"The woofer flapping issue is prevalent among people with small woofers that are tuned to go as low as possible. They have to move a lot farther to produce the same results as a large woofer." Only if the bass response is artificially boosted, as in your system. Otherwise, the smaller woofer will just roll off at a higher frequency than a bigger woofer. It’s even possible in some cases that the smaller woofer will be less likely to dance due to upstream perturbation, both because the smaller woofer can't respond and because of the enclosure damping, as in an acoustic suspension cabinet. This was mentioned earlier.
"Once digitized it is a simple matter to program in a subsonic filter with a very steep slope sidestepping the issue altogether." There is no free lunch in audio; fiddling with the signal in the digital domain will have a cost in SQ that some would not tolerate and others would find OK. Anyway, if one wants a subwoofer system, one can build an active low pass filter with a 24-db/octave slope out of discrete components that work in the analog domain, with potentially less of a sonic penalty. 24db/octave should be sufficient for cleaning up the higher frequencies.
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You won’t see the woofers dancing if they are in an enclosure that constrains motion at subsonic frequencies. You also won’t see them dancing if the preamplifier or amplifier in the upstream chain cannot pass subsonic frequencies, e.g., if you use a transformer coupled tube amplifier or if you have engaged a subsonic filter on your preamplifier. In either of those two cases, the woofers are protected from damage AND you won’t experience the audio distortion associated with the woofer dance. However, if rumble is present due to some sort of feedback (acoustic mechanical or electrical) at the source, these palliative measures do not prevent or cure it. In any system, the presence of rumble does not always cause the woofer dance. Audionoobie, if you have done the experiment of borrowing another TT to see whether the problem persists, and if the problem does persist, then I give up,... you need a rumble/subsonic filter, best inserted between your phono stage and your linestage.
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Two questions for arrowhead: (1) By what mechanism do you think a phono pre or a pair of ICs can cause rumble (except in the case where the phono stage has very low bass frequency extension, enabling it to transmit rumble frequencies to the downstream components, but that's where the rumble or subsonic filter works)? And (2) why do you focus on the tonearm and not mention the platter bearing as a source of rumble? I do agree it's a nice idea to take the TT to another system and determine whether rumble follows along with it. I had suggested inserting a known good TT into the OP's system, for the same reason.
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As others already pointed out, the physically visible woofer pumping is typically seen when the woofer is mounted in a ported or vented cabinet that does not backload the woofer at extreme low frequencies. You’d never see it with acoustic suspension type speakers for example. ( I don’t know about Zu speaker designs or how your Tannoys are enclosed.)
incidentally we’re seated in the Kennedy Center, and my first live post covid concert is about to commence.
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Well if the OP's tt is not broken, and if moving the tt in his listening room or improving tt isolation do not have any effect, up or down, and if his LPs are not warped, then he does need a subsonic filter, preferably a good one.
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I think dynamic and crusty have very good ideas about what might be going on. I was an early fan of transmission line woofers. Specifically I bought Irving M Fried (IMF) speakers in the early 1970s, because I thought they had unusually natural low bass response, and they were darn good above those frequencies as well. When I was once in the presence of IMF himself, he pontificated that all preamplifiers should have a subsonic filter built-in. This is because the transmission line is a ported design in which the woofer is completely free at low frequencies. And on occasion my woofers iused to pump visibly in those days. I ignored it. I still use transmission line woofers to support the low bass response of my Beveridge 2SW speaker panels. (80Hz cutoff). The TL cabinets I am using I built in something like 1971. Because I am aware of this pumping tendency, I am often staring at the KEF B139 woofers built into my transmission lines, while I am listening to music. They are pretty much static at all times, visually, from my listening seat. No pumping has been observed yet. Could be because this system is in my basement, with concrete flooring over earth, or it could be because my turntables are well isolated and quiet. And I don’t mess with warped LPs unless the warp is minuscule and I love the particular recording. |
That’s why there’s been some mention of the KAB rumble filter which is a well made add-on. |
Chak, If the 1200GR is without construction or operational flaws, I quite agree with you. If it has a construction defect, like an off-balance platter or bad bearings or something rubbing on something, then it is better to identify that problem than to put a rumble filter bandaid over it, I think. For that matter, I am guilty of suggesting other types of bandaid, so my own advice is also flawed in that way. One would like to examine the TT in isolation. The only thing I suggested that might help to eliminate problems in the Technics is to temporarily replace it with another known good TT, and determine whether the woofers still act up. If that test is passed, then maybe the rumble filter is the way to go or any of various forms of enhanced TT isolation first.
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That's very interesting. You mentioned you are using a 1200GR turntable. Do you own or can you borrow a different TT, just to see if the problem is the same with a different source TT? I am wondering whether there is some sort of defect in your 1200GR. That seems very unlikely, given that Technics make thousands of them and have vast experience building TTs with a very low error rate, but unlikely events do occur. If you own a stethoscope, place the bell on the plinth and listen for excessive noise; that might be an indicator of a problem. (It's also interesting that a Pangea shelf is not as stable as an IKEA product not even made to support a TT.)
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I generally prefer to go from the simple to the complex, in trying to solve any problem. Simple is to either move your turntable in your listening room or suspend it, or both. Complex is to add a subsonic filter. If you add a subsonic filter, then a steep slope at a frequency below 20Hz is desireable, but 24db/octave is to be preferred over 18db/octave, because of phase anomalies that may affect upper frequencies. (6db/octave is also good but much too gradual for slope in this case. 12db/octave and 18db/octave are less "good" compared to 24db/octave.) I will bet that moving the TT will solve your problem or at least ameliorate it sufficiently so as not to be a bother.
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Before purchasing a rumble filter, I would first see whether changing the location of your turntable in relation to the speakers has any effect on the phenomenon you are observing. Also, the addition of some isolation to the turntable base might help the problem. You might try to suspend the Technics turntable in someway. There are “feet” available for that. I would put all this ahead of adding a rumble filter in the signal path, only because the rumble filter is not without its sonic penalties. There is no free lunch in Audio. The only other thing I would add to the discussion is that someone mentioned DC voltage as the possible culprit. That is impossible since DC by definition has no frequency. Significant DC voltage delivered to the speaker voice coil could damage it, but it is not causing the piston like movements you are observing. |
