LP rumble destroying woofers? - diyAudio
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Old 7th February 2010, 01:28 AM   #1
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Default LP rumble destroying woofers?

Is it possible for LP rumble to destroy woofers? My mother in law has blown 3 Scanspeak 18W/8545K woofers and I'm trying to find out why. I've checked her amp, but it seems to be working fine (I've run it up to clipping with a load and seems no ill effects + very little DC on the output). She does play some LPs so my current thought is if she if playing these at moderate volume the subsonic rumble may be causing them to exceed maximum excursion.
Any thoughts?
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Old 7th February 2010, 01:50 AM   #2
Loren42 is offline Loren42  United States
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Do a post mortem on one of the blown drivers.

If the voice coil is discolored, then excessive Wattage is suspect. If the voice coil has deformed, scratched, or unwound, then excessive excursion is suspect.

If the latter proves true, a filter would be in order.
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Old 7th February 2010, 04:52 AM   #3
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Thanks...will do.
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Old 7th February 2010, 07:00 AM   #4
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Depending on the make and model of the turntable you could remove the feet and replace with tennis balls or racket balls. I've done this on many tt set up and works 70% of the time. Or you can move either speakers up so there fronts are behind or put tt back beyond front of speakers
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Old 7th February 2010, 07:40 AM   #5
Mooly is online now Mooly  United Kingdom
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Rumble causes damage to the cone surround mainly... particularly on old foam surrounds that are disintegrating. To burn a VC out would take some doing I would have thought.
Can you see excessive cone movement playing a record ? If the cone hits the end stops the distortion is obvious.
Is it always the same channel that fails ?
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Old 7th February 2010, 08:37 AM   #6
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I'd agree that rumble damages the surround, but I expect it would cook the voice coil, if the level has high enough.
When the voice coil leaves the magnetic gap, its inductance changes dramatically (I think it goes down, but I'm not sure). So, if a rumble came along that made the coil leave the gap, the inductance would drop, huge current through the coil, cooked.
So, looks like a rumble filter is on order.
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Old 7th February 2010, 09:08 AM   #7
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OP has not said what design the woofer is in. Since the woofer in question has a low Fs i.e. resonance and a high VAS indicating a 'loose' suspension, it would be easy for the coil to leave the gap with some designs notably OB
As Chris suggests a HP filter is needed.
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Old 7th February 2010, 08:45 PM   #8
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Quote:
Originally Posted by chris661 View Post
I'd agree that rumble damages the surround, but I expect it would cook the voice coil, if the level has high enough.
When the voice coil leaves the magnetic gap, its inductance changes dramatically (I think it goes down, but I'm not sure). So, if a rumble came along that made the coil leave the gap, the inductance would drop, huge current through the coil, cooked.
So, looks like a rumble filter is on order.
The inductance change will not significantly affect the VC current. VC inductance is only significant at high frequencies, usually above the operating range of the driver. At low frequencies, the VC current is set by two factors:

1- The DC resistance of the voice coil.
2- The impedance peaks at the system resonance(s).

Most bass driver systems (driver/enclosure) are designed to be excursion limited, not power limited. That is, in most cases, at the lowest frequencies the driver will reach its excursion limits before reaching its heat limits.

For the current scenario, I guess we wait for the results of the failed driver post-mortem examination to decide the cause.
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Old 8th February 2010, 04:23 AM   #9
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Quote:
Originally Posted by smithy666 View Post
My mother in law has blown 3 Scanspeak 18W/8545K woofers and I'm trying to find out why.

Any thoughts?
Tell her to turn down the Acca Dacca?
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Old 8th February 2010, 04:40 AM   #10
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Quote:
Originally Posted by smithy666 View Post
My mother in law has blown 3 Scanspeak 18W/8545K woofers...
Any thoughts?
She loves the sound of bass (music). (/she uses drums to communicate!) Curious.

Last edited by Inductor; 8th February 2010 at 04:43 AM.
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