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Old speakers: bass causing abnormal vibration, What could it be and should I bother fixing it?
ChronoSphere
post Apr 7 2013, 14:16
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I have an old system of stereo speakers of mostly a nostalgic value (my dad had them when I was a kid) that started showing the following behavior:
on heavy bass, one of the speakers produces clanging noises as if the speaker is hitting something from the vibration.

It only really affects one of the speakers and only on really heavy bass. I don't know anything about speaker's internal workings but could it be that some kind of mount loosened over time and is not holding the speaker in place? Or is it just "normal" wear and tear and there is nothing to be done?
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julf
post Apr 7 2013, 14:46
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If it really is a metallic noise, then probably a speaker mounting bracket or a reinforcement bracket has come loose. If it is a scratching sound it's a different matter...
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ChronoSphere
post Apr 7 2013, 17:22
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Yes, it's not a scratching noise, sounds pretty much metallic. I didn't try to open the box yet, not even sure it's possible without having to break it apart. From the front, only the normal movement of the membrane is visible.
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julf
post Apr 7 2013, 18:04
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QUOTE (ChronoSphere @ Apr 7 2013, 18:22) *
Yes, it's not a scratching noise, sounds pretty much metallic. I didn't try to open the box yet, not even sure it's possible without having to break it apart. From the front, only the normal movement of the membrane is visible.


How is the speaker element itself attached? Taking it out might be the easiest way to inspects the innards of the box.
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ChronoSphere
post Apr 7 2013, 18:33
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One more addition, the behavior only occurs on higher (~45%+) volume.

I just took the bigger one out (man are they heavy), they are fixed to the front plane with four screws. There are no apparent cracks or anything on the supporting mount. About the only thing that is suspicious is that one of the 4 screws cannot be tightened, probably the winding is giving out.

It doesn't seem to be related to the noise I'm hearing. I removed the whole (metal!) backplate including the integrated amp and the noise mostly stopped. Now all I hear is slight crackling from the tweeter my younger brother poked in. I think the sound was coming from the backplate. The insulation(?) between it and the backplate has long disintegrated, so maybe even tightening the screws doesn't help.

Just making sure, as long as the speakers are in passive mode, the capacitors etc are "safe" to touch, right? I'd want to dismount the tweeter and try to pop the membrane back into spherical form. Would that be a bad idea?

This post has been edited by ChronoSphere: Apr 7 2013, 18:33
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julf
post Apr 7 2013, 18:41
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QUOTE (ChronoSphere @ Apr 7 2013, 19:33) *
Just making sure, as long as the speakers are in passive mode, the capacitors etc are "safe" to touch, right?


Yes, no high voltages involved, and any charge gets quickly dissipated through the speaker anyway.

QUOTE
I'd want to dismount the tweeter and try to pop the membrane back into spherical form. Would that be a bad idea?


Tricky - I would try to do it from the front, using a small needle to grab the dome.
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ChronoSphere
post Apr 7 2013, 18:48
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QUOTE (julf @ Apr 7 2013, 19:41) *
Tricky - I would try to do it from the front, using a small needle to grab the dome.
Hmm, without piercing it, though? I'm not sure how this one works but I can't imagine it would like a hole pierced into it to grab the dome.
I thought it would be possible to push the dome from the inside, but the thing looks solid...
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trout
post Apr 7 2013, 19:47
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There are other methods for "fixing" a pushed-in tweeter dome, like using tape or vacuum suction. Here is one of many discussions on the topic : http://www.diyaudio.com/forums/multi-way/1...ter-pushed.html .

(I said "fixing", in quotes, because reversing the "dent" doesn't necessarily mean the tweeter will function properly / as new.)
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julf
post Apr 7 2013, 20:13
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QUOTE (ChronoSphere @ Apr 7 2013, 19:48) *
Hmm, without piercing it, though? I'm not sure how this one works but I can't imagine it would like a hole pierced into it to grab the dome.


The dome doesn't need to be air tight. There won't be any real ill effect from a small needle hole.

QUOTE
I thought it would be possible to push the dome from the inside, but the thing looks solid...


Most dome tweeters are totally sealed at the back.
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ChronoSphere
post Apr 7 2013, 21:25
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Hmm, it seems to be something different than the transformer mounting. I reassembled the whole thing and the sound is back. It does go away once i remove the whole backplate. I don't know if pictures would help you, but I can take some if you tell me what you want to see...

