Underpowering cab?

Do a search, underpowering is bunk. Clipping a SS amp is, however, a very nasty thing. That has lead to the inaccurate reference to "underpowering". There are quite a few threads on this that explain it in greater depth.

UH OH.

Geez! This subject is just like the IRS.. Can never seem to get rid of it... no matter how much we all try and want/wishing it to disappear for good.

Myth_103 said:

I've been hearing a lot about how "underpowering" your bass cab is a lot worse and can blow a speaker a lot faster than by overpowering it. I never did really know too much about that. However, I don't understand this because everyone talks about how the SVT-CLs and SVT-VRs are so wonderful, but how could they possibly power an 8x10 safely if that was the case? I play through an SWR Megoliath 8x10, but have been told underpowering it could damage the speakers. Would a SVT-CL damage my speakers?!

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Clipping from an underpowered amp might damage your tweeter, but it will not damage your woofers.

As for tweeters - you could play a piccolo through a cabinet, never approach clipping and still blow it. High freq content overpowering it is what blows it. If a tweeter is rated 50w, and you put 55w power through it - it blows. Turn down the treble if it sounds harsh.

Perhaps the best way to put it is that having an underpowered RIG is a great way to blow stuff up. It doesn't matter what the ratio is between your amplifier's rated output and your cabinet's rated power handling. The shape of a waveform won't damage a driver, whether it's square, triangular, sinusoidal, or anything else.

There are two main modes of failure of drivers:

1. Thermal failure of the voice coil. This happens when the voice coil can't dissipate the power driving it.

2. Overexcursion. This happens as a function of driver design, cabinet design, and equalization. It can and often does happen at power levels well below the cabinet's thermal rating.

There is no harm in using an amplifier rated for less output than your cabinets are rated for in handling. Nothing will be damaged by the combination. The wrong tool for the job is the real problem.

bassian

My first bass amp was a 35 watt rickenbacker. I competed (poorly) at medium volume practice gigs. I had to crank it up to compete, and blew the 15" driver. Replaced it (at a shop for lots of $$ at the time) with a 15" black widow. best poart of a year later, the driver was shot. repeated with new black widow (on my fathers dime) and approx 6 months of loud rehearsal later it was shot again.

this was the early 80's. the tech who replaced the driver as well as others I spoke to all attribute it to sending a clipped lousy signal to the driver, therby blowing it.

if this is bunk, what can you attribute the 3 blown drivers to?

35 watts aint a lot of watts for a black widow.

thanks in avance.

Crockettnj said:

if this is bunk, what can you attribute the 3 blown drivers to?
.

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Overpowering. An amp is rated at a certain level of harmonic distortion, say 0.1%. You can get a lot more than the amp's rated power, albeit with high distortion levels; twice the amp's rating isn't at all unusual, It will sound like crap, and a prudent person would turn it down, but prudence isn't a trait generally displayed by teenagers.

Amps also are capable of generating transients as much as 10dB higher than their rated output, and that means that 35 watter might have been capable of 350 watt transients. Those transients would have been of very short duration, but each transient would cause addtional heat to be created in the voice coil, and that heat takes time to be wicked away by the magnet structure. If repeated transients added heat faster than it could be wicked away eventually enough heat would build up in the coil to result in driver failure.

Rattman said:

Geez! This subject is just like the IRS.. Can never seem to get rid of it... no matter how much we all try and want/wishing it to disappear for good.

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In honor of tax season and me not being able to leave the office the next 2 weeks, I dub this sig material!

billfitzmaurice said:

It will sound like crap, and a prudent person would turn it down, but prudence isn't a trait generally displayed by teenagers..

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Also Bill, many of the younger players seem to like the sound so they may not be aware of the problems that they can be creating.

BassIan said:

There are two main modes of failure of drivers:
1. Thermal failure of the voice coil. This happens when the voice coil can't dissipate the power driving it.

