Dustcover Blues


Most of you probably know that I have always championed the use of dustcovers on turntables even during play, the goal being to protect the record from the environment and shield it from sound. For the first time in my audio career I have stumbled into a problem with this and other than not putting the dustcover down I have not come up with a solution. 

Yesterday I was playing Herbie Hancock's Secrets and I cranked it on my favotite song. After about 30 seconds the room started to rumble. My subs were putting out a remarkably clean 20 Hz as if I were playing a test tone. Feedback! Just turn the volume down a little and it disappears. Turn the volume back up and within 30 seconds it starts up again. Did I screw up my cartridge set up? I veiwed the tonearm during the feedback and it was rock solid. Usually with low frequency feedback you can see the tonearm shaking. I played the resonance tracks on an Ortofon test record and both lateral and vertical resonance were centered on 9 Hz With the feedback going and the house shaking I wanted a better look at the cantilever. On lifting the dustcover the feedback stopped!  The dust cover is attached to the plinth which is isolated from the sub chassis (tonearm and platter mounted on this) by four springs. The resonance frequency of this suspension is 2 Hz. Nothing above 2 Hz can pass directly through to the platter and tonearm. What is going on here? Any of you scientists out there have a clue? My best guess is that I am dealing with a type of Helmholtz resonation. The dust cover is lowered on four hard rubber pads, one at each corner. There is a 1/16" slot all the way around. This combined with the weight and dimensions of the dust cover creates a resonance at 20 Hz. To get it going I have to turn the volume way up. 

Today when I get home I'll play around with it to see if I can figure it out. Any ideas would be appreciated. 

E30bfe46 c951 4f07 8d5f 08b0a596e00dmijostyn

What happens to the sound when you remove the dust cover?

My turntable's dust cover can't be used when playing a record.  And while I appreicate the desire to safe guard the record - I have yet to see damage to a record playing on a turntable in my house.

You can try moving the turntable to a low bass null point in your room. Otherwise, maybe a frame with silk streched over it to keep the dust out while playing the record.

ETA:  Don't say nothing can pass above 2hz.  Springs will vibrate at harmonics just like loudspeaker cones and musical instruments. Just lower in amplitude.

Try replacing the rubber pads with sorbothane dots.   

Does not a rumble filter stop it?

There is little likelihood of anything cut below 40 Hz on a quality master anyway.

Alfred said: "What me worry?"

Just remove the cover when playing an LP. That's what I do.

@vinylzone , The springs are dampened. The sub chassis is a 1" thick aluminum plate. Think SME 30/2

@jasonbourne52 , IMHO that defeats the purpose of a dust cover. Records never have a neutral charge. Just playing the record develops a small amount of static charge. Static always attracts dust like a magnet. Small particles can be sucked right into the groove. Keeping the records covered and discharged are the best ways to maintain clean records. If you are doing a good job you should never have to clean your stylus. (hardly ever)

@fuzztone , I have an 80 dB/oct rumble filter 3dB down at 18Hz. The feedback is just above. It does take a vigourous bass line to get it going but the thing about feedback is that it is self perpetuating and it does not require a 20 Hz tone to get it going, like blowing across the top of a bottle. 

Gosh, what a great time to have not called cable elevators porn, and to have not dismissed with scorn every tweak you do not understand and insulted all of us who use them. Because if you had not done those things then I would be happy to help and tell you exactly what is going on and how to fix it.

 

Duh!!!

You demonstrated for yourself that your problem goes away when you lift the dust cover, yet your excessive fear of dust and static electricity and feedback induce you to want to change anything else but removing the dust cover during play.With all due respect, don’t you think that is a bit silly?

Not to mention the fact that there is static electricity buildup even with a dust cover down, and the moment you left the dust cover the LP becomes a magnet for dust.

Kudos to you, however, for having the cojones to admit to this human frailty.

@jasonbourne52 

Just remove the cover when playing an LP. That's what I do.

 

It feels a little strange to get used to but that's what I do now too.

The problems with using any dust cover, any of them, can be sidestepped by removing it.

At the last show I went to, not a single demo room was using a dustcover with their turntable.

In fact, in the case of the Rega 10 (truly a sight to behold in action), I'm not even sure whether they could have even if they had wanted to.

Did you ever measure your room with EQ Wizard (it’s free software) ? You can see what's going on in your room with low frequency.

Apparently that is an insufficient derumbler for your quest.

You could try sub DSP.

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"Cognitive dissonance".  But Mijostyn is smart enough to change his ways, without a therapeutic intervention.

Well, then, maybe you can seal your dustcover in such a way that it acts like a bell jar, and evacuate any air from it before you spin up the record (don't forget the remote control cuing lever).  No air, no feedback.

Nature abhors a vacuum, I always say.

This is soooo much fun.

Millercarbon, you have zero idea what is going on not to mention you are the last guy I would ever take advice from. Cheers!