The dome didn't want to bend back completely, but the crackling is now much less than before.

edit: ok, so I took the backplate from the working speaker, and put it into the defective one - the same thing happens. Now it's not a metallic sound anymore though, it sounds like two pieces of rubber or plastic rubbing on each other. As if putting the backplate on causes the bass to become too strong and it's pushing the membrane in recoil... or something huh.gif

This post has been edited by ChronoSphere: Apr 7 2013, 21:49
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Arnold B. Kruege...
post Apr 8 2013, 03:29
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QUOTE (ChronoSphere @ Apr 7 2013, 09:16) *
I have an old system of stereo speakers of mostly a nostalgic value (my dad had them when I was a kid) that started showing the following behavior:
on heavy bass, one of the speakers produces clanging noises as if the speaker is hitting something from the vibration.

It only really affects one of the speakers and only on really heavy bass. I don't know anything about speaker's internal workings but could it be that some kind of mount loosened over time and is not holding the speaker in place? Or is it just "normal" wear and tear and there is nothing to be done?



Look inside the box. Something might be loose. If it is, tighten it up.

You acess the inside of most loudspeaker systems by removing the largest driver with a few screws.
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DVDdoug
post Apr 8 2013, 21:39
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QUOTE
edit: ok, so I took the backplate from the working speaker, and put it into the defective one - the same thing happens. Now it's not a metallic sound anymore though, it sounds like two pieces of rubber or plastic rubbing on each other. As if putting the backplate on causes the bass to become too strong and it's pushing the membrane in recoil... or something
If you move the woofer in & out (carefully) by hand, do you hear/feel any rubbing? Sometimes a woofer can sag out of alignment and you can get rubbing of the voice coil (or voice coil form) in the magnet gap.

Sometimes you can minimize that by rotating the woofer. (As a test, you can turn the whole speaker upside-down.)

Sometimes the voice coil can start to come loose and rattle (metallic sound).

Speakers can be reconed. (The speaker cone, voice coil, and suspension are replaced. The magnet & basket are re-used.)

Or you can replace the woofer, which is often cheaper. The problem with replacing the woofer is that the original woofer was (hopefully) matched to the cabinet design, the crossover, and the tweeter. If you can't get an exact factory replacement, the speaker characteristics will change, and if you don't replace the pair, the left & right won't sound alike.

Reconing should give you a speaker closer to the original, but I'd probalby recone both of them to keep them both the same.
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Paul_ 2012
post Apr 9 2013, 01:27
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QUOTE (ChronoSphere @ Apr 7 2013, 15:16) *
I have an old system of stereo speakers of mostly a nostalgic value (my dad had them when I was a kid) that started showing the following behavior:
on heavy bass, one of the speakers produces clanging noises as if the speaker is hitting something from the vibration.

It only really affects one of the speakers and only on really heavy bass. I don't know anything about speaker's internal workings but could it be that some kind of mount loosened over time and is not holding the speaker in place? Or is it just "normal" wear and tear and there is nothing to be done?

I bought my speakers some time ago second hand. First one and then a month later the other bass speaker developed what I called at the time a 'death rattle' only with deep bass notes, which was distinctly metal on metal, very much as you have described . I assumed that they had been thrashed, it was more cost effective for the dealer to fit a new speaker than to have it repaired.

This post has been edited by Paul_ 2012: Apr 9 2013, 01:28
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julf
post Apr 9 2013, 10:23
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QUOTE (DVDdoug @ Apr 8 2013, 22:39) *
If you move the woofer in & out (carefully) by hand, do you hear/feel any rubbing? Sometimes a woofer can sag out of alignment and you can get rubbing of the voice coil (or voice coil form) in the magnet gap.


That was why my first question was about the sound - metallic rattle vs. scratching. Cured a couple of KEF139's by turning them upside down.

This post has been edited by julf: Apr 9 2013, 10:23
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ChronoSphere
post Apr 9 2013, 18:57
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Thanks for all the replies.

I tried turning it upside down, the sound remained the same. I guess it sounds closer to scratching now that I tightened the transformer mount. Here are a few samples, recorded with a clip+

Bad speaker: click
Bad speaker (highpass): click
Good speaker: click
Combined: click

I think you can hear it best at the start of the sample, the ... *skrskrskr*-like post-bass oscillation (?). I don't really get why the noise only comes back with the speaker box completely assembled...

I don't think replacing a driver of a soviet era speaker with an identical model would be possible laugh.gif
Reconning sounds like an idea, if it's really the cone that shifted. But then I'd hear a pure metallic sound, right?

