Matrix 804 bass driver rasp

Hi,

I have come into some B&W matrix 804's second hand, i got them home and noticed ( listening to dire straits: 'your latest trick') that a woofer in one of them seemed to be rasping slightly, like somthing nearby was resonating, although nothing is.

Closer inspection pointed to the lower bass woofer (part no zz8613), i removed it from the cabinet and noticed the screw holes are a bit threaded in the wood, so someone has rather clumsily had the driver out on the past and tightened it back in too much...

And the woofer, when held in my hand, still makes a slight raspy sound i can see anything flapping on it. So i am baffled (no pun).

Whats this a sign of? The drivers look in good shape, could it be the crossover? or am i simply burying my head to the possibility some fool has tried to play dubstep through these and damaged them?


Advice and help is appreciated.

Thanks.
 
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After some internet research I am coming to the conclusion that someone has turned up the bass too much, leading to a damaged voicecoil in this woofer. It measures 5 ohms at DC which i think may be a tad low in an 8 ohm woofer, probably indicating shorted windings..The flipping eeeejuts!

The tweeters and rest of the speakers sound perfectly fine.

I dont know if it's worth asking B&W for a replacement or just returning the speakers to the seller, the price was fairly low I thought, at around £250 GBP.

PPS another thing, and that is the kevlar mids seem to be traversing in and out a bit more than the black bass drivers during music, is this an ominous sign? i would expect them to be both pumping around the same amount when listening at reasonable levels.
 
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I'm not saying, just talking that same thing would happen if changing connections between mid and woofer, so something is wrong with those drivers. (They are) Ready for an inspection. At that price I would keep them but you might be in trouble by now if a lot of work is in front of you (not money for new drivers but time and know-how). Maybe another member can help you because to be fair I can't or it will take a long time.
 
5 Ohms DC resistance might possibly be still allrigt, the best would be to compare with the other one.
Keep in mind that some drivers suffer from some cone sag over time - causing the voicecoil to rasp against a pole-piece. There are case where this can be cured by turning the respective driver by 180 degrees and therefore causing the gravity to pull in the opposite direction.

Regards

Charles
 
Hi,

Thanks for your responses. I am erring on the side of keeping them, just because they were quite a good deal.

Interesting point on cone sag, although I was holding the driver in my hand, dust cap facing the ceiling and it still did it, so no such luck. I will get the DMM on the other speaker today and compare, that will be the deciding factor I think. I wouldn't dare try and rewind this coil myself, I think its a difficult process even with the knowhow.

I am also calling B&W for a quote replacement, if anyone knows sources of older matrix drivers I'm all ears as i'm not optimistic B&W will hold spares for this speaker.

Interestingly B&W manufacturing are based in my home town, Worthing :)

Cheers
Craig
 
Ok i am sure that the driver is damaged, it made some nasty knocking sounds during a song passage, i guess the coil movement is extra non linear with some portion of the coils shorted and it lost control and bottomed out.

Replacemet from B&W £163 built to order
Or wembley loudspeakers in London offer a repair for under £40.

Whats the opinion on the options?
If repaired will it look and sound like it should afterwards?
They must have to pull the rubber surround out which could get ugly :S
And how the heck do they match the cone if they replace it?

Cheers
Craig
 
Try a match repair (first) that they check both drivers for same final R/impedance (the repaired one with the second you have). Maybe that's not possible. Like measuring both drivers for a match pair.
(Second) If not possible then it doesn't matter because or you buy a new matched pair or you buy just one. If you buy just one (more expensive) you also can send it for repair for less. It will be just fine with original recones.
Maybe another member with experience on repairs can say different, I don't know for sure.
 
Manual is available, as usual with B&W 804 Matrix. Always interesting.
http://www.bwgroupsupport.com/manuals/bw-service

Looks like the plastic (Cobex) bass does the bafflestep boost, while the Kevlar does bass/midrange. You'd need to check, because the schematic isn't clear.

This is not a design I've ever spent time on, so I'm not sure what the matching requirements for the two woofer's parameters is. But I'd guess you might fit another similar driver if it fits the baffle cutout and chamfer. Time for the measuring tape. :)
 

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the kevlar mids seem to be traversing in and out a bit more than the black bass drivers during music, is this an ominous sign? i would expect them to be both pumping around the same amount when listening at reasonable levels.
Now, from the LAYOUT Steve just posted that's reasonable that the main driver is not presented with a high pass crossover, ""while the Kevlar does bass/midrange"" in this case it's obvious and explains why.
 
@Inductor, good suggestions thanks, i'm not sure how they operate in driver repairs. Maybe the broken one is enough for them? if they get the same gauge wire and put on the same number of turns. Ill have to call them and find out how they operate in an ideal world.

I'm more worried about how they replace the spider, cone and surround without changing stiffness, weight and aesthetics.

@system 7, thanks for the link, I see that the lower bass driver gets a further high pass filter beyond the mid. This makes me wonder how the bass driver was damaged before the Midbass driver! Whats a bafflestop boost?

Cheers
Craig
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I've had a metal shard (such as a burr from a screw/bolt head) actually in the air gap in the magnet pole piece. This was on B&W703's (mine would you believe). The problem really showed up when feeding the drive unit from a low frequency sine (say 1 or 2 Hz) as you could hear it rubbing badly.

Fortunately, it was visible right on top of the pole piece and easily removed with a plastic stick and some sticky tape.
 
sounds nasty, my speaker anatomy isnt up to scratch but doesnt the spider stop crap from getting down the gap.

What are the 700 series like?

I got this pair because I heard the 800 series were very good speakers, I have to say they are very nice even with this distortion, its generally not audible. Then again I had a cheap pair of KEF Q series before, so wouldnt take much to impress me :rolleyes:
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I've been very pleased with them.

The metal filing was where I've arrowed. Being metal it couldn't "fall" into the gap, it seemed to get attracted to where the field was strongest.

All very open and easily visible where the voice coil emerges from the maget assembly.
 

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Whilst Listening to Antonio Forcione with a Martini and a copy of Jules Verne 20,000 Leagues, I regained determination to find this damn rasp..
Using newly acquired 'High Def' vision (begrudgingly gave in to a pair of glasses recently) I have re-inspected the driver and found what is clearly the issue.
...A great big CRACK in the Cobex cone; invisible from the front of the driver as it is covered by the dust cap and looks normal but, from the back a large crack is visible in the cone underneath the dustcap.

Im guessing this can be repaired but whats the best method / resin / long term solution to fill this crack and restore this driver to glory?

Cheers!
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I doubt anything hard (resin... as in resin based glue) would survive the vibration/flexing long term. Only guessing though. You could perhaps make it airtight with a PVA or impact adhesive which is flexible but I suspect the whole dynamic of the cone will be totally compromised as its not working as a "whole".
 
Thanks Mooly,

I have started reading a bit on the net, some seem to suggest bike tyre repair glue, lots are warning against brittle glues and too rigid a glue as you say.
some seem to advocate some tape behind the crack and some sort of glue as well.

I need to settle on the best method and try it out, the result it certainly going to be an improvement at least.

Cheers
 
I went for some clear silicone sealant and carefully cut postit note.

Smear a small amount of sealant on back of the crack (using a paintbrush works well), stick neatly cut postit note over crack (I had to cut to a cresent shape to cover the crack near the base of the cone), apply thin layer of sealant over and around edges of postit. The result is presumably light-weight and looks to be a robust fix provided the sealant holds long term.

The rasp is gone now and has withstood numerous test songs with plenty of LF content.

A new lease of life!