Woofers shake uncontrollably at moderate volumes with turntable source; why?
April 25, 2011 9:39 AM   Subscribe

When increasing the amplifier volume beyond a certain level, my speakers' woofers begin to shake spasmodically when my turntable is the source. However, the same problem doesn't occur with a digital source at higher volumes. What's goin' ahn?

I've witnessed woofers shaking like this before when presented with low frequencies at volumes higher than they can handle, but the digital equivalents of the music I'm playing indicate that the source of the instability is not the audio. My turntable is decoupled by both a vibrapod/acrylic plinth sandwich and a dual plinth/sorbothane design. Suggestions?
posted by alexandermatheson to Technology (10 answers total)
Sounds like it could be a wow & flutter problem (i.e. unwanted sub-sonic junk getting to the speakers from the turntable).

You might consider running the TT through a low pass filter ... but then again, that''s adding another link in the reproduction chain, which adds another opportunity to degrade the signal etc etc etc ...
posted by Relay at 9:56 AM on April 25, 2011

I'm betting your vinyls are less compressed (dynamically, not talking about bit rate or anything) than your digital sources (see loudness war). Short version, many cds and mp3s suffer from very compressed dynamics these days, and something like the initial attack of a kick drum is not going to be that much louder than the rest of the drum's sound. For a variety of reasons explained in the above link, this isn't done as often on vinyl, so even though the digital source may SEEM louder overall, the vinyl source is going to have a number of very short attacks from bass and drums that are un- or less-compressed and actually much louder for very short periods.

A very compressed master of a song can turn up the WHOLE song to be louder, at the sacrifice of losing some of those extra punchy hits. An uncompressed track may have some very punchy and loud parts, but the quiet parts are not brought up and as such it sounds quieter.
posted by Benjy at 9:59 AM on April 25, 2011

Does it happen even in "silent" parts of the music? Then mechanical rumble, for sure.
posted by StickyCarpet at 10:00 AM on April 25, 2011

This sounds more like low frequency feedback, if your turntable is a few years old it may be worth replacing the suspension springs.

Getting a more rigid turntable table can help, as can adding or removing mass. The effectiveness of plinths does vary for different t/tables (its a case of matching the resonant frequency) you may well find it works better without one (or both) of the plinths you have.

Moving the speakers further away is another option.
posted by Lanark at 10:12 AM on April 25, 2011

I've seen conventional tonearms, particularly those with spring provided anti-skate compensation, suddenly go "resonant" at very low frequencies, when the volume of sound from speakers in the same room got high enough. Generally, anti-skate force is only a fraction of a gram at the phono cartridge (generally equal to cartridge tracking force), so it doesn't take much air movement to couple low frequencies from the speakers directly to the tonearm, regardless of any mechanical acoustic path through the turntable. Straight tracking servo controlled tonearms like the Rabco SL-8 and SL-8E were developed to eliminate this issue back in the 1970s, and various manufacturers including Technics have made integrated tracking tonearm/turntable combinations in the time since. And some conventional tonearms equipped with weight and pulley systems for delivering anti-skate compensation are fairly immune to this issue, by design.

For conventional tonearms with this problem, you can sometimes change this behavior by adding a little mass at the cartridge shell mount, in the form of a bit of modeling clay, and then rebalancing your tonearm for the new added weight, while adjusting your anti-skate compensation accordingly. Overall, you're sacrificing some compliance for improved resistance to airborne low frequency resonance, and your tonearm won't track warped or out of round vinyl as well it did without the added shell mass; but, your speakers may live longer and your amp may run cooler. You can also move the turntable farther from the speakers, to raise the volume at which this behavior begins.

Systems powered by tube amps with large output transformers, or solid state amps with large DC blocking caps on the output are less likely to contribute to these issues, too. The problem is most apparent in systems where the power amp output stage is DC coupled.
posted by paulsc at 10:53 AM on April 25, 2011 [1 favorite]

Sometimes these rumble problems stem from the furniture the turntable is on, even with decent isolation. You could try moving the turntable to the floor.
posted by rtimmel at 11:49 AM on April 25, 2011

Definitely sounds like a feedback or resonance problem. Where is the placement of your turntable relative to the speakers? Even if you dampen the vibrations picked up by the base and tonearm, in my experience the needle itself can still pick up feedback directly if it's close enough to the speakers.
posted by iivix at 7:11 AM on April 26, 2011

Feedback/resonance seems to me the most likely explanation; thanks, all. The speakers are below and just beyond the edge of the table on which the components sit, so speaker-cartridge feedback/resonance seems very possible. As to Benjy's point on compression of digital music, I very much agree: the more dynamic, less super-compressed quality of vinyl is a factor in my preference! The woofer rumble does seems to appear only when presented with the louder passages (at the threshold volume), but lasts beyond a strong snare hit, for example.

I'll orient the speakers further away and report back if necessary. Thanks again.
posted by alexandermatheson at 12:22 PM on April 26, 2011

Closed the dust-cover to see whether it would prevent shaking by lessening speaker-cartridge vibrations, but no observably difference. Perhaps that sheds some light on the problem?
posted by alexandermatheson at 12:37 PM on April 26, 2011

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