Lewm, you really do not want to get me started. Remember those pictures you showed me? I use a conductive sweep arm during play which discharges the record as best as it can be done. I still don't like playing with the dust cover up but that is my neurosis.

Having analyzed the problem more completely this afternoon I can add some details. The feedback is being set off by bass drums in certain songs that have great low end extension. Comparing the feedback to test tones it is right at 24 Hz. If I pick the dust cover up the feedback stops. If I press firmly down in the middle of the cover the feedback stops. It gets even better. If I put weatherstripping around the bottom edge the feed back won't get started. How cool it that! Through a thin slot at the bottom of the dustcover 24Hz is creating a pressure wave in the space under the dust cover. The flexibility of the top is also involved. Pressing down on the top does not close off the slot at the bottom of the dustcover. Only the weatherstripping closes it off and I do not have to press on the dust cover at all. I can not get it to feedback with the weatherstripping on at all, it just looks ugly.  I will have to come up with a better looking solution. What I inadvertantly have is a 24 Hz musical instrument. 

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Good idea, tvad.  And it allows Mijo to reduce his tranquilizer dose.  But we know that any remedy that leaves the cover in place is working because the resonance has been altered in frequency.  The resonant energy generated by the presence of the dust cover is still "there", at some other frequency.

Mijo, what pictures?  Did I send you one of my mug shots?

@mijostyn May I inquire on the importance of using the dust cover while using your turntable? (I have a Rega P8 - using my dust cover isn’t an option and prior to that I removed the dust cover to avoid excess vibrations in closing it.)

Everyone else does.

How can you know that?

Much love for vinyl covered diving weights

 

How about clear silicon caulking applied to the bottom edge of the dust cover?

 

I solve the static electrical charge issue with a small humidifier, mason jar size, not too far from my tables, never dust, static electricity, or resonance from an unsprung cover.
At home we play about 4-5 hours a day, clean the stylus maybe once a month with a small air blower.
I have been doing this since the mid 70s, and none of the records I originally bought have a pop.

now I have a usb microscope and can see how dirty the records get and when they get a pop that I can't clear with a soft brush, they go in the ultrasound recordcleaner.

 

 

@lewm , worse. I'm already maxed out on the tranquilizers. I'm trying not to get too far into the Islay.                                                                                                         

 @tvad , that will work. As Lew suggests that will move the frequency the dust cover vibrates at up higher. There is a confluece of issues at work here. It is not just the dustcover vibrating but pressure waves under the dustcover are exciting the tonearm directly. You can bang on and vibrate the plinth all you want and you will not be able to hear it with a record playing. That path is cut off. Heavy bass drum gets the cycle started, the pressure waves under the dust cover vibrate the tonearm at a frequency it is sencitive at for some unknown reason, generating more signal at the subwoofer and around we go. Sealing off the opening at the bottom of the dust cover stops it. So it is a Helmholtz effect. The next question is why a correctly set up arm/cartridge combination should be sensitive at 24 Hz. It may also be sensitive at other frequencies. 24 Hz is just the one being generated by the dust cover. For fun I am going to install a different cartridge this weekend tp see if it changes anything. 

@mrklas , I have always used a dust cover during play. Even at the age of 13 I had my TD 124II mounted in a cabinet under a hinged dust cover that I made myself. I have used a conductive sweep arm since the late 1970's. Records collect static electricity very easily. They will actually automatically charge themselves! The paper label donates electrons to the vinyl, the vinyl goes negative and the label positive. You can see this yourself. Wool is at the very top of the triboelectric series. If you take some fine wool thread, tie it around the end of a pencil and leave a two inch tail you will have made a very sencitve static electricity detector. Take any record out of it's cover and wave the tail over the vinyl and it will be strongly attracted. Wave it over the label and it will be repulsed. Dust and pollution are for the most part positively charged and the record attracts them just like the wool thread. 20 minutes out in the open is plenty of time to collect all sorts of things which your stylus will happily grind into your records. It must be a problem because a large proportion of audiophiles spend a lot of money on cleaning devices, fluids and chemistry. All I use are a conductive sweep arm and a dustcover. I never have to clean my records and I hardly ever have to clean my stylus. All I have is a Spin Clean in case somebody brings a dirty record over. I do not buy used records. If I did I might have something more ellaborate. The trick of having a clean record collection is not to let them get dirty in the first place. Then you get, "But, they come filthy from the manufatcurer." Other than some incidental surface dust they do not. I know this because after playing many new records I have stared at my stylus under magnification and there is never anything on it. No dust, no goo, no mold release, no plasticizers, nothing. Don't beleive me. Do it for yourselves. 

I will report back after changing the cartridge.

In your first paragraph you describe the very reasons why I never use a dust cover. Also, I think you might be kidding yourself about the benefits, but I wouldn't argue that point. What you're observing is nothing special or out of the ordinary in terms of the problem with a dust cover. It's more or less the argument I posed to you long ago.

