What caused this...?

K

Karl Barnes

Sophomore
Aug 28, 2012
105
0
0
London, England
Hi,

I've got a head scratcher here and could do with some useful opinions.

I've got some blown subs and want to know why and how they've blown and more importantly, how to prevent doing the same damage to them again.

Here's the scenario.
Arrive and set up for small theatre gig (100 people). Setup takes a couple of hours, so all the equipment is at room temperature by the time things are switched on.
The PA is switched on and tuned to the room. No problems there. It sounds so good that the LXeng comments on it sounding good.
It's not massively loud or bass heavy. OK, so the sub amp knobs are 3/4 of the way to max. There was more to give.
Then we hit the problem.
A mic cable is unplugged by one of the band, in order to repatch something. This cable is to a phantom powered mic.
You guessed it. BOOM!!!!!
No subs. Actually very distorted sounding subs, but unusable either way.

I've got them back to the workshop and am trying to work out what has happened.
From the attached pics, the damage looks thermal.
There's bubbling on the inside of the coil (highlighted in red).
bubbles.jpg

Also, there are several vertical scratches around the outside of the coil that look like that's where they are in contact with the magnet (highlighted in red).
scratch1.jpgscratch2.jpg

That's the distortion explained.

I've spoken to the manufacturer and I've given them the model numbers of the speakers and the spec of the amp involved and they have commented that the amp is a good match for the rating of the speakers. For the record, the subs are Peavey Hisys Subs (twin 15", 4ohm, 700w RMS) powered by a T.Amp Proline 2700 amp (1350w at 4ohm).

One thing I have noticed is the internal wiring of the speakers.
cable.jpg

They appear to use 1mm cable. I use 2.5mm cable from the amp to the rear of the speaker.
From what I can see of the speaker wiring, the cabinet is wired in parallel. The yellow and blue cable goes to 1+ and 1- on the Speakon on the rear of the speaker and the red and black cable is connected to the + and - terminals of the first speaker in order to feed the second speaker.
Would the change from 2.5mm cable to 1mm cable result in a build up of heat in the smaller cable that is passed on to the speakers, causing the voice coils to expand and rub on the magnet?
Is that what caused the bubbling?
Is that something that can happen in a short BOOM event or is it something that takes longer to manifest itself?

Procedures can be revised with regards to patching with the PA on (i.e. death threats, etc), but is there anything else that can be done?

I know that this can be chalked up to experience, but what exactly have these subs experienced.
Any help would be appreciated, as this is something I am keen to not have repeat on me.

Thanks.

Karl.
 

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Re: What caused this...?

Hey Karl,

This isn't a thermal failure, for sure - that coil is squeaky clean. Looks like a failure from either arcing or rubbing of the VC against something else. I would also guess that this is not due to just one event, it takes more than one phantom pop to wreck something as sturdy as a woofer without also obviously tearing the suspension apart. Probably just the straw that broke the camel's back. Your amp power looks good to me, as long as you have a proper HPF and LPF in place I would write it off to wear and tear and pop in a new basket.
 
Re: What caused this...?

Bennett Prescott said:
Hey Karl,

This isn't a thermal failure, for sure - that coil is squeaky clean. Looks like a failure from either arcing or rubbing of the VC against something else. I would also guess that this is not due to just one event, it takes more than one phantom pop to wreck something as sturdy as a woofer without also obviously tearing the suspension apart. Probably just the straw that broke the camel's back. Your amp power looks good to me, as long as you have a proper HPF and LPF in place I would write it off to wear and tear and pop in a new basket.
Click to expand...

The problem here is that the baskets that blew were only two years old and only had only done around a dozen gigs.
Two of the magnets were replaced with the baskets two years ago, by an authorised service center. The other two magnets are as old as the cabinets.

What would cause the arcing?

Could storage/transport be a factor?
The cabinets have castors on the rear and are moved from A to B on their back, with the speakers pointing up. They are also stored this way.
 
Re: What caused this...?

Karl,

There was a full output thump and then your coils had arcing and rubbing signs on them. I don't know what other kind of cause you expect to find! I have never seen a loudspeaker failure as a result of storage, and the only transport related failures I have seen have been sharp drops that shifted magnets. Arcing is caused by a voice coil not being held centered in the gap, usually during extreme excursion, while high voltage is running through it. Your amp is capable of about 75v at about 18amps before clipping into that 4 ohm load, so there's a lot of electrons flying around in there!
 
