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Author Topic: Burning Voice coils, what the heck is going wrong???  (Read 14158 times)

Jeremy Bridge

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Burning Voice coils, what the heck is going wrong???
« on: August 23, 2004, 05:22:33 pm »

Okay, so I am completely at a loss here.  I recently built 12 labs, and am very impressed with thier sound.

Thier first show was 1 month ago, and they preformed stunningly, or so it seemed.  QSC PL 4.0's per pair of subwoofers, so only about 1400 per cabinet or there abouts.

Then this past weekend thier second show same amplifiers, a weekend festival.  About 3 hours after setup I noticed a strange sound, wouldn't you know it three subs were blown (one sub in each enclosure).  

So we were down too 8 cabinets (we only took 11).  The funny thing is I swear it was louder than with all 11.  The array was very long (all 11 in a row, so I thought maybe it was simply due to directivity?).  I had triple checked the wires to be sure everything was in phase.

2 hours later another sub goes down.  Same problem burnt voice coil in one driver.

So here I am at a complete loss, I have no idea why in the world I would burn 4 voice coils.  We were not even running them as hard as the show before.  PL 4.0's per pair of cabinets no clipping at all.

Can anyone offer any sugestions?  I will repair everything but obviously don't want this to be a recurring nightmare Sad

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Mac Kerr

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #1 on: August 23, 2004, 05:39:35 pm »

Two questions:
1 - In the show where they performed stunningly did you have all eleven? is there a chance the ones that blew weren't there the first time?
2 - What do you have before the amps? Is there any chance something was clipping, or a limiter was limiting?

Ok three, is there any chance there might be a couple of bad amps putting out DC? Were any of the blown speakers on common amps?

I guess that was really six.

Standing by,

Mac
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Mark Seaton

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #2 on: August 23, 2004, 06:02:55 pm »

The other question we need to ask is if the drivers are truly burnt or did they tear or bottom from over-excursion.  Does the voice coil measure as open circuit?  While nothing is impossible, I would be suprised to hear that you burned a LAB without sustained clipping of the PL4.0.
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Mark Seaton
Seaton Sound, Inc.
"Make no little plans; they have no magic to stir men's blood..." - Daniel H. Burnham

kevinnemrava

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #3 on: August 23, 2004, 11:25:28 pm »

I can step in here - the unit before the lab subs was my DBX 260:

what are the settings jer?

any crazy EQ?

if you where useing that subharmonic synth, I might have to show up and kick your ass!


I asked jer earlyier today, and he assures me that their was not any clipping in any other stage. Cause I was thinking that if you use to much input to the drive rack the A/D converters will clip. And apreantly no cliping in any other spots either (ie mixer i/o, the graphs ect.)

I am paticurly interested in the crossover slopes and types used as well as teh parametic eq's and graphic eq;s used.

I know that jer was useing the auto eq function, and we all know that you have to take them with a grain of salt, but what I am wondering is if teh auto eq function deided to boost saw 20 hz by 10dB. that would be bad.


talk to you soon- big K
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Graeme Goodacre [Centauri

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #4 on: August 24, 2004, 08:07:57 am »

As I mentioned in another thread, if we start pushing things to the limit and beyond, failures will be inevitable.

The rated thermal limit power for the Lab12 voice coil is 400W, making 800W per box total.  While this would suggest a music power (and hence appropriate amp power) of around 1600W, the average dissipation will be very dependant on the actual program material.  In any case. 700W per driver is going to get them damn hot!  Are these Labs version1 ? - I believe there was very little clearance between the back of the driver and the cover in these boxes, which could restrict the cooling air flow slightly.  A small restriction here cut make a large difference to voice coil temp.

The drivers will also reach their Xmax at around 400 to 500W, depending on number used, so running much more than this can stress them mechanically as well (as some have found out).

Whilst the LabSub is undoubtedly a great performing box (especially for DIY), it still must be run within reason.  I would call this a 800W to 900W box just from the Xmax figures alone.

Cheers
Graeme
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Jeremy Bridge

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #5 on: August 24, 2004, 10:29:17 am »

#1 There was no clipping in any component to my knowledge.
#2 I did not use the auto eq function
#3 24db @ 85hz LP, 48db @ 30hz HP
#4 -5db @ 105hz, Q = 1.38
#5 Problem did not follow amplifiers
#6 Labs were purchased 5 mnths ago therefore I would assume they are the new version as they came right from Eminence
#7 There was only one sub that had not been used at the previous show and it was one of the ones that was toast, but that leaves a few more.

#8  People consistently talk about putting over 3000 watts per enclosure, I would think that 1400watts of clean unclipped Powerlite power should be fine.  There must be another problem.

I am taking all the subs apart this weekend to double check everything, I hope I find an answer?

Any other sugestions?
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Mac Kerr

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #6 on: August 24, 2004, 11:52:42 am »

Jeremy Bridge wrote on Tue, 24 August 2004 10:29

#1 There was no clipping in any component to my knowledge.
#2 I did not use the auto eq function
#3 24db @ 85hz LP, 48db @ 30hz HP
#4 -5db @ 105hz, Q = 1.38
#5 Problem did not follow amplifiers
#6 Labs were purchased 5 mnths ago therefore I would assume they are the new version as they came right from Eminence
#7 There was only one sub that had not been used at the previous show and it was one of the ones that was toast, but that leaves a few more.

#8  People consistently talk about putting over 3000 watts per enclosure, I would think that 1400watts of clean unclipped Powerlite power should be fine.  There must be another problem.

I am taking all the subs apart this weekend to double check everything, I hope I find an answer?

Any other sugestions?
You didn't say whether there was a limiter. If the signal is heavily limited before it gets to the amp, there will be no "clipping", but the end result is nearly the same. How did you determine the voice coils were burned? Do they measure as an open circuit? If it turns out that they have been damaged by over excursion check and recheck the polarity of all wiring.

Mac Kerr
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Jeremy Bridge

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #7 on: August 24, 2004, 04:20:55 pm »

I assume the voice coils are burnt because I can see the color of them through the space in the basket above the top plate, they are dark brown (unless they use some really dark varnish perhaps?), and they measure open.

There was no limiting or compression present at the time which they were blown.

I apreciate everyones help in troubleshooting this with me.
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Al Limberg

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #8 on: August 24, 2004, 04:38:55 pm »

Well now maybe we have some progress.  The voice coils on mine appear rather dark as copper goes right out of the box.  It's a very heavy (26 guage I think) wire and the coil is wrapped both inside and outside the former.  On the other hand, if your Labs aren't properly sealed, you could be experiencing over-excursion of the cones and voice coil rub that leads to the open condition.  That wouldn't require any clip or limiting, and even the 30Hz HPF might not protect you all that much.  I feel like I'm preaching to the choir sometimes, but properly sealing the speaker chamber is critical.

Al
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Jeremy Bridge

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Re: Burning Voice coils, what the heck is going wrong???
« Reply #9 on: August 24, 2004, 04:45:03 pm »

No sir, preach away Smile

I thought I was very dilligent in sealing the chambers, I used ample ammount of west system epoxy (with filler), and I also used silicone in all corners on both the inside and joints of the module.

2 of the subs rattle horribly and scrape allong the gap, however the rest don't seem to rub at all.

I have also never actually heard the subs ever bottom out, but that doesn't mean they haven't of course.
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