Speaker defect?

[Richard from Madison]

Hello! I am a long time viewer of ASR but just recently joined. I recently bought a pair of MoFi Sourcepoint 888's. I've been having a little trouble acclimating to them - my old Vandersteen 2C's were a little rolled off in the highs and my tinnitus seems to be aggravated a bit by the more extended highs of the MoFis - they sound a little rough on top. I have seen this before from other speakers but I'll save that for another discussion. I decided to break out my old UMIK-1 and REW to see if there are any issues that are measurable.

What I'm seeing is that the right speaker measures fine but the left speaker has a spike in third order distortion starting around 1.4 kHz and peaking at 3% around 2 kHz. My laptop is running directly into my power amp and I swapped amplifier channels just to be thorough. The problem follows the speaker. The levels, etc., were unchanged between measurements.

Can anyone think of a scenario where this is NOT a speaker defect? Thanks!

 

[raindance]

1.6K is the crossover to the tweeter and there'll be some overlap just below so I'd suggest it's a faulty tweeter.

Did you also swap the speaker locations in the room just to be sure?

I'm dealing with a similar issue with my Wharfedale Lintons. One tweeter is louder, has elevated distortion and sounds quite coarse.

 

[Flaesh]

Can anyone think of a scenario where this is NOT a speaker defect?

I can't.

 

[Redacted]

Could it be a failing component in the XO?
Another test might be to swap the XO boards.
Sure, dying tweeter is most likely, but....

 

[levimax]

Most likely some kind of a speaker defect but I would check to make sure all the driver screws are tight and confirm there is nothing on top of the speaker or something the speaker is sitting on top of that could be vibrating. As mentioned swap out speakers to confirm the issue first.

 

[Richard from Madison]

1.6K is the crossover to the tweeter and there'll be some overlap just below so I'd suggest it's a faulty tweeter.

Did you also swap the speaker locations in the room just to be sure?

I'm dealing with a similar issue with my Wharfedale Lintons. One tweeter is louder, has elevated distortion and sounds quite coarse.

It occurred to me to swap the speakers but at 96 lb that is the last thing I'll get to. Luckily dragging them around on a beach towel has worked pretty well on hardwood floors.

It seems highly unlikely that the speaker location could have anything to do with odd order harmonic distortion starting at the crossover point, but I do like to cover all the possibilities.

 

[Richard from Madison]

Could it be a failing component in the XO?
Another test might be to swap the XO boards.
Sure, dying tweeter is most likely, but....

I'm at the point where I'll ask Crutchfield and MoFi what they want me to do. A bad driver would be convenient because I could swap that out if they shipped a new one (assuming they don't customize the crossover values for the particular drivers, which some manufacturers do). I'd rather avoid two more 100 lb freight shipments if possible. These speakers got flown to the wrong hub and were delayed for a week with no tracking updates. Gave me a bad taste for the whole process.

A crossover replacement would be a little too much disassembly for a new speaker with a non-zero chance of damaging something in the process. I would have them ship a new speaker before I'd do that.

Thanks for your input!

 

[Flaesh]

were delayed for a week with no tracking updates.

At this time, scammers could replace a working speaker with a faulty one!

 

[mglobe]

You might want to swap the speaker cables at the amp just to make sure the issue isn’t upstream of the speakers.

 

[Richard from Madison]

You might want to swap the speaker cables at the amp just to make sure the issue isn’t upstream of the speakers.

Already been done, thanks! And I swapped the channels coming from the laptop in case the laptop had an issue, unlikely as that may be. And I did swap the location of the speakers just to be thorough. My assumption from the beginning was a bad driver but I don't have the luxury of having one on hand to swap so my task at this point is to give MoFi as much data as possible. My first real job, decades ago, was as an electronics technician.

 

[Richard from Madison]

I sent a link to this thread to MoFi and they are going to send a replacement driver. No muss, no fuss. I'll update the thread once it's installed.

 

[Flaesh]

It would be interesting to know the cause of these odd harmonics, especially at low volumes.
Just for example, a cheap tweeter from Ali-Express:

 

[Richard from Madison]

It would be interesting to know the cause of these odd harmonics, especially at low volumes.
Just for example, a cheap tweeter from Aili-Express:
View attachment 538405

I would be fascinated to know as well. I did another test 5 dB louder and the distortion started at 800 Hz instead of 1.4 kHz. But the 2 kHz peak was exactly the same amplitude as it was at the lower level. The fundamental got louder but the third harmonic distortion did not, it just spread out to a lower frequency. As a percentage at 2 kHz it's actually lower than it was at a lower playback level. I can think of a lot of possible driver defects, loose windings, bad glue bonds, etc., but I have no idea what would cause this.

I think I need to run distortion tests on all of my speakers going forward. The symptoms can be subtle, you might live with a problem for years without knowing.

 

[Richard from Madison]

MoFi sent the replacement driver and the problem is resolved. In case anyone is curious, here is a picture of the driver. According to the invoice it's the same unit used in the SourcePoint 8. Because of the neodymium magnet it's very light, the shipping weight was just over 4 lb.

Replacing the driver was a bit of a trial, it didn't want to come out. I ended up finding a machine screw that barely fit in the holes in the basket and threading it in to push the driver out. Also, the crimp terminals on the speaker wires have locks you need to press before you can pull them off.