Re: What caused this...?

Bennett Prescott said:
Hey Karl,

This isn't a thermal failure, for sure - that coil is squeaky clean. Looks like a failure from either arcing or rubbing of the VC against something else. I would also guess that this is not due to just one event, it takes more than one phantom pop to wreck something as sturdy as a woofer without also obviously tearing the suspension apart. Probably just the straw that broke the camel's back. Your amp power looks good to me, as long as you have a proper HPF and LPF in place I would write it off to wear and tear and pop in a new basket.
Click to expand...

Hi Bennett,

Its doesn't look like a thermal failure, but I have seem some thermal failures that look like this, mostly compression drivers. The glue becomes weak and bubbles, this pushes the VC out of shape and things being to rub, but there is no discolouration of the VC. Most drivers will discolour the coil long before it fails but not all.
 
Re: What caused this...?

Bennett Prescott said:
Karl,

There was a full output thump and then your coils had arcing and rubbing signs on them. I don't know what other kind of cause you expect to find! I have never seen a loudspeaker failure as a result of storage, and the only transport related failures I have seen have been sharp drops that shifted magnets. Arcing is caused by a voice coil not being held centered in the gap, usually during extreme excursion, while high voltage is running through it. Your amp is capable of about 75v at about 18amps before clipping into that 4 ohm load, so there's a lot of electrons flying around in there!
Click to expand...

Bennett,

Thanks for the explanation. That was exactly the sort of answer I was looking for.
That covers what happened.
Now, does anyone have a practical way to prevent this from happening again?
Would changing the crossover from a basic HPF/LPF unit to something like a Driverack offer any more protection?

Karl.
 
Re: What caused this...?

My guess was based on the idea that a big transient with higher frequencies present was more likely to jump out of the coil and arc across the windings. It's good that you have a crossover, and so my guess could well be wrong.
 
Re: What caused this...?

Peter Morris said:
Hi Bennett,

Its doesn't look like a thermal failure, but I have seem some thermal failures that look like this, mostly compression drivers. The glue becomes weak and bubbles, this pushes the VC out of shape and things being to rub, but there is no discolouration of the VC. Most drivers will discolour the coil long before it fails but not all.
Click to expand...

Hey Peter,

Certainly true, but my impression is that Peavey builds good stuff, and the rubs are quite localized, so I ASSume this was not a thermal failure.
 
Re: What caused this...?

Actually, that had got me thinking.
If the glue was bubbling, forcing the coil out of shape, what would cause that?
Heat would have been my first guess, but in the absence of that, this is suggesting a manufactoring fault with the glue.
All the speakers were replaced at the same time, so it's probable that the replacements came from the same batch.
I'm not aware of any faulty batches that are out there.
However, they're stored in exactly the same conditions as the amps and top speakers, as well as the X32 they run from (garage).

If it is heat, there's only one place that would come from. The amp.
It was turned up further than I have had it before, but it wasn't at max.
Being a new venue for us, the system was tuned according to the layout of the venue.

With the amp rated at twice the RMS value of the speakers, if the amp was running too hard, would that have caused the coils to heat and distort in shape?
Having the amp up too high would then suggest that we didn't have the rig for the gig.
Would that be the case, or am I just paranoid?

Karl.
 
Last edited:
Re: What caused this...?

Glue heating up and bubbling generally happens with slam-the-limiters long term abuse, though. Something like a sudden spike from unplugging phantom power would tend to just burn a spot (or spots) in the voice coil or the fly leads.
 
Re: What caused this...?

Bennett Prescott said:
Hey Peter,

Certainly true, but my impression is that Peavey builds good stuff, and the rubs are quite localized, so I ASSume this was not a thermal failure.
Click to expand...

Yes, Peavey did build good stuff, I always consider Peavey good value and reliable ... Its not a fail I have seen previously from them, what doesn't make sense is the bubbles on the VC former, its usually heat but who knows.
 
Re: What caused this...?

