Speaker making subsonic movements

Discussion in 'Hi-Fi and General Audio' started by darynngarrett7, Oct 1, 2021.

  1. darynngarrett7

    darynngarrett7

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    When on phono input, my bass driver on one channel is moving in and out, i estimate at some very low frequency as i can see it but not hear it. This is only on the turntable input, without the stylus even on the record. Swapping the leads round still leaves the problem on the right channel only. Unplug the tt altogether and the phono input is fine, just the usual gentle hiss. To make matters worse, this sometimes does not happen. Even at its worst its usable, i just worry about the poor cone. Speakers and amp are old, owned 30 years. Recent cartridge change and amp recap. The gear itself, speakers epos es14, amp audiolab 8000c pre and rotel rb850 power. Turntable nad rega 2 clone with AT vm95ml cartridge. All of the above regularly making lovely music ...just this one problem
     
    darynngarrett7, Oct 1, 2021
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  2. darynngarrett7

    Sergeauckland

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    This is sadly normal. Your loudspeakers have bass reflex loading, so at very low frequencies there's little restoring force on the cone, and any low frequency signals will move the cones.

    The problem is that many phono stages in amplifiers, or indeed separate stages have no rumble filtering, so low frequency rubbish gets through unattenuated. This can be made worse by any mismatch between the cartridge's compliance and the arm mass, which can reduce the resonant frequency to warp frequencies. Ideally, the arm/cartridge resonant frequency should be around 9-11Hz, above the 3-7Hz warp frequency and below the 20Hz or so lowest audio frequencies.

    I would firstly check that the arm/cartridge resonant frequency is around the 10Hz mark, using any of the commonly available test records. At first glance, I don't see any great mismatch between your cartridge and arm, but that depends on which version of the Rega 2 your NAD is a clone of. If it has the arm with a removable headshell, that may have been changed to a heavier one, and that could make the problem worse.

    Secondly, looking at the 8000C manual, I don't see any mention of a fixed or switchable rumble filter, and with the phono frequency response quoted as flat (-0.5dB) at 20Hz, I doubt there is one, as otherwise I'd expect 20Hz to be down a couple of dB.

    As to damage to your loudspeakers, it shouldn't be doing any damage, but is clearly reducing the voltage output capability of the amplifier, so you won't get as much useful (i.e. audible) power from the amp before clipping.

    The real solution to all this is to buy an external phono stage with a built-in rumble filter, or change the pre-amp to one that has one, or just to live with it. Loudspeakers with sealed bass won't exhibit the wild cone movements as there's a restoring force all the way down to very low frequencies, but the problem of wasted voltage excursion will still be there, just rather less visible.

    S.
     
    Sergeauckland, Oct 2, 2021
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  3. darynngarrett7

    chris_w

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    put 3 or 4 squash balls or simular undernether the TT if the problem disapears you have a sub sonic mechanical feedback. if that does not fix it then try and place a blanket/towel/cardboard box over the TT, if the problem disapears you have a sub sonic acoustic feedback. try moving the TT to a different place. Rumble filters are really a last resort fix as they also round of the bass to much.
     
    chris_w, Oct 2, 2021
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  4. darynngarrett7

    Sergeauckland

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    Sorry, I disagree. ALL phono stages should have a rumble filter, as subsonic noise is mostly caused by the arm/cartridge resonance as the cartridge is a seismic detector, which inevitably and unavoidably has a resonant peak at LF There's also a resonance with the stylus tip mass and the compliance of vinyl, which should also be rolled off by an HF filter, but that's a separate issue.

    That's not to say that feedback won't make things worse, but uncontrolled cone movements when playing vinyl have nothing to do with feedback as they occur even when playing a silent groove, as they're inherent in vinyl playback.

    Unfortunately, there's an audiophile view that filters are a bad thing, whilst anyone who knows how these things work and why resonances occur, know they're necessary. If nothing else, they'll stop amplifier headroom and loudspeaker cone excursion being wasted on spurious LF.

    Your suggestion about squash balls or blankets applies only to feedback at mid or higher frequencies as the resonant frequency of the squash balls and turntable will be much higher than the 3-10Hz of the loudspeaker cone movements so won't isolate that low. Ditto the blankets, they'll absorb mid and high frequencies, probably above 500Hz or more, so again won't do anything about the cone movement.
    S
     
    Last edited: Oct 2, 2021
    Sergeauckland, Oct 2, 2021
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  5. darynngarrett7

    darynngarrett7

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    Hi and thanks for all the above...will prob give the squash balls a go. The only thing I ever owned with a rumble filter was an Amstrad Tower System in 1980. Clearly Sir Alan was ahead of the curve. Strangely the dreadfulness of the tape deck on that was what led me into hi fi addiction. From memory the bsr turntable was almost ok...dont shoot me, I said almost. There was a scratch filter too and power meters. Happy days!
     
    darynngarrett7, Oct 2, 2021
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