Subwoofer stopped playing - need help.

Gardenstater

I have a Martin Logan Dynamo Sub that stopped working a couple weeks ago. When the gain is turned all the way up there is still no sound and if you place your fingers on the cone you can feel it twitching ever so slightly. Turning the gain down even a little, there is no cone movement whatsoever.

Well at first I thought the amplifier was probably at fault, because the woofer *seemed* to pass the basic tests of: 1) Manually exercising the cone with my fingers, 2) Connecting a 9V battery to the woofer, and 3) Checking the DC resistance of the voice coil.

So.......I went and connected a Dayton 200W Plate Amplifier to the sub. I even went to the trouble of fabricating a back plate that seals, with a pass through seal for a wiring harness to the externally located Dayton Amp. Well would you believe it still didn't work?? Exactly the same behavior. This amp tests out fine on a Mordaunt Short subwoofer that is also 4 Ohm impedance rated.

So obviously something is actually wrong with the woofer driver. Here's how the tests pan out when I repeated them:


1) Manually exercising the cone results in pretty good range of motion with no sensation of any voice coil rubbing whatsoever. I'd say the woofer is a bit stiff but it has a very substantial surround on it so I think it is normal.

2) Connecting a 9V battery (measuring 9.5V) to it with the proper polarity results in click or snapping sound and the cone rapidly moves outwardly about 1/8" and then stops abruptly, which is quite a bit less than the full range of movement I got when manually pushing on the cone. Reversing the polarity gives the exact opposite result. When I repeat the test with a more worn out 9V battery (measuring 8.8V) I get a clicking noise but the cone will barely move at all, with just the slightest amount of visible motion, regardless of the polarity of the connection.

3) Checking the DC resistance of the voice coil gives a fluctuating reading between 2.9 and 3.7 Ohms which doesn't quite seem right to me, that it should be so unstable. I mean it never settles at all.

So, just wondering if anyone has any helpful suggestions on what could be the problem or any remedies to the situation. Martin Logan says it is very unusual and they are checking upon driver availability but the subwoofer has been out of production for quite some time so they may not have any at all and I don't know how repairable this driver will turn out to be, even if a replacement voice coil is to be had any where.

Thanks!

Gardenstater

Thanks. That makes sense to me too. You know....it didn't really seem like the Mordaunt Short driver was moving very much either, at least not much more than 1/8". However, the size of the surrounds on the Mordaunt Short drivers (2 8 Ohm impedance units in parallel) are tiny compared to the huge surround on the Martin Logan driver and I feel they are probably not nearly as high excursion. For a little while the Mordaunt Short stopped working and I was about to curl up in the fetal position. Definitely something strange is going on here and I will revisit all this after a recharge in the morning

westmassguy

Sounds like the voice coil has intermittent shorting. Not a good sign

mhardy6647

westmassguy wrote: »

Sounds like the voice coil has intermittent shorting. Not a good sign

Perhaps one of the tinsel leads from the terminals to the voice coil is 'loose'?
I concur with the above-quoted expert's opinion (fwiw): "Not a good sign."

Gardenstater

Mordaunt Short Driver (8Ohm nom. impedance, DCR 6.1Ohm):

9.5V battery: about 1/8" extension of cone (14.8W)
8.8V about 1/32" (12.7W)
8.0V minimal, barely felt any movement (10.5W).

E=iR P=i^2R P=Ei

I think the next step is to test the Martin Logan woofer (and the Mordaunt Shorts) with a frequency generator through my main amplifier and try to measure the dB spl's (if any) with an app on my phone.

The unstable DCR readings were due to a low 9V battery in my DVM. Now getting stable 3.0-3.1 Ohms on Martin Logan

Gardenstater

mhardy6647 wrote: »

westmassguy wrote: »

Sounds like the voice coil has intermittent shorting. Not a good sign

Perhaps one of the tinsel leads from the terminals to the voice coil is 'loose'?
I concur with the above-quoted expert's opinion (fwiw): "Not a good sign."

