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Old speakers: bass causing abnormal vibration, What could it be and should I bother fixing it?
ChronoSphere
post Apr 15 2013, 23:50
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Hmm, reading the specs of these speakers, the woofer seems to have some kind of electro-mechanical feedback system to correct the bass output or something along these lines. Do you have any idea what they mean and could a failure of this sensor lead to the erroneous behavior of the driver?
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MikeFord
post Apr 16 2013, 01:22
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QUOTE (ChronoSphere @ Apr 15 2013, 15:50) *
Hmm, reading the specs of these speakers, the woofer seems to have some kind of electro-mechanical feedback system to correct the bass output or something along these lines. Do you have any idea what they mean and could a failure of this sensor lead to the erroneous behavior of the driver?

Two types were primarily used, baloney, ie nothing dressed up in the marketing literature, and a extra coil on the voicecoil that is supposed to provide feedback to the amp in relation to the cone motion. Neither should make any noise.

If you have a stethoscope handy, even a cheap one, you can use the open end of the rubber tube to listen to smaller areas and perhaps locate the source of the sound. Even a plain rubber hose works, its a trick mechanics use to find problems.
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splice
post Apr 16 2013, 11:39
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QUOTE (ChronoSphere @ Apr 15 2013, 15:50) *
Hmm, reading the specs of these speakers, the woofer seems to have some kind of electro-mechanical feedback system to correct the bass output or something along these lines. Do you have any idea what they mean and could a failure of this sensor lead to the erroneous behavior of the driver?


If it did have some sort of feedback sensor, and if the sensor had failed, it might be overdriving the woofer at the lowest frequencies where the feedback would normally be most effective, and banging the voicecoil into the backplate.


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Don Hills
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Arnold B. Kruege...
post Apr 16 2013, 20:16
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QUOTE (ChronoSphere @ Apr 15 2013, 18:50) *
Hmm, reading the specs of these speakers, the woofer seems to have some kind of electro-mechanical feedback system to correct the bass output or something along these lines. Do you have any idea what they mean and could a failure of this sensor lead to the erroneous behavior of the driver?


Which raises the question - do the speakers hook to the power line, or are they typical passive speakers?

Another observation - speakers with negative feedback loops tend to fry more easily than usual if overdriven. When the driver runs out of excursion the internal power amp tries to correct that and ends up sending a lot of power to the speaker driver.

This post has been edited by Arnold B. Krueger: Apr 16 2013, 20:19
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ChronoSphere
post Apr 17 2013, 00:36
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QUOTE (MikeFord @ Apr 16 2013, 02:22) *
Two types were primarily used, baloney, ie nothing dressed up in the marketing literature, and a extra coil on the voicecoil that is supposed to provide feedback to the amp in relation to the cone motion. Neither should make any noise.

If you have a stethoscope handy, even a cheap one, you can use the open end of the rubber tube to listen to smaller areas and perhaps locate the source of the sound. Even a plain rubber hose works, its a trick mechanics use to find problems.
Well the sensor in these drivers is placed under the "dome". I don't really have a stethoscope or a rubber tube, but from what I see, it somehow seems to be producing the correct movement, but then suddenly vibrates at the end of the pulse, coming to a standstill, which kinda feels like what splice said:
QUOTE (splice @ Apr 16 2013, 12:39) *
If it did have some sort of feedback sensor, and if the sensor had failed, it might be overdriving the woofer at the lowest frequencies where the feedback would normally be most effective, and banging the voicecoil into the backplate.
Though I'm not really sure it's that. I don't really know how to show it to you, would filming it help?

QUOTE (Arnold B. Krueger @ Apr 16 2013, 21:16) *
Which raises the question - do the speakers hook to the power line, or are they typical passive speakers?

Another observation - speakers with negative feedback loops tend to fry more easily than usual if overdriven. When the driver runs out of excursion the internal power amp tries to correct that and ends up sending a lot of power to the speaker driver.
They have both modes, active (integrated amplifier) and passive (external amplifier). I don't know if this sensor is active in both modes, or only in active mode. I do have the electrical scheme of the speakers if that would help you, my ability to read them is quite limited.
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splice
post Apr 17 2013, 17:07
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The sensor will only be active in "active" (integrated amplifier) mode.
If it sounds normal when being driven from a normal amplifier but bad when using the built-in amplifier, there are several possibilities:
- Sensor has failed.
- Sensor has come loose from the voice coil.
- Wire from sensor to amp has failed.
If it also sounds bad when driven from a normal amplifier, then possibly:
- Voice coil damaged
- Sensor loose (and rattling)
- Air leaks (use door/window sealer strips to seal around the back panel)


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Don Hills
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ChronoSphere
post Apr 17 2013, 17:22
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I already sealed the cabinet, there was some air leaking but now it doesn't.
Sensor doesn't seem to be loose, I tried shaking the driver and there is no noise coming from it. Another weird thing is that the noise only appears when installed into the cabinet, if I dismount it and drive at the same volume, nothing abnormal happens.

So, what is left is voice coil damage? I never tested the integrated amplifier(s) actually...
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