new voice coils lasted 4 mins kicker L7 - diyAudio
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Old 3rd January 2010, 12:32 AM   #1
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Default new voice coils lasted 4 mins kicker L7

i replaced the coils in my 12 inch kicker L7 i went out today to give them a try and at 40 hertz it took about 4 mins till i had smoke coming out the port in the box.

what looks like has happened is the coil former is made out of paper and the coil actually came off of the former and got all squashed down inside the magnet. i bought these speakers blown up and the stock kicker coil seems to have a coil former made of resin. and the burnt coil i pulled from it was just burnt in 1 spot but still intact. this replacement coil seems to be burnt a little but is like 50 feet long and all unwound.

i saw that kicker (and im asumming others) have speakers that have replaceable motors, and i thought i could do something like this with a set of
L7s. i machined a outer ring for the surround and one for the spider. the only thing is i still need to glue the coil in place. im new to this but im thinking 4 mins is a unusually short lifespan for a speaker. i dont mind replacing these coils i kind of expect to.

for the initial test i ran the speaker with a 40hz tone for 4 mins with a jbl bp1200.1. i realize this amp is over kill but the sub didnt distort or bottom out or complain at all. (till all the smoke, then it just died).

so my questions to the community are.....

1. is it normal to have formers made of this plasticky/paper stuff? or resin or is there even any difference
2. should i go punch this guy in the stomach for selling me an inferior coil or should i buy another one from him? and this time just relax with the 40 hz test
3 does anyone know where to buy reputable coils from?
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Old 3rd January 2010, 05:14 PM   #2
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can anyone recomend a forum where i might get these questions answered?

is this a normal way for a speaker to fail? or do they usually just burn which creates a break/open in the coil?

is there anyway to see this coming so i dont do the same thing a second time?
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Old 3rd January 2010, 05:18 PM   #3
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I'd say 40Hz would be low enough to start warming up coils. If you used it at high power, chances are the coils have cooked that way.
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Old 3rd January 2010, 05:35 PM   #4
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I wouldn't use a paper former in a high powered speaker. If you think about it, the former will act as a heatsink to help pull heat from the coil if it is made of something appropriate. Paper, kapton and aluminum are common. I'd want aluminum for a high-powered sub.
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Old 3rd January 2010, 05:36 PM   #5
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Quote:
Originally Posted by scampo77 View Post
went out today to give them a try and at 40 hertz it took about 4 mins till i had smoke coming out the port in the box.
You say that the amplifier is "probably overkill"

Ok, let see if we can figure things out here. 40Hz is probably somewhere near the port tuning frequency of most car subwoofers. You say that the speaker didn't bottom out? Well this is probably due to the fact that the driver was being severely damped by the port at that frequency, so it wouldn't move that much - allowing you to feed it probably more power than it could handle

As the driver wasn't moving that much a second factor comes into play! If the voice coil isn't moving very much then there isn't going to be a lot of air pumped in & out of the voice coil to cool it - this is the reason you cooked it.

You'll probably find that the voice coil former is made of Kapton, it does look a bit like paper...

Next time be a bit more careful Music will never generally cause a problem as it's not a pure sinewave test tone...
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Old 3rd January 2010, 07:47 PM   #6
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thanks everyone who responded i am really in need of some answers.

so should i order the same coils that i just blew up?

i had a really hard time finding anyone, anywhere who would sell me coils. i would LOVE LOVE LOVE!!!!! to find coils with aluminum coil formers where do i even start looking for something like that?

how do i know if the former i have now is kapton? (ive enclosed a picture) is kapton the standard or should i even bother looking for something stronger?

the pictures i sent are the old coil and former (the intact one) and the new one you can see that the old coil came right off. can anyone tell if this replacement coil is a lesser quality one?
Attached Images
File Type: jpg 100_0125.jpg (78.9 KB, 216 views)
File Type: jpg 100_0121.jpg (70.4 KB, 212 views)
File Type: jpg 100_0128.jpg (101.6 KB, 208 views)
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Old 3rd January 2010, 07:52 PM   #7
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what is the Purpose of the Paper over the Kapton Former?....to provide a better surface to bond the voice coil on?
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Old 3rd January 2010, 07:56 PM   #8
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yes that is the part that sits inside the spider. if you look closely at the one still in the motor you will see it is barried inside the motor and you can see where the glue is
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Old 3rd January 2010, 08:01 PM   #9
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Something needs to change.

Only you can say if the speaker is underpowered or the amp is overpowered but they're not matched and if you want to play 40Hz again then just buying the same coils will result in the same destruction.

As e-h said, the systems probably OK for music as you don't get 4mins of full on 40Hz but if you're competing for loudness then you need a speaker with greater power handling or maybe go to multiple drivers to split the power.
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Old 3rd January 2010, 08:03 PM   #10
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Looks like what i said earlier was about bang on

You can plainly see where the poor voice coil overheated the most due to lack of cooling, in fact it looks like it just about managed to burn off the insulation on the wire itself

What you want is a standard coil for the driver, IE a direct replacement for the drive unit in question. I have never replaced a voice coil or re-coned a driver, i didn't know you could actually buy just a voice coil assembly without the cone & spider, outer suspension etc. Replacing them & aligning them is a bit like an art, you need to know exactly what you are doing - if you don't, you're probably wasting your time.

In future if you are going to test things, do it at a frequency where there is a good deal of cone movement for little input. That way you won't overpower the voice coil & you'll get to hear if there is any voice coil scrubbing on the magnet poles. With music you shouldn't have a problem, but as you've found a constant signal with little cooling =
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