Nothing visibly wrong with tinsel leads in either drivers:

Mordaunt Short:





Martin Logan:

mlistens03

What is your input to the subwoofer amplifier? I've had sub outputs on AVRs go bad in the past. If you have a different AVR or something else to plug into it, try that. You could even plug your phone in with a 3.5mm to RCA cable and use a tone generator app. That tends to be my go-to for testing subs.

Other than that, I don't know what to tell you. Good luck! Martin Logan IME has great CS so should it be the sub they will surely help you out.

-Micah

Gardenstater

mlistens03 wrote: »

What is your input to the subwoofer amplifier? I've had sub outputs on AVRs go bad in the past. If you have a different AVR or something else to plug into it, try that. You could even plug your phone in with a 3.5mm to RCA cable and use a tone generator app. That tends to be my go-to for testing subs.

Other than that, I don't know what to tell you. Good luck! Martin Logan IME has great CS so should it be the sub they will surely help you out.

-Micah

I am using the speaker level inputs. The wires are right on top of the wires that go to my main speakers and they were working just fine and were undisturbed, except to check for tightness. I tested the speaker wires from my Crown D150 amp to the subwoofer amplifier inputs with a DVOM and no shorts or opens. Measure 0 Ohms. So if these wires have mysteriously refused to conduct low frequency audio signals there is something REALLY weird going on. I'm going to try some other wires but have to rig something up......basic lamp cord is what it's going to be instead of my Litz cables that are in there now.

My audio system is connected to a USB Dac which is connected to a Computer. Won't be any problem to run an online signal generator into the DAC to do testing.

mlistens03

Gardenstater wrote: »

mlistens03 wrote: »

What is your input to the subwoofer amplifier? I've had sub outputs on AVRs go bad in the past. If you have a different AVR or something else to plug into it, try that. You could even plug your phone in with a 3.5mm to RCA cable and use a tone generator app. That tends to be my go-to for testing subs.

Other than that, I don't know what to tell you. Good luck! Martin Logan IME has great CS so should it be the sub they will surely help you out.

-Micah

I am using the speaker level inputs. The wires are right on top of the wires that go to my main speakers and they were working just fine and were undisturbed, except to check for tightness. I tested the speaker wires from my Crown D150 amp to the subwoofer amplifier inputs with a DVOM and no shorts or opens. Measure 0 Ohms. So if these wires have mysteriously refused to conduct low frequency audio signals there is something REALLY weird going on. I'm going to try some other wires but have to rig something up......basic lamp cord is what it's going to be instead of my Litz cables that are in there now.

My audio system is connected to a USB Dac which is connected to a Computer. Won't be any problem to run an online signal generator into the DAC to do testing.

Ok, cool. Have you tried the line-level inputs? I'd highly doubt that the speaker level inputs are the problem, but it wouldn't be much trouble to test methinks.

As far as other cables go, I wouldn't bother trying another set unless I had them on hand, because as far as I know, that couldn't happen, you know probably.

Once again, good luck! -Micah

mhardy6647

wait -- what are you (the OP) feeding to the drivers to test them?
Are you using an audio-frequency sine wave generator (or even a phone app)?

Gardenstater

After I try substituting new cables for the existing ones between the amp and the subwoofer amp, yes that is the next thing I will try. Up until now, it has been music from Qobuz, CD, and TIDAL, which played just fine on the main speakers which are hooked up in parallel with the subwoofer via the speaker inputs on subwoofer.

Gardenstater

mlistens03 wrote: »

Gardenstater wrote: »

mlistens03 wrote: »

What is your input to the subwoofer amplifier? I've had sub outputs on AVRs go bad in the past. If you have a different AVR or something else to plug into it, try that. You could even plug your phone in with a 3.5mm to RCA cable and use a tone generator app. That tends to be my go-to for testing subs.

Other than that, I don't know what to tell you. Good luck! Martin Logan IME has great CS so should it be the sub they will surely help you out.

-Micah

I am using the speaker level inputs. The wires are right on top of the wires that go to my main speakers and they were working just fine and were undisturbed, except to check for tightness. I tested the speaker wires from my Crown D150 amp to the subwoofer amplifier inputs with a DVOM and no shorts or opens. Measure 0 Ohms. So if these wires have mysteriously refused to conduct low frequency audio signals there is something REALLY weird going on. I'm going to try some other wires but have to rig something up......basic lamp cord is what it's going to be instead of my Litz cables that are in there now.

