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Old 25th August 2008, 12:46 PM   #1
Dan2 is offline Dan2  South Africa
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Default Please help with PA speaker

we got a second hand PA system for our church, which consist of 2 wharfdale pro 300w 15" speakers, and matching amp.

about 3 months ago the fuse in one of the crossovers blew (LF side). i thought maybe someone had feedback issues with the system and so i replaced the fuse.

about 2 weeks ago i noticed the speaker wasn't working again, so i figured someone blew the fuse again. i checked and the fuse was fine. so i took out the driver and found that the cone was "sucked in" and didn't move at all. i managed to pull the cone back out, but now it doesn't move freely and feels very grainy when i move it with my hand. my guess is that the voice coil burnt up.

before i go and buy a new speaker, i really want to find out how this happened. i cant understand how the fuse didn't blow but the speaker did (and i got the exact fuse for that speaker model).

another thing, being a church, we don't abuse the system, the amp's gain is on abt 20%.

PS: would it be possible to repair the speaker???
thanks in advance for any help
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Old 25th August 2008, 01:42 PM   #2
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Power the amp up and check for DC on that channels output. Sounds like it put out DC sucking the cone in and frying the coil.
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Old 25th August 2008, 06:20 PM   #3
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Well, we are dealing with subtleties here. Did it seem as if the cone were pushed (or pulled/sucked) ALL the way back to the limit of it's travel? Or, was it simply back slightly from the center or neutral position?

To some extent, it sounds as if the speaker has been driven beyond its Xmax; the limit of its ability to travel. If that happens with the cone moving forward, it can literally come out of the voice coil gap and can miss the gap as it tries to go back in. This certainly can cause physical damage.

If it is moving backward, the voice coil mechanism and cone can literally crash into the magnet structure and cause physical damage, or in this case, become stuck.

Now we must ask, what would make the speaker cone move to such an extreme? Because this is an extreme. Most speakers will be very distorted and certainly very LOUD before so massively exceeding its range of travel.

One has already been suggested. If somehow, perhaps when the amp gets hot, a DC (direct current) voltage is applied to the speaker.

Assuming the speaker is wired correctly, a positive voltage will make the cone move forward and a negative voltage will make the cone move backward. But to move the cone to and beyond its full extent of travel, it would have to be a high voltage - 30 volts, 40 volts, 50 volts, etc....

The alternative would be for an extremely loud sound impulse to drive it beyond its limits, but we are talking a lightening strike in the room loud; loud in the extreme.

Though, I don't think feedback could do this. Feedback is usually high pitched squeal, and being high pitched, even at extreme volumes, the cone doesn't move that far forward or backward.

Now, it could have been some type of Rumble, which is low frequency feedback. But, for a simple PA application, a deep bass or sub-bass rumble, while possible, is unlikely.

So, indeed we have a mystery. Part of the reason I'm rambling on is I want to impress on you the extreme nature of what it takes to drive a speaker to that extreme range of movement. Any sound would have to be at near earthquake proportions to cause this kind of damage.

But again, it hinges on how far back the speaker cone was. If it was just back slightly from the neutral no-signal position, then I would say, you had an old speaker in which the insulation on the voice coil wire broke down and shorted causing a current overload, and the melting of the voice coil.

But if it was physical at or near its limit of travel, then either an extreme electrical or physical event cause it. Somehow an extreme AC or DC voltage was applied. The alternative is that some curious kid just pushed it back to see how far it would go, and when it jambed, he ran for it.

I would leave the amp on for a while so it is good an warm, then use a simple volt meter to measure for ANY DC voltage at the output. If there is, then the amp needs to be repaired or replaced.

If by chance you know someone in the community who has an oscilloscope, you could have them place that on the speaker outputs and that might indicated if there was a sub-harmonic/sub-sonic AC signal. It would also confirm any DC offset voltage.

Just out of curiosity, is this an old TUBE PA amp. Tubes generally work off much higher voltages than transistor amps. If it is a tube amp, then likely it is transformer coupled to the speaker. It could be an intermittent short in the output transformer that is periodically applying high voltage to the output.

As to the speaker, it may be possible to have it reconed. If you are near a big city, then likely there is someone somewhere who recones and repairs speakers.

The probably doesn't help, but should give you some idea of the possibilities.

