JBL L150 Sub Bass Mystery

[DJ5D]

Hi, I'm restoring some JBL L150s and I've run into a mystery.

I refoamed all the speakers successfully, but then I noticed that the sub bass response was extremely poor in one of the cabinets. A 40Hz test tone revealed that the passive radiator was putting out almost nothing. (note: i did not test anything prior to the refoaming)

To troubleshoot, I swapped out all the speakers with the other cabinet. No effect.

So as far as I can tell, it's not the speakers themselves, or the wiring, or the amplifier. All i'm left with is that the internal crossover board must have a bad component.

Any suggestions on further troubleshooting?

 

[LHorn]

I'm by far no expert but is the cabinet sealed well? I would think with a passive it would be critical.

 

[Katalyst]

To troubleshoot, I swapped out all the speakers with the other cabinet. No effect.

Did you also swap the passive radiator?

 

[Chris Brown]

I agree we need more detail on exactly what you swapped, particularly in regards to the passive radiator. Did you swap the passive radiator at the same time as the woofer (same woofer always paired with same PR) or did you also try mixing them?

Does each passive radiator have the same number of weight discs attached to the rear? This directly affects the tuning of the speaker.

A properly sealed cabinet is critical, as was said. One spot to check is the small panel in the rear where the cables connect, as it can easily come unglued and pressed inward.

You can always bypass the crossover for testing purposes. Connect a speaker wire from the main speaker terminals inside the speaker to the woofer. This would potentially send a full-range signal to the woofer but if you are simply testing with a 40hz test tone, it doesn't matter.

 

[DJ5D]

Yes, I swapped both the active and passive speakers in different combinations, with the same result- that's why i am fairly certain that the speakers are not the issue. I haven't tried anything yet with the weight discs, though.

The cabinet is sealed well enough that when you push on one speaker with your finger you can see the other move. Is this an effective test?

I will try bypassing the crossover next, that's a great idea, thank you!

 

[DJ5D]

I tried bypassing the crossover....going straight from amp to the speaker....and still, negligible sub bass frequencies. Possibly the wires? I'm going to try bypassing the crossover on the good cabinet next, just for fun.

 

[David_NC]

If I'm getting this right, one channel / speaker always has less bass. Can you verify that the amp / source isn't the issue? Use one speaker wire and test each woofer on its own, and then test the other woofer (with that same wire) and see if they sound the same. There's only so many things that this can be. You've determined it's not the drivers or PR. Bypassing the crossover removes that from the list of suspects. You say there's no air leaks, and I'd check that a bit more, but I'd verify that it's not something with the source or amp first (if you haven't already done so).

 

[qguy]

To troubleshoot, I swapped out all the speakers with the other cabinet. No effect.

Just to be clear, when you swapped the drivers, did the problem stay with the same enclosure or it followed the drivers?

if it stayed on the same enclosure, swapped the crossovers. if the problem is still on the same enclosure, then the problem is not in the loudspeakers

 

[qguy]

is the louder speaker in a corner ? Corner placement increases bass response

 

[Chris Brown]

I tried bypassing the crossover....going straight from amp to the speaker....

Clarifying the terminology a bit would be helpful for those following this thread and trying to get an idea of what you have or haven't done. Each entire L150 is a "Speaker", which includes a "Tweeter", a "Mid-range", a "Woofer", and a "Passive Radiator". Referring to just the white 128H woofer on it's own as a "speaker" is a bit like referring to a motor vehicle "engine" as a "car", and does make it slightly more difficult to know exactly what you are doing when reading your posts.

As has been touched on above, this could easily be a room issue, for a number of reasons. In many ways, the lower the frequency, the more the room is going to be a factor. This is because the length of the sound wave increases dramatically as the frequency decreases. For example, a 220hz sound wave is just over 5 feet long, but a 40hz sound wave is over 28 feet long. Perhaps the sound from that one cabinet at that location is reflecting off a nearby wall and cancelling out the initial wave at that frequency before it even has time to fully mature.

Easiest way to test this, and to verify that it's not a cabinet issue, would be to simply swap the two cabinets between their existing locations, assuming you have not done this already.

 

[DJ5D]

qguy & chris, you guys nailed it. I swapped the cabinets themselves, and it indeed was a room issue.

one of the corners of my basement is solid concrete, and though the thought had crossed my mind that it could be a factor, I had no idea it would make that huge of a difference. A cabinet placed in that corner produces easily triple the amount of sub than anywhere else in the room.

I think that this is possibly the largest amount of unnecessary troubleshooting i have ever done....*sigh* ...I'm going to need new screws for the woofer and passive radiator mounts, since I've stripped the heads by taking them on and off so many times with a cordless driver.

Thanks for the help guys! Next task, belt replacement on a TEAC A-510 cassette deck.

 

[David_NC]

qguy & chris, you guys nailed it. I swapped the cabinets themselves, and it indeed was a room issue.

one of the corners of my basement is solid concrete, and though the thought had crossed my mind that it could be a factor, I had no idea it would make that huge of a difference. A cabinet placed in that corner produces easily triple the amount of sub than anywhere else in the room.

I think that this is possibly the largest amount of unnecessary troubleshooting i have ever done....*sigh* ...I'm going to need new screws for the woofer and passive radiator mounts, since I've stripped the heads by taking them on and off so many times with a cordless driver.

Thanks for the help guys! Next task, belt replacement on a TEAC A-510 cassette deck.

Hey, it was a learning experience. You didn't have to replace anything to resolve it, so it wasn't a total bust!

 

[Katalyst]

Hey, it was a learning experience. You didn't have to replace anything to resolve it, so it wasn't a total bust!

Amen to that.

/QUOTE]

I'm going to need new screws for the woofer and passive radiator mounts, since I've stripped the heads by taking them on and off so many times with a cordless driver.

Learned that leason myself. Now I use the manual driver and work on my Popeye arms.

 

[vintagerocks]

Ditto on manual screwdriver! Too easy to slip off the screw with power drill and punch through a brand-new foam surround. Been there, done that!

 

[imral3]

Glad you got it all "fixed"!!

 

[donprice]

I think that this is possibly the largest amount of unnecessary troubleshooting i have ever done....*sigh* ...I'm going to need new screws for the woofer and passive radiator mounts, since I've stripped the heads by taking them on and off so many times with a cordless driver.

Not sure if these are the correct size for your L150, but I'll never use a phillips head (or similar) screw in a speaker again. The hex socket head means you don't have to push down to get a grip on the fastner which means much less chance of poking the driver, stripping the head, or pushing/spinning out the tee-nut. You might need to add a small washer to make sure it doesn't pull through the hole in the driver which can be an issue on larger drivers (15" & 18")...not sure about a 12.

https://www.mcmaster.com/91251a203

You may also want to look at adding a couple drops of CA glue (or epoxy) to anchor the t-nuts a little better.

Good luck!