My audio system is connected to a USB Dac which is connected to a Computer. Won't be any problem to run an online signal generator into the DAC to do testing.

Ok, cool. Have you tried the line-level inputs? I'd highly doubt that the speaker level inputs are the problem, but it wouldn't be much trouble to test methinks.

As far as other cables go, I wouldn't bother trying another set unless I had them on hand, because as far as I know, that couldn't happen, you know probably.

Once again, good luck! -Micah

Thanks for the input. No I can't try the line level inputs because my system currently has no preamplifier, or at least I'm not sure if the output from the DAC has sufficient voltage and would require some adapters to try. That's something to try if nothing else works though.

Yeah I concur on the cables. I will definitely call it a "paranormal event" if it turns out to be the case, but I will check it out.

mlistens03

Gardenstater wrote: »

mlistens03 wrote: »

Gardenstater wrote: »

mlistens03 wrote: »

What is your input to the subwoofer amplifier? I've had sub outputs on AVRs go bad in the past. If you have a different AVR or something else to plug into it, try that. You could even plug your phone in with a 3.5mm to RCA cable and use a tone generator app. That tends to be my go-to for testing subs.

Other than that, I don't know what to tell you. Good luck! Martin Logan IME has great CS so should it be the sub they will surely help you out.

-Micah

I am using the speaker level inputs. The wires are right on top of the wires that go to my main speakers and they were working just fine and were undisturbed, except to check for tightness. I tested the speaker wires from my Crown D150 amp to the subwoofer amplifier inputs with a DVOM and no shorts or opens. Measure 0 Ohms. So if these wires have mysteriously refused to conduct low frequency audio signals there is something REALLY weird going on. I'm going to try some other wires but have to rig something up......basic lamp cord is what it's going to be instead of my Litz cables that are in there now.

My audio system is connected to a USB Dac which is connected to a Computer. Won't be any problem to run an online signal generator into the DAC to do testing.

Ok, cool. Have you tried the line-level inputs? I'd highly doubt that the speaker level inputs are the problem, but it wouldn't be much trouble to test methinks.

As far as other cables go, I wouldn't bother trying another set unless I had them on hand, because as far as I know, that couldn't happen, you know probably.

Once again, good luck! -Micah

Thanks for the input. No I can't try the line level inputs because my system currently has no preamplifier, or at least I'm not sure if the output from the DAC has sufficient voltage and would require some adapters to try. That's something to try if nothing else works though.

Yeah I concur on the cables. I will definitely call it a "paranormal event" if it turns out to be the case, but I will check it out.

Do you have a headphone jack to RCA cable? If you do, you could just plug the sub straight into your phone. If you don't they aren't hard to find, One could probably be had for less than $5. This is how I do my testing because it removes any variables from the amp or DAC or receiver or anything else, all you have is the sub in question and the (theoretically) known working phone.

On the cable front, I knew it all along! Aliens DO exist! I told everyone but they all thought I was crazy.

Gardenstater

Martin Logan Sub is back in action. Turns out that it WAS some sort of freaky issue with the Litz cable that was feeding the speaker level inputs in the subwoofer amplifier. Making a pair of 13 gauge lamp cord cables solved the problem. I can't explain it. The cable was testing fine by DVM, but it may have been frequency (low frequency) dependent fault. Anyway, I'm glad to have it fixed. Now maybe this house needs a paranormal investigation team, and I need a valium

mlistens03

Gardenstater wrote: »

Martin Logan Sub is back in action. Turns out that it WAS some sort of freaky issue with the Litz cable that was feeding the speaker level inputs in the subwoofer amplifier. Making a pair of 13 gauge lamp cord cables solved the problem. I can't explain it. The cable was testing fine by DVM, but it may have been frequency (low frequency) dependent fault. Anyway, I'm glad to have it fixed. Now maybe this house needs a paranormal investigation team, and I need a valium

Who ya gonna call??

Ghost Busters!

I'd imagine it was probably something to do with one of the connections and not the cable itself... perhaps replacing the cables fixed the issue.

edit: Of course replacing the cables fixed the issue, but I meant that moving/replacing the cables fixed something else.

