

It assumes that every cab can safely handle the amp if set with the master and bass knobs both dimed: very few cabs made will be able to hand that kind of power and low end excursion simultaneously, and clearly this particular cab couldn't.

 

[40Hz]

Sometimes I feel some need to be a little bit science nitpicking.
Noved air volume means nothing but moved air volume in any regards just to perceive (audible) loudness cause SPL is the derivaton of air particle velocity rather than moved air voluime.
For example a gust of wind moves lots of air volume but the moved air is only audible if the moved air moves the leaves of a tree or anything else that does produce audible SPL.
Moved air volume on its own does NOT automatically produce audible SPL.

Agree.

But in my earlier post you're quoting I also said:

There’s a lot more to it than that. And I ran roughshod over the actual technicalities and physics a bit. But the takeaway is: if you want more volume, you want more speakers or cabs - not more watts - almost every time.

I shudder to think what the reaction might have been if I said air particle velocity. I tried many moons ago to get into an a rational discussion here with somenbody about what SPL means, and it provoked a major argument. So once bitten, Twice shy.

 

[Ross W. Lovell]

In theory, cabs should be able to withstand bursts of power double that of their RMS rating.

The flip side is that amps are usually capable of also putting out approximately double their RMS rated output in bursts. Those smarter than me can weigh in, but I believe that whether we're talking amps or cabs, the formal calculation for burst or "max" power is double RMS (or is it RMS is half of burst...)

I'm not going to go into the difference between thermal and mechanical limitations here, but suffice to say that from an RMS standpoint, as long as the values aren't manipulated or misrepresented they're at least somewhat useful as far as a cab's thermal rating being aligned is concerned.



Unless I'm mistaken, it's not like an amp starts clipping and all of a sudden immediately produces double the watts. Amps are given their RMS rating at a certain, usually still "clean" level of distortion (THD). Go above that, and you get more power with more distortion - audibly more as you get higher and higher.

Depending on how you run your amp, you might only get those bursts on transients, and so they're pretty fleeting, which is why they might not be very noticeable, but they can still tax a speaker.

Let me clarify.

I have always gone by the rule that it's safer for my cabinet to be rated at twice the amp's RMS rating.

Better safe than sorry.

With regards to when it hits peak, i dont really care when, but I'd like to think my cab is prepared if it's rated for twice the RMS?

Am I wrong in thinking that?

 

[Kro]

Or is this all OK, because they are generally not maintaining a constant RMS value as in my example, but the RMS is actually comprised of the aggregate of all the instantaneous peaks and valleys?

I personally subscribe to the theory that most speakers in modern cabs will reach mechanical failure given overpowering with typical program material prior to them exceeding their RMS rating and frying.

My own experiences seem to bear this out - as while I've never fried any voice coils, I have smacked a few backplates and creased a few cones from exceeding xlim.

It's not always the case, but again IME, mechanical failure is probably a much bigger concern for most users.

I may be a bit on the verge of my understanding with a few of my latest posts here, so as always I'm genuinely happy to be corrected.

 

[mcnach]

~3 o'clock on both the master and bass knobs IS a pretty extreme setting.

An amp rated for 800 watts at 4 ohms will also put out around 500 watts at 8ohms.

It depends.

The master on its own means little, it'll depend on how the input gain has been set and of course the actual input level.

Bass control at 3 o'clock is indeed pretty intense, from my experience with those amps. However if the input signal lacks a lot of bass, it may not actually be all that demanding. Unlikely, as I cannot imagine a Jazz bass on its less bassy setting lacking so much bass.

But I agree with you, I also suspect that we probably have a case of bass knob cranked up a little too much in combination with trying to push the volume too much (for that level of bass and into a single cab). Although without being there, it's hard to tell for certain. It is possible that there was something wrong with the cab to start with. But that bass knob in that position would make me nervous (unless I were playing quietly).

 

[Old Garage-Bander]

I personally subscribe to the theory that most speakers in modern cabs will reach mechanical failure given overpowering with typical program material prior to them exceeding their RMS rating and frying.

Good point. Thanks for the added comment.

 

[MDBass]

I personally subscribe to the theory that most speakers in modern cabs will reach mechanical failure given overpowering with typical program material prior to them exceeding their RMS rating and frying.

My own experiences seem to bear this out - as while I've never fried any voice coils, I have smacked a few backplates and creased a few cones from exceeding xlim.

It's not always the case, but again IME, mechanical failure is probably a much bigger concern for most users.

I may be a bit on the verge of my understanding with a few of my latest posts here, so as always I'm genuinely happy to be corrected.

100% accurate.

 

[Kro]

Let me clarify.

I have always gone by the rule that it's safer for my cabinet to be rated at twice the amp's RMS rating.

Better safe than sorry.

With regards to when it hits peak, i dont really care when, but I'd like to think my cab is prepared if it's rated for twice the RMS?

Am I wrong in thinking that?

Well, I suppose you could say better safe than sorry infinitely depending on how you look at it:

"I like my cabs to have three times the amp's rating... better safe than sorry", or four or five times, if you follow me. You could also say that you'd rather have your cab rated at just slightly over, than slightly under your cab's RMS rating for the same reason.

As has been said in this thread a few times already by other members, there's no clear cut answer as to what will definitely be safe: it's a sliding scale of risk - and it never gets to 0... unless you don't hook up an amp.

For me personally, I prefer to not have an amp exceed my cab's rating - simple as that.

