Dustcover Blues

Audioguy, very incorrect. The dust cover was initially intended to keep dust and polution off of everything including the record. Why do records get noisier after repeated play? Groove wear does not make noise. It makes distortion. Why do people feel the need to spend a lot of money cleaning their records? I can afford any record cleaner on the market but don't feel the urge to buy one. I use a electrostatic loudspeakers and would think dirty records would bother me as much as anyone else. I am also very fastidious. 

Brandon2, that is what most people with a dust cover will tell you. I hear slightly better focus with the dustcover down at louder volumes. I would be the first to tell you this might be psychological. At low volumes I can not hear a difference either.

@lewm , Several reasons why I do not think that tracking noise is the problem. My cartridge produces very little tracking noise. You can barely hear it with your ear up to the cartridge and it is only very high frequenies that you hear. Pressing down on the dustcover and sealing off the opening at the bottom of the dustcover stops the feedback. Neither of these would affect tracking noise. It starts with loud low fequency sound and that is were the energy initially comes from. You are dealing with very long wavelengths, 30 feet plus. As the  pressure increases around the dustcover air rushes into the slot at the bottom lifting the cover up then as the low pressure part of the wave passes, air rushes out from under the dustcover dropping it. The same frequency is reproduced and you get a positive feedback loop. The big question in my mind is why is the tonearm/cartridge picking this up? It is over an octave away from the resonance point. Is air moving in and out from under the dust cover actually moving the tonearm. There is a venturi effect. The counterweight might be acting as a sail and it is always close to the edge of the dustcover wear velocities would still be high. 

@mapman , Do you hear a difference with the dust cover up or down?

The microphone is now positioned on the Sota's platter and we are getting warmed up to run these curves.

If you cut off your subwoofers does the problem persist?

@mijostyn I have not but frankly never directly compared. I’ve had the Linn for years and no issue. Others prior perhaps. I should mention table is in front of and to the left of speakers and there is also a sub about 5 feet away firing in its direction. The basement of my current house is by far the best room structurally I have ever had my hifi in.  Much tougher if not at foundation level. 

@lewm , disappears instantly with the subs turned off and the Soundlabs run full range. But I can not run the Soundlabs that loud alone without pushing them into distortion and rapping the stators. They also can not radiate bass that low anyway at even moderate levels. I cross them out at 120 Hz and the reduction in distortion is distinctly audible, even by millercarbon. I do have to try putting Townsend springs under them. I'll cut holes in the ceiling next week. Kidding aside, at 100 dB they cruise along like most systems at 85 dB. My subs as a single unit will produce useful output right down to 10 Hz. The rumble filter cuts them off at 18 Hz where, as you can see they are still going strong.

@mapman , I do not think your Linn is suspended which is why it does so much better on a solid floor. Try playing with the dustcover up and down and tell us what you think. As you can see by the curves I posted there is no question that dust covers attenuate sound getting to the tonearm. In my case up to 10 dB which is 1/2 the volume.  

Man! You really like to turn it up. Most people that come into my listening room where the 845PXs are located (otherwise known as "our living room") complain to me that I listen too loud. My sound labs probably go lower in the bass region than yours do without subwoofers, because of greater square area of radiating surface, but I do realize that with subwoofers you’re probably getting deeper base than I do. on the other hand I am hearing wonderful continuous bass down as low as I think there is any important music.

You once had an issue due to your room frequency response correction paraphernalia.  Could it be that the same circuitry is boosting the bass in the problematic region, thereby either exacerbating the problem or actually causing it?  Also, if the problem is caused by the Helmholtz Resonator mechanism, wouldn't you expect that shutting down the subwoofers and running the ESLs full range would not so much cure the problem (as you observed) but only reduce the intensity and clarity (because now the ESLs are straining to deal with the still present spurious LF input).  The Atma-sphere amplifiers can certainly get down that low and lower.  The Helmholtz hypothesis is happening at the turntable, so shutting down the subwoofs per se should not "cure" the problem. (By the way, if you are crossing over at 120Hz, I would call them "woofers", not subwoofers, but that's semantics. I think of a true subwoofer as a woofer that comes in at below 50-60Hz, to choose an arbitrary cut-off.)  In my Beveridge system, I cross over to my home-made TL woofers at 80Hz, with an 18db/octave slope.  Have you entertained the notion that the bass energy dumped into the room at high SPLs on bass-heavy passages is feeding back to the dust cover itself, maybe setting it into motion which affects the cartridge stability in the groove? As you know, bass frequencies are encoded largely by horizontal motion of the stylus/cantilever.