Broken JM Lab Subwoofer? (1 Viewer)

[timelinex]

My new house has a home theatre with 2 JM Lab Cobalt 800s Sub's. Both of them aren't 100% and one of them is definitely "broken" and since they were good/expensive I wanted to know where to start learning how to fix it if possible versus replacing it with a cheaper option. What happens with it is that if they sound goes above 1/8th of it's volume or so, it just makes a screeching sounds instead of the bass sound. The other speaker is mostly fine but if it goes above 70% of it's capacity or so, then the same thing happens. So with the second one, it realistically never happens in movies, while the first one I have had to disconnect because almost any amount of bass makes it happen.

Most searches I have made have resulted in how to fix a broken sub that just doesn't make any noise anymore. But they definitely still make a sound, and it's pretty loud and annoying. I think a decent way to explain the sound would be if someone put the front cover on the sub up against the actual cone and it's just hitting it and vibrating off it. Not sure if that's a great explanation though.

I know this is a basic question and you guys can't walk me through everything, but if you have any idea what it wrong, or the actual name for this phenomenon is (and not just "blown speaker" as I would know it). I would appreciate it, that way I can start my search somewhere.

Thanks

 

[JohnRice]

Your second description of the problem sounds like it's probably a damaged voice coil in the driver. You basically have two options. You can have the voice coil replaced, or you can replace the entire driver. I don't know which will be easier to get done. Replacing the voice coil is a little specialized and it might be difficult to find anyone who can do it. It think that's probably a little unusual to have done, except with particularly distinctive speakers. If you replace the driver, you can't just put in a new one of the same size. There are parameters to sub drivers which are extremely important to duplicate as closely as possible.

There are some sub experts here, and hopefully at least one of them can contribute more specific help.

 

[Jonathan Burk]

Your best bet is probably to replace the driver. The problem is that it looks like it has a 13" driver! That is an extremely unusual size, so finding an exact replacement will be difficult.

You'll also need to find out what kind of power that amp was putting out. Contact JM Lab (which looks like it's now owned by Focal?) and see if they have any specs for the amp.

In the end, if you can figure out the amp specs, and the resistance for the current speaker (8ohms or 4ohms, probably 4), you might be able to replace it with a 12" driver. Take out the existing speaker and measure the diameter of the cutout. Then browse here to see if you can find a speaker with the same resistance and power handling abilities that will fit:

Parts Express 12" Subwoofers

Then measure your speaker box and get the internal volume, and then measure any ports and get their volume, and then email the support at Parts Express with those measurements and ask them to verify the performance of any speakers that will fit.

If you can get all those variables to fit, then you can replace the driver!

You could also look at making the cutout bigger and going up to a 15" driver (or making a spacer to raise the driver off the plane of the speaker surface). All the other consideration would still apply.

Good luck!

 

[timelinex]

Thank you for the info! I will look into all the things you mentioned.

Would it be worth contacting a repair place or do you think the cost to repair "professionally" would likely make it a better option to just buy new ones?

 

[JohnRice]

Driver replacement in a subwoofer is complicated. There is a lot more to it than the impedance and size of the drivers. There is also the T/S Parameters. Match them, and you're good to go. Get a mismatch, and you get garbage performance.

 

[JohnRice]

I would at least explore the option of having the voice coil rebuilt. Don't expect for it to be cheap, but it guarantees a perfect match of the driver, since it's the same driver. Who knows. Focal might offer that service.

 

[Jonathan Burk]

Driver replacement in a subwoofer is complicated. There is a lot more to it than the impedance and size of the drivers. There is also the T/S Parameters. Match them, and you're good to go. Get a mismatch, and you get garbage performance.

Yes, if you send Parts Express the box dimensions, they'll run the TS parameters for you for a driver and let you know what kind of performance you'll likely get.

 

[JohnRice]

Yes, if you send Parts Express the box dimensions, they'll run the TS parameters for you for a driver and let you know what kind of performance you'll likely get.

There ya go. Of course, you need to account for the internal bracing as well.

 

[Mr645]

If you can find a replacement driver that is designed to work in the size and type of enclosure, replacing it then becomes rather easy. I would start by taking out the driver and looking for a model # on it and search for a new one of the same model.

