what could cause B&W CDM7NT low output

[klamsoss]

About a month ago I picked up a set of B&W CDM7NT speakers for my dad. He loves his CDM1NTs, but wanted more bass and wasn't happy with a few different subs he'd tried. I found a nice pair not too far away and I made the pick up. We brought them home, set them up, and gave them a test run that sounded great. Pretty much exactly what my dad wanted!
Until...
A couple weeks ago my dad commented that he had to turn his amp up "really high" to get them to play at even standard listening levels. With his Fisher KX-200 he could turn it up to between 9 o'clock and 10:30 or so and get good, room-filling sound with the CDM1NT, but he's going to 12 o'clock or even higher to get the same level out of the CDM7NT.
The CDM7NT is rated at 90dB/1w/1m while the -1NT is only either 88dB or 89dB.
I asked him to try a different amp so he hooked up his Musical Fidelity A300 150wpc integrated and had the exact same experience. Normal listening almost never even got to the 9 o'clock position with the CDM1NT, but with the 7s, he's at 10-11o'clock for normal listening all the time and if he wants to turn it up a little, he's almost at 12 o'clock. That's WAY higher than I've ever seen that amplifier set, even with Thiels, Monitor Audio, other B&W speakers, etc.
All drivers appear functional on the CDM7NTs, the response is equally subdued on both speakers, and neither is displaying a particularly notable distortion in any frequency band. Multiple amps and multiple different sources have been tried all with the same result.
The particularly confusing part about this is that they appeared to work just fine, and have the expected output upon initial setup.

Ideas?

 

[Erik Tracy]

That does sound odd.

I also have B&W CDM1NTs with a Musical Fidelity A5 integrated and can get rocking loud at 10-11 o'clock on the volume dial with most CDs.

Is your dad using the same source components? I know until I got an external phono preamp, there was different levels between CDs and LPs using the internal phono section of the Musical Fidelity and 'equivalent' volume would be more like straight up 12 o'clock.

 

[klamsoss]

Multiple source components, same outcome regardless.

 

[RichPA]

Speaker sensitivity ratings aren't necessarily accurate. If both speaker sound the same, I don't think there's any problem there, and there's no problem with having the volume at 12 o'clock or above - in fact, the channel tracking is probably better there than at a lower level.

 

[klamsoss]

@RichPA I've auditioned enough speakers to be generally aware of the disparity that sometimes exists between the rated sensitivity and the practical reality. You're 100% right on that front. And yeah, it's not uncommon for me to have my own Fisher up around 12 o'clock when listening, especially if I'm not using my 96+dB Tekton Lores.

The issue here is more that the -7NT uses identical tweeter and mid range drivers to the -1NT and seems to be putting out on the order of -12dB relative to the -1NT. That makes no sense to me. Further, that the speakers seemed to be equal or slightly higher in output on first listen, but then mysteriously diminished drastically has me scratching my head.

Using a 60wpc parasound amp, we had the volume knob up to 3 o'clock, on the ragged edge (clipping was occasionally barely audible) and the speaker still wasn't putting out more than slightly higher than typical listening volume. This same amp pushed the Thiel CS2 to noise complaint levels at around 1 o'clock.

I'm 100% certain this isn't a case of not understanding the power needs of the speaker. There is something more to the issue.

Is there a possibility that some damage has been done to a voice coil when there was a (very brief) red-plating incident with the Fisher?

Can anyone think of another reason why these speakers might be putting out much less sound than they should, or recommend a method for testing them to identify what, if any, issue might exist?

With no game plan, my approach will basically be to disassemble both the CDM1NT and the CDM7NT to test impedance and response of individual drivers. I'm not particularly hopeful that this will yield an explanation, though.

 

[F1nut]

Are you sure the tweeters are working? If they are good then I'd say the crossovers have suffered damage.

 

[klamsoss]

The tweeters are functioning normally, as far as I can tell, aside from the reduced output.

I was thinking that there might have been some sort of damage to the crossovers that's allowing for a bleed off of current prior to it reaching the drivers. That's the only thing that makes sense to me, but I have no evidence that this is the case. One would think that the current being bled off wouldn't be equal between the two speakers, and that there would have to be some sort of evidence of that current being bled off (heat, most likely, and probably a burning smell?). Perhaps there is, internally. I guess I'll need to take these apart regardless.

 

[RichPA]

It's hard to imagine crossover damage that would result in both speakers sounding the same. But in this review the -7NT and -1NT were very similar in sensitivity: https://www.soundandvision.com/content/bw-cdm-nt-series-speaker-system-ht-labs-measures . The impedance measurements are also similar. So I guess you do need to take them apart and investigate.

 

[klamsoss]

@RichPA that's a great piece of tech info! surprised I haven't run across it in my research. Many many thanks!!!!
This data also shows that the measured sensitivity is proportional to the manufacturer reported sensitivity. We thought the CDM7NT would be a better match with the lower powered Fisher my dad has based on that. This has been a thoroughly disappointing venture, rather than a fun and exciting investment in improved sonics.

I agree that the odds of both speakers suffering exactly the same damage and it resulting in the same diminished output in both are similar to the odds of me winning the lottery. The multimeter doesn't lie, though, so I guess I'll find out when I start poking around.

 

[klamsoss]

After measuring the working CDM1NT drivers and the remaining working CDM7NT drivers it's clear that the problem is that both midrange drivers on the 7NTs are to blame. They both measure open.

Now, the big question is how the heck both of the midrange drivers blew at basically the same time?
Any ideas?
My dad insists that he had the speakers wired up properly when they were biwire, and when using jumper plates (I have my suspicions). Even if he had them reversed, that would just reverse the phase, wouldn't it? If he had accidentally switched one set of leads while biwiring, would that blow the coils on one set of drivers but leave the other two intact?

The aim here is to prevent a repeat of the offending action and ensure these speakers last for many years to come.

 

[klamsoss]

I just wanted to update this thread with the final resolution in case anyone in the future looks at it. Ultimately, the drivers were the culprit. I can't identify why they got smoked, but they did. After replacing them with manufacturer-supplied brand new units, the speakers work exactly as expected... exactly as they should. They appear to have slightly higher overall output than the CDM1NTs when at the same notch on the volume control (consistent with the 2dB higher rated sensitivity), they have notably fuller and more extended bass (as you would expect from a speaker with a separate bass woofer and larger cabinet volume), and they retain the spatial and midrange characteristics of the -1NTs (not surprising since they use exactly the same drivers in exactly the same configuration).

The biggest possible "drawback" to my dad's CDM7NTs (other than the inexplicable failure of the drivers, which is looking more like it was user error... there's no other explanation...) is that they are a bit on the short side for many/most chairs and people. With bookshelf speakers, you are automatically using a stand of some sort, so I (and probably most people) tend to end up putting them closer to ear level. I have found that MANY "floorstanders" are a bit squat, though. My Tekton Lore 2.0 speakers sit atop 6" platforms that bring the low-mounted tweeter more inline, and (maybe) reduce some of the floor reflections of the low-mounted ports. the B&W DM602.5 speakers that I use as mains for our home theater are a bit on the short side and only appropriate because the couch we have down there is especially low, and we tend to lay back heavily slouched. The Polks I own are probably the best, but due to their MTM arrangement, they still benefit from a slight lift with normal height chairs/couches. Even my Thiel CS2s sound better on 2" platforms.