Moving it by hand doesn't reveal any rubbing. I will try soldering it over to the other box to be completely sure it's the speaker itself.

edit: typos

This post has been edited by ChronoSphere: Apr 9 2013, 19:01
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pdq
post Apr 9 2013, 19:10
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QUOTE (ChronoSphere @ Apr 9 2013, 13:57) *
I don't really get why the noise only comes back with the speaker box completely assembled...

Perhaps tightening the mounting screws distorts the speaker frame enough that the coil now rubs inside the magnet.
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ChronoSphere
post Apr 9 2013, 19:49
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That doesn't seem to be it. It seems to be air escaping between the backplate and the casing, at least when I loosen the screws the sound is louder and I can feel air escaping through the slits. It doesn't really explain why the other speaker doesn't have this behavior, unless they're actually not the same on the left and the right... and the one on the "left" (aka "bad") simply has more oomph.

I now soldered it unto the right box and the sound is still the same, so it seems somehow related to this one driver after all. I will try putting new insulation between the backplate and the case, and see if it changes anything.
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ChronoSphere
post Apr 10 2013, 11:42
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Well, I'd say it's a partial success. The annoying sound is now much lower and only starts appearing on volumes I don't feel comfortable listening to. I guess the rest of it comes from the one speaker being more worn out than the other one =/

They're still usable at normal volumes with the majority of the tracks, and that's enough for me. Thank you for all the help.
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MikeFord
post Apr 10 2013, 20:26
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If its just low loud bass, could be voicecoil hitting the backplate due to excursion limits.

Swap the drivers left to right, if the noise follows the driver, bad driver which depending on what it is might easily be replaced.
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MikeFord
post Apr 10 2013, 20:28
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If its a wood cabinet and one of the woofer mounting screws is stripped out, put in a T nut or some thread reinforcement device, or just use a thicker screw.
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ChronoSphere
post Apr 10 2013, 20:50
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Yes, I did both, no change. So it's probably a defective driver. I doubt it's worth replacing it though, I know too little about driver characteristics in general and finding out the exact specs is a bit hard. I actually found out the company who made these still exists and they're listing this driver as "EOL, no modern analogues exist". Specs are also not listed.

Edit: these are the speakers btw: click
They mention the 3 drivers for the high/middle/low frequencies and two passive filters, 700Hz and 5000Hz

This post has been edited by ChronoSphere: Apr 10 2013, 20:57
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DVDdoug
post Apr 10 2013, 21:46
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...I doubt it's worth replacing it though, I know too little about driver characteristics in general and finding out the exact specs is a bit hard.
Of course that's up to you... If it was me I'd probably replace them with something. Those are cool-unique looking speakers, and with the nastalgic value it would be worth it to me. I assume you are not in the U.S., but just for example, Parts Express has 8-inch woofers starting at $11 USD. (It looks like about 8-inches.) If you replace the woofer with a new "cheap" woofer and it sounds worse, you can always put the old one back in.

You can usually get the Thiele-Small parameters for any new woofer, and with WinISD (FREE!!!) you can predict bass performance in your existing cabinets, if you wish to make the effort.

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ChronoSphere
post Apr 10 2013, 23:17
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It is really tempting to be honest.

The driver is 25cm in diameter (24,5cm if not counting the mounting itself), which is 9.8 inches according to google. I think the problem would be the peculiar form of the mount. You can see it if you search this page for "75ГДН-5", though mine actually look exactly like the "75ГДН-1-4" (despite being labeled 75ГДН-5)

I'd probably have to modify the cabinet to fit a new driver somehow. From my understanding a "closed box" design has to be more or less airtight to not suffer from "wheezing"? I might need some kind of a guide to different cabinet types and what is required :x

I'm sure there are some German members around here, so maybe someone could point me to a good place to get the drivers? I tried looking at amazon.de, but the biggest they have is 20cm. The funny thing though, the one I found has the same peculiar form mine have ^^

edit: uhm. mellow.gif I think I just found a more recent version of the radiotehnika site. Whichever the official one is lol. Anyway, the other site has the full specs in English (though the site is still latvian/russian only): click

This post has been edited by ChronoSphere: Apr 10 2013, 23:25
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MikeFord
post Apr 11 2013, 11:40
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QUOTE (ChronoSphere @ Apr 10 2013, 12:50) *
Yes, I did both, no change. So it's probably a defective driver.

Could you clarify that?

Swapped L and R inputs to speaker and sound stayed in same speaker?

Swapped woofer to other cabinet and bad sound followed the driver?
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ChronoSphere
post Apr 11 2013, 11:50
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I swapped the woofer to the other cabinet and the sound followed the woofer. The screw now also holds the speaker in place fine.
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