Karl Barnes said:
Actually, that had got me thinking.
If the glue was bubbling, forcing the coil out of shape, what would cause that?
Heat would have been my first guess, but in the absence of that, this is suggesting a manufactoring fault with the glue.
All the speakers were replaced at the same time, so it's probable that the replacements came from the same batch.
I'm not aware of any faulty batches that are out there.
However, they're stored in exactly the same conditions as the amps and top speakers, as well as the X32 they run from (garage).

If it is heat, there's only one place that would come from. The amp.
It was turned up further than I have had it before, but it wasn't at max.
Being a new venue for us, the system was tuned according to the layout of the venue.

With the amp rated at twice the RMS value of the speakers, if the amp was running too hard, would that have caused the coils to heat and distort in shape?
Having the amp up too high would then suggest that we didn't have the rig for the gig.
Would that be the case, or am I just paranoid?

Karl.
Click to expand...
It doesn't matter where the amp was "turned up to". Even with the level control almost all the way down it can still put out full power (with a few exceptions in some cases)

The knobs on the front have nothing to do with the amount of power the amp can produce.

I don't remember you mentioning what filters you were using-freq/slope/type etc. Switching to a process only does good IF the processor is set up properly. If not-it can make things worse.

As has been said earlier this DOES NOT look like a thermal or heat failure. The coil would show signs of discoloration. Instead it shows spot burns-which are most commonly associated with trash in the gap of the speaker that has rubbed the insulation off and made it easier to spark and burn out.
 
Re: What caused this...?

classic driven out of gap - once out in free air long enough and the coil turns into a light bulb, but a short time will make the enamel blister or bubble - plus once out of the gap mechanical stresses can cause the coil to twist/distort/misalign when it reenters the gap hitting or scrapping the plate or pole piece. Not good. This is why dual spider designs take an out of gap experience a bit better.
 
Re: What caused this...?

How bout this as a scenario? Was the channel that the mic was connected to hi passed? Possible a large LF spike caused the initial damage to the voice coil and then after the coil was distorted did the rubbing occur. Running subs on an aux will keep only the channels that you want real low end in the subs.
 
Re: What caused this...?

In addition to what everyone else has said, have a look at the other side of the driver.

Are there a couple concentric rings (folds) in the cone?

If there are, this would absolutely mis-align the coil in the gap and cause the rubbing you see.

I am guessing this is what happened:
1. Your single event (the phantom powered mic unplug - kabang) banged your coils out of the gap faster than the suspension of the woofer cone would allow it to move.
2. This caused the cone to fold. It is like your drivers were trying to turn themselves inside out, but they couldn't on account of the spider.
3. Now the coil is mal-aligned in the gap and is rubbing all the insulation off. You start hearing distortion.
4. It doesn't take a lot to rub the insulation off when the coil is constantly rubbing against the magnet in the gap, so then it starts arcing. You wonder what is that distortion and listen to it for a minute or two in disbelief wondering what to do.
5. You finally shut it off and pull the loudspeakers out and see what is going on.

This may not be what actually happened, but it is my guess.
 
Re: What caused this...?

Glenn Adams said:
classic driven out of gap - once out in free air long enough and the coil turns into a light bulb, but a short time will make the enamel blister or bubble - plus once out of the gap mechanical stresses can cause the coil to twist/distort/misalign when it reenters the gap hitting or scrapping the plate or pole piece. Not good. This is why dual spider designs take an out of gap experience a bit better.
Click to expand...

Hey Glenn, that's not classic out of the gap. THIS is! :lol:

This one is so good that a friend of mine saved it up just so I could see it when I visited. This is an 8" coax, never seen one get like this before!

IMG_7983.jpg
IMG_7986.jpgIMG_7987.jpg
 
Re: What caused this...?

Rob Spence said:
What specific crossover are you using?

What are the High pass and Crossover frequencies?


Sent from my iPad HD
Click to expand...

Hi, it's a Behringer CX2310 (don't judge me...) running 2 way with the crossover set at 120Hz.

I've sent the above pics to Peavey who have replied saying that it's probably dirt in the magnet gap.
I could believe that if it wasn't for the fact that all four cones suffered from the problem at exactly the same time.
Maybe I'm just that unlucky...

To look at the cones, you can't see any damage. No folds, tears, etc.
They are almost pristine, as they are only a couple of years old.
It's only when you take the magnets off that the damage to the coil is visible.
 
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