 

[timelinex]

Ok guys, before I take it further, do you guys see anything specifically wrong with it:



At first glance, all the internals look solid. Nothing is loose. Nothing is unglued. Nothing sticks out as being the reason. But maybe the video helps you guys.

 

[JohnRice]

99.9% certain that's a burned voice coil.

 

[timelinex]

Doing some research, that seems like an expensive fix and considering the age of these speakers, it's probably worth just replacing.

Is there a reason they installed 2 fairly large sub's in my home theater? The room is like 20ftx20ft. Seems like way overkill compared to just running one nice sub?

EDIT: Never mind, I just read more into it and I guess alot of people prefer 2 over 1, even when the room isn't that large.

EDIT 2: You are right that I think maybe replacing the entire driver with something compatible might be the best option for the money.

 

[JohnRice]

Sub drivers, good ones, are quite expensive. It comes down to finding someone who can rebuild it and finding out the cost. I'm genuinely not trying to push you one way or the other, just trying to educate you. A rebuild guarantees a perfect match, but can be more expensive. A new driver may or may not be cheaper, won't be a perfect match and will (probably) involve some customizing to get it installed.

 

[timelinex]

I measured everything today.

The actual hole: 11.5"
The cutout including mounting area: 13"
Exterior dimensions: 16.75" tall, 15.5" wide, 20.75" deep
Wood thickness: it's hard to tell if its uniform everywhere, but it seems like its about 1.5" thick

Using the parts number on the driver I was luckily able to find an old Parts Express listing HERE .

I think that should give me all the info I need to try and find a replacement. I posted in another place that specializes in Subwoofers and lets see if they can help with finding an alternative.

I appreciate all the guidance you have provided thus far. I am starting to see the light at the end of the tunnel!

 

[JohnRice]

The specs on that driver don't look very impressive, and the driver itself looks a little anemic for a sub. You can probably get a much better driver. You just need one that will work well in the enclosure. Parts Express can probably help you with that.

 

[timelinex]

Looks like someone ran some comparisons and here are some replacements that would work.

Peerless XXLS 835017 seems like a good option and it looks like the specs say it goes down to 19hz, which is pretty good (if I'm readying the specs correctly!)

 

[aceinc]

First thing I would do is push in and out on the cone (not the dust cap) and listen carefully for a scratching sound, especially on unit 1 (the one that barely plays). If it does, you have a bad voice coil. The video isn't 100% conclusive IMO. If you are certain it is the driver, before replacing it, use the link below to find the folks closest to you and get a price for repair;

https://www.madisoundspeakerstore.com/speaker-repair

BTW, I have built a variety of subs for HT use with good results. If the drivers are shot, the cost to repair is a lot and you like to tinker you could build a nice pair of subs for a whole lot less than buying store bought ones, and they would probably sound better & last longer.

 

[timelinex]

First thing I would do is push in and out on the cone (not the dust cap) and listen carefully for a scratching sound, especially on unit 1 (the one that barely plays). If it does, you have a bad voice coil. The video isn't 100% conclusive IMO. If you are certain it is the driver, before replacing it, use the link below to find the folks closest to you and get a price for repair;

https://www.madisoundspeakerstore.com/speaker-repair

BTW, I have built a variety of subs for HT use with good results. If the drivers are shot, the cost to repair is a lot and you like to tinker you could build a nice pair of subs for a whole lot less than buying store bought ones, and they would probably sound better & last longer.

Just tried this and its completely silent when I push in and out

 

[aceinc]

Do you have any other amplifier around that you could hook the really bad driver directly to?

There is a very small chance that what I am about to suggest could cause damage to the amplifier, but it would need to be a really crappy amplifier for it to damage it, or the driver would need to be a dead short which based on your description and video it is not.

Take the driver, attach it to any amp and play something with some decent bass through it. Start at low volume and ramp it up.This will remove the amplifier in the sub from being the issue. If it plays well, then there is an electronics issue, if it buzzes and rattles, then it is the driver.

The amp doesn't need to be beefy a cheap receiver would work just fine.

 

[timelinex]

Can I just hook up the one from the not too bad subwoofer to the really bad subwoofer? If it's still really bad and not a little bad, then it indicated the driver still?