Steve/bluewizard
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Old 26th August 2008, 03:37 PM   #4
Dan2 is offline Dan2  South Africa
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ok let me try to answer these questions:

the speaker was not pushed in by someone, they are mounted quite high up and have a grill over the front.

the cone was quite far back, but i don't think it was ALL the way back.

i highly doubt the speaker was driven beyond its xmax cos they are very powerfull and the gains on the amps are set very low. if someone did that they would have to know how the whole lot was set up (very unlikely but maybe possible).

unfortunately i forgot to take my multimeter with me to test the amp, so i must still do that. i did however take a quick look at the schematic of the amp (it a Haffler amp, and has schematics in the book that came with it)
There is no decoupling cap on the output, and rail voltages are +- 96V, so if one of the transisters shorted there would be a high dc on the output.
and it is not a tube amp, just a transister or mosfet amp (didn't check)

i don't know if this helps, but the tweeter is still working. i guess it would have been protected by the high pass filter.


is the fact that the fuse blew on that same channel any indication of a problem? 'cos after i put a new fuse in the speaker worked for a couple of months.

and one more thing (if this helps): the amps sometimes stay on for a long time. being a church, the (ahem) older folk use the PA and forget to turn it off
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Old 27th August 2008, 06:50 AM   #5
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Dan,

Cut the cone, spider and voice coil assembly out of the woofer frame. Examine the voice coil. If the coil wire is cooked evenly along its entire length, then the woofer simply cooked because of too much long term power.

If the coil wire is cooked near one end, but not the other, then the amplifier is passing high voltage DC to the woofer. The cone motion will be in the direction of the cooked wire.

Look at the end of the voice coil former. If it is bent over or cracked, then the voice coil was probably driven very hard into the backplate of the woofer. This is usually from high level bass transients. DC will rarely cause this damage.
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Old 27th August 2008, 08:07 PM   #6
badman is offline badman  United States
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Default Good advice

Jack's got it right. And Jack, glad you're still participating beyond the driver sales. I keep slapping my hand away from ordering some of the 15"s, those are impressive value.
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Old 27th August 2008, 11:43 PM   #7
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I've cooked an unfair share of EVP-X18B subs. one was a probably well-meaning dj thinking there was plenty more in the amp and winding the gain up into extreme clipping (aka 100v p-p square wave)... the other was more my fault, they're rated at 600w RMS and i had 1400W of amp behind each one. The amps weren't clipping but the bass was fairly hard on the limiters all night so the average power in the speakers was probably 800-900 watts, plus it was 35C weather (Aussie NYE party). In any case, the symptoms were the same as you experienced.
The excess heat distorts the voicecoil former (aluminium) and causes the coil to rub on the edge of the gap. eventually either the coil wears through or fuses and the speaker is locked solid.
Recone kits aren't too expensive and are relatively easy to fit (at least in the subs they are, the gap is big to allow more manufacturing tolerance so alignment is relatively straightforward. Get advise from someone before attempting it if you're not sure, or you can pay a company to do it but you'll pay the price of the kit and the same again to fit it.
Two things will cook a speaker like that: 1 - too much average power due to operator fault (excessive clipping or running too hot on the limiters); 2 - Too much average power due to an amp fault (blown transistors, excessive DC offset, etc). If the horns are still OK then it could be the latter as clipping often kills the compression driver too...
On my impression of the Wharfedale Pro gear (I've got the relatively budget end stuff, EVP-X15 tops, EVP-X18B subs and LX-12 tops that I use for foldback) is that they sound really good with a bit of EQ, I have often had people comment on the sound quality. They probably wouldn't handle heavy road use as they are constructed of MDF, light portable use or installation use is fine though. Power handling is good but do stick to the music power rating or less if running on limiters. I use a Driverack PA for foldback EQ and a Deadringer DEQ2496/DCX2496 combo for the tops (digital from the mixer until the crossover outs) and through a few old jands amps (about 3kw total).
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Old 27th August 2008, 11:48 PM   #8
PB2 is offline PB2  United States
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Something simple to consider, is to be sure that you have a low cut filter somewhere in the chain. Vocal pops on the mic can send the driver into huge nearly DC excursions, and you need some filtering to prevent this. Curious to hear about what you find with that voice coil.

Pete B.
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Old 28th August 2008, 12:26 AM   #9
imix500 is offline imix500  United States
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Also, since the system is available to the general public it could be turned on/off out of sequence. It would be difficult to actually cause a voice coil to lock up from the resulting thumps, but I suppose it could happen.
Even though the input attenuation pots on the amp are set low (they don't affect the gain of the amp itself) it wouldn't be out of the realm of possibility that a piece of misbehaving gear applies near rail voltage to it's line output when turned on/off which would still present quite a spike on the amps output if it's on when it shouldn't be.
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Old 1st September 2008, 08:10 AM   #10
Dan2 is offline Dan2  South Africa
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i measured the output of the amp for dc and found that there isn't any. i then switched my multimeter over to ac and it jumped around 1-2v. unfortunately i cant get hold of a scope so i can't do any other kind of measuring
i have an old 12" PA speaker that hardly works, so i could use that to do more testing (at least if it blows it won't be a loss)

i think i am going to try get the speaker professionally repaired - i don't trust myself with that kind of job!

as far as the sequential turning on/off of the mixer and amps... well it does happen quite often that the amps are turned on before the mixer - but i am not the only one using the system so there's not much i can do about that.

as for the low-cut filter, there is a 80hz low cut switch on every input channel of the mixer. as far as i know they are all on, but also easy enough to turn off too
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