In theory, like I noted earlier, a cab should be able to handle twice it's RMS rating in bursts without thermal failure - so if your personal rule of thumb has you doubling the cab rating just for that reason, you likely don't need to... in theory. Good loose speaker spec sheets even list program, burst, or max specs alongside RMS - commercial cab manufacturers usually don't though for whatever reason.

And to those that say that an amp should have twice as much power on tap than the cab is rated for for reasons of headroom, I say: get more cab. Then your amp won't have to work so hard in the first place!

 

[2F/2F]

It depends.

The master on its own means little, it'll depend on how the input gain has been set and of course the actual input level.

Bass control at 3 o'clock is indeed pretty intense, from my experience with those amps. However if the input signal lacks a lot of bass, it may not actually be all that demanding. Unlikely, as I cannot imagine a Jazz bass on its less bassy setting lacking so much bass.

But I agree with you, I also suspect that we probably have a case of bass knob cranked up a little too much in combination with trying to push the volume too much (for that level of bass and into a single cab). Although without being there, it's hard to tell for certain. It is possible that there was something wrong with the cab to start with. But that bass knob in that position would make me nervous (unless I were playing quietly).

That's along the lines of my thinking.

3 o'clock on bass and 2:30 on master are not "extreme" if the pre-amp is being handled with kid gloves – as it was here. If the pre-amp is cooking...sure, those could be dangerous settings. And in any case, I'd think a potentially harmful group of settings would make itself heard before the speaker fails.

The Jazz definitely isn't as bassy as my P, but I wouldn't say it lacks low end.

 

[Kro]

That's along the lines of my thinking.

3 o'clock on bass and 2:30 on master are not "extreme" if the pre-amp is being handled with kid gloves – as it was here. If the pre-amp is cooking...sure, those could be dangerous settings. And in any case, I'd think a potentially harmful group of settings would make itself heard before the speaker fails.

The whole gain structure thing I wouldn't worry about personally, seems to me what you were doing was just fine. As far as EQ, it's all relative. Unless I'm mistaken, and depending on amp, equal EQ boosts across the board typically would just equate to a volume boost - maybe with some interesting peaks and valleys, but an overall boost in signal nonetheless.

Issues IME really come into play when you boost lows over mids to the point where they're masking midrange frequencies critical to actually being heard, and where you start turning volume up to get the mids-volume you need it. From your pic I see you're boosting mids as well, which in conjunction with the bass boost, likely meaning that the net effect is primarily very attenuated high frequencies.

For more detail, lets take a look at the manual for that PF800 and see what's actually going on. Bass knob is capable of +14dB @ 40Hz, and depending on mid selection, with mids you can dial in up to +10dB - based on your picture, for you at 220Hz (the lowest mid setting). I can't say for certain what levels you were at because I'm not aware of the taper that those pots use, but I think that does help to give a better idea of what was actually happening with the amp.

Look, there's nothing wrong with using settings like that if the cab(s) you're running can handle them, and I'm still not convinced that that's what the issue is, but lows/low-mid boosts in those realms could potentially be significant depending on what you actually need to hear yourself well.

 

[JimmyM]

Let me clarify.

I have always gone by the rule that it's safer for my cabinet to be rated at twice the amp's RMS rating.

Better safe than sorry.

With regards to when it hits peak, i dont really care when, but I'd like to think my cab is prepared if it's rated for twice the RMS?

Am I wrong in thinking that?

Seems a bit drastic, but is relatively prudent.

 

[JeezyMcNuggles]

Those are cranked too. Low mids never go below 9 o'clock on my B-100R. It's the one tone knob on that amp that I never move. And it's no replacement for the bass knob, tonally.

At any rate, the B-100R handles it, and has for literally thousands of playing sessions since '96 or '97, when I got it new. But the point of acquiring the PF-800 was to not have to dime stuff...so I wasn't doing it on that amp.

9 o'clock cuts them. Much less low end. Much

 

[2F/2F]

9 o'clock cuts them. Much less low end. Much

That was a brain fart on my part in posts 93 and 96. I knew what I meant; I just typed the exact opposite end of the clock. I meant 3 o'clock.

I noted this in a followup post, but perhaps I should fix the original posts as well.

 

[JeezyMcNuggles]

That was a brain fart on my part in posts 93 and 96. I knew what I meant; I just typed the opposite. I meant 3 o'clock.

That's more like it. That's exactly the spot I was thinking

 

[ScrewDevil]

I honestly expected this thread to go in a totally different direction just based on the title! Lol. How disappointing, hundreds of off color jokes just lying dormant...

 

[agedhorse]

I was under the impression, possibly mistaken, that my speakers should be able to take twice the RMS Rating of my amp? Anything less and if the amp somehow clips it's going to make TWICE the watts IPP?

I know very few single speakers that would take double that amp's rating, but I'm sure the others will educate me on what new stuff is out there.

It depends very much on the cabinet. SOME cabinets have both mechanical and thermal power ratings in their published specs. This will give you a much better and more accurate perspective of the speaker's real power handling.

 

[gregbackstrom]

the proof is in the pudding on that.

Another butchered proverb that makes no sense! (Unless it’s a rum pudding, of course!) Try this: “The proof of the pudding is in the eating.”

While we’re at it, horses don’t chomp at the bit — they champ at the bit.

Okay, now back to carelessly blowing innocent drivers!

 

[Pbassmanca]

Someday, I'm going to miss this thread title.....