2. Overexcursion. This happens as a function of driver design, cabinet design, and equalization. It can and often does happen at power levels well below the cabinet's thermal rating.
.

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I think that one of the problems on these types of discussions is that all the focus seems to aimed at the voice coil (overexcursion, heat dissipation); another important factor contributing to over excursion is cone suspension fatigue, after a while due to either improper drivers or use; cone edge fatigue takes place, the surround becomes soft allowing too much movement resulting in overexcursion.

Bassman62 said:

I think that one of the problems on these types of discussions is that all the focus seems to aimed at the voice coil (overexcursion, heat dissipation); another important factor contributing to over excursion is cone suspension fatigue, after a while due to either improper drivers or use; cone edge fatigue takes place, the surround becomes soft allowing too much movement resulting in overexcursion.

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True. A bass driver is very susceptible to damage from exceeding xmech. A typical bass driver has a 4mm xmax and 8mm xmech, and it's not all that difficult to push the driver to double the xmax, reaching xmech and smacking the coil against the rear plate, especially if you have an inefficient cab design where xmax can be reached with only 20% or less of the driver power rating.

Bassman62 said:

I think that one of the problems on these types of discussions is that all the focus seems to aimed at the voice coil (overexcursion, heat dissipation); another important factor contributing to over excursion is cone suspension fatigue, after a while due to either improper drivers or use; cone edge fatigue takes place, the surround becomes soft allowing too much movement resulting in overexcursion.

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Absolutely possible and probable over the lifetime of a driver. As the suspension wears, of course its compliance increases. The parameters of the driver change over time. This will also cause the driver to interact differently with its enclosure.

My second failure mode made no specific reference to the voice coil. Overexcursion, as you and Bill have said, can have a variety of effects on a driver.

Crockettnj, Bill already covered your comment, but I'll state that I strongly agree with him. The shape of a waveform will not damage a driver. If a driver cared about the acceleration of its cone, would it not be damaged simply by supplying it with a higher frequency signal? A woofer goes into cone breakup modes to reproduce higher frequencies, though they're very directional. Above that, it simply doesn't reproduce significant signal.

Feed a square wave (or something close to that - any signal with a lot of high frequency content from a distortion box, or from excessive clipping over a power amp) to a multi-driver cabinet (ie a 2-way bass cabinet). The woofers will reproduce what they're capable of (for the sake of argument 2khz) and won't have any appreciable output beyond that in any direction. The harmonic content above that will be handled by the high frequency driver(s).

Your drivers probably blew because of overpowering or overexcursion. If the enclosure wasn't very well suited to the replacement drivers, they could very well have exceeded Xmax or Xmech as you played. Did you ever take the driver apart to examine what caused it to fail? It's pretty easy to identify overpowering or mechanical failure by seeing the damage to the coil/former.

I run a YBA-200 200 watt tube amp into an Avatar 4x10 Neo (I think rated at 1000 watts.) This isn't problematic, right? When I push the tubes they compress, especially the low end and it becomes apparently overdriven, but this is ok?

GODSENT said:

NONE OF THIS MAKES ANY SENSE!!!!

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In short: no significant danger running a 300W amp into a cab rated at 1200W.

Jeralya said:

I run a YBA-200 200 watt tube amp into an Avatar 4x10 Neo (I think rated at 1000 watts.) This isn't problematic, right? When I push the tubes they compress, especially the low end and it becomes apparently overdriven, but this is ok?

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Yes, it is. You stand no more chance of hurting your woofers than does a guitar player running A 22 watt Mesa at full tilt into a 100 watt ten. Just be sure to turn the tweeter down or off, if you've got one, as it can be hurt by distortion.

I've got a single Ashdown 15 that seems to be blown or something? It has no movement when plugged in and makes absolutely no noise!? I blew an SWR 15 once but it still works - just distorts at high volumes and at certain frequencies... the Ashdown however seems to be completely dead? what happened to it - is it the voice coil or what???

Note:// no intention to Hijack, just thought this falls along the line of the discussion