Either way, I'm glad you got it working! Now just sit back and enjoy the tunes.

Gardenstater

mlistens03 wrote: »

Gardenstater wrote: »

Martin Logan Sub is back in action. Turns out that it WAS some sort of freaky issue with the Litz cable that was feeding the speaker level inputs in the subwoofer amplifier. Making a pair of 13 gauge lamp cord cables solved the problem. I can't explain it. The cable was testing fine by DVM, but it may have been frequency (low frequency) dependent fault. Anyway, I'm glad to have it fixed. Now maybe this house needs a paranormal investigation team, and I need a valium

Who ya gonna call??

Ghost Busters!

I'd imagine it was probably something to do with one of the connections and not the cable itself... perhaps replacing the cables fixed the issue.

Either way, I'm glad you got it working! Now just sit back and enjoy the tunes.

I thought it might be the connection too and so I had already removed the wires and tested them and re-installed them. The connections were super tight before and after because I always use a socket on the binding posts. Also, remember that the sub had been working flawlessly for many weeks and then all of a sudden it was not working, without any of the connections being disturbed at all. It wasn't an intermittent issue. The sub just stopped working. Perhaps it shall remain a mystery.....

Gardenstater

mlistens03 I would say the same thing if I were you. Believe me though, I had checked and re-checked the connections and they were super-tight. I always use a socket wrench (carefully) on the binding posts. They were tight even when the sub mysteriously stopped working, after working flawlessly for about 5 weeks. Then when I checked the cables with a DVOM, I completely removed them and re-attached and the sub was still not working. Perhaps it shall remain a mystery but I think it has something to do with the fact that the cables were Litz cables. Oh and by the way, they did not have crimp connections but they had soldered on spades.

Yep, listening to "Echoes" right now with my first cup of coffee. Good to have it back in order. Now on to the 7B crossover mods that got put on hold due to this whole debacle.

Gardenstater

Update: The plot thickens because I also found an issue with the binding posts on the Martin Logan amp where the plastic actually stood a bit proud of the contact shoulder for spades or ring terminals and the contact could've been a tenuous one against the shaft of the binding post more than the shoulder, no matter how much the binding post nut was tightened. Ya really can't make this stuff up. After cutting away plastic with an xacto knife:



Martin Logan amp working fine and sounds so much better than the Dayton 200W plate amp. Must be true that they have some special inverse EQ circuitry compensating for the rolloff of the woofer:

Gardenstater

I told Martin Logan that they had a manufacturing defect in the binding posts that they supplied with this amplifier. The nut is bigger in dia. than the contact shoulder and the plastic sticks up above the contact shoulder. Doh !!!!

F1nut

One word....bananas

mhardy6647

F1nut wrote: »

One word....bananas

https://youtu.be/VfrShu_Lp2A

Gardenstater

Bananas have their limitations. You can't put more than one cable on an amplifier terminal using only bananas, for example, unless you want to use hokey bananas that accept bananas. Thanks for the advice but the point is I found the problem and fixed it and notified Martin Logan of the manufacturing defect. Working fine now and I don't expect any further problems. Live and learn and I will examine future binding posts with a sharp eye.

PS: Another advantage of Spade and Ring Terminals is that even the inexpensive ones you get at Home Depot or Lowes are made of Copper.

F1nut

PS: Another advantage of Spade and Ring Terminals is that even the inexpensive ones you get at Home Depot or Lowes are made of Copper.

Those posts aren't copper.

Gardenstater

Not at the moment they aren't. Anyway you could take that logic one step further and say......might as well use brass wires then.

mhardy6647

There is no sin associated with stacking banana plugs.
Just wanted to mention that.



DSC_4469 (2) by Mark Hardy, on Flickr

F1nut

Gardenstater wrote: »

Not at the moment they aren't. Anyway you could take that logic one step further and say......might as well use brass wires then.

Well, that would definitely affect the signal, IMO.

Gardenstater

mhardy6647 wrote: »

There is no sin associated with stacking banana plugs.
Just wanted to mention that.



DSC_4469 (2) by Mark Hardy, on Flickr

I don't judge