@lewm , As I just mentioned I have been using a dust cover continuously since 1967. This is the very first time I have had any issue wih a dust cover and it  is a fluke which I will correct. It also happens at volumes which most people can not dream of acheiving at 24 Hz including you. Very few people would ever notice it. Perhaps I should not have mentioned it as I knew the anti dust cover crowd would have a field day. But, it is an interesting phenomenon which should be easy to correct and it merits discusiion. I have ordered some self adhesive felt cloth that I will cut into strips and position all around the periphery of the plinth under the edge of the dust cover. This will be relatively attractive and will stop the Helmholtz effect without harming the dust cover which is very attractive as it is. Otherwise the turntable sounds wonderful and I am very pleased with the Soundsmith Voice cartridge. Don't listen to me. My hearing sucks. 

@mijostyn thanks for sharing your experiences and process.  Keeping records clean from the start is certainly easier and cheaper than buying an ultrasonic record cleaner!

@mrklas , Yes, if the dustcover is hinged to the plinth of an unsuspended turntable moving the dustcover will make the tonearm skip. This is one reason so many manufacturers do not offer a usable dust cover and many do not use one. Trying to put an unhinged dustciver over a turntable in operation is a PITA and asking for it.

The way around this problem is to get or build a dust cover that is larger than the turntable and hinge it to the cabinet the turntable sits on. To keep it up use a prop just like with some car hoods. If you were really handy you might be able to adapt a gas filled strut to work. If the cabinet is as heavy as it should be using the dust cover with reasonable care will niot make the turntable skip. This is just one of the advantages having a suspended turntable. You can do just about anything to the plinth and it won't bother playback.

@mijostyn Looking forward to new thread comparing different brands of diving weights atop the dust cover. 

Being Friday, I am really hoping to hear @tubebuffer 's thoughts on the question. 

As a last resort, you could limit listening to panflute recordings until you land a solution. Hang in there! Cheers,

Spencer 

anti dust cover crowd 

Sorry, but I was totally unaware that there was a "pro dust cover while playing the record" crowd.  I've never met anyone who has done this and I can't recall ever seeing a record playing with a dust cover on.

 

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I haven't had a dust cover on a turntable since the late '70s. I lived in Tucson (home of dust) for 20 years before moving back up to the Pacific North West.

@tvad

I actually started getting into audio in the latter 70’s. Granted, it was a cheap table/system, but the first time I tried playing a record on my Sanyo rack system, I realized feedback was a problem and never played another record with the dustcover down. When I started looking at the higher end stuff in the early 80’s, I never saw, or even had a demo where the dust cover was left down. My first high end table, a Logic DM101 had a dust cover, but I removed it from the hinges, and just placed it on the table while it wasn’t in use.

 

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@tvad 

You astound me with your powers of deduction.

@vinylzone , This is the very first time I have seen a dust cover contibute to feedback. What it does demonstrate is that a tonearm can pass on airbourne vibration that is outside of it's resonance point. It can not be the record vibrating because it is vacuum clamped. Under most conditions the dustcover attenuates sound getting to the tonearm by as much as 15 dB at some frequencies. This can be demonstrated by putting a measurement microphone under the dustcover. This weekend while I am farting around I will do this and take screen shots of the curves generated inside and outside the dust cover which I will put up on my system page. I will use a short sine wave sweep from 10 Hz to 20 kHz. My arguement is dustcovers can improve the performance of your turntable and help to keep your records clean also diminishing stylus wear. Dustcovers are hearing protection for your cartridge. Mark Dohmann related to me in an email that he is working on a dust cover for his Helix turntables to be used during play. 

When I press down on the dustcover the feedback does stop probably because I am closing the gap at the bottom not because I am damping the dustcover. Closing the gap with felt strips is a more elegant way of solving the problem. 

If the table is subject to vibrations, dust covers could easily exacerbate the problem. Best to avoid vibrations reaching the table always but if not possible and dustcover exacerbates things just take it off when playing. Dust cover vibration is not an issue for me these days personally. My table is rock solid. 💪

 

@mapman , What are you using? Even if a table is "rock solid" attaching a dustcover directly to the part of the turntable that holds the platter and tonearm would be more prone to causing trouble. Historically, the tunetables that came with dust covers were suspended such as the Thorens TD 125, The Linn LP 12 and the Sota Sapphire. They were isolated from their dust covers. I favor this type of construction. It is not as flashy as some designs but isolating the platter and tonearm from the rest of the environment has significant functional and sonic advantages. Mass loaded turntables can work very well if located on a very sturdy rack on a concrete floor. I do know of people putting turntables like the Kuzma Stabi XL DC on MinusK platforms. 

@mijostyn Linn Axis table with Denon Dl103r cart on low profile solid cherry table at foundation level, concrete foundation with thin dense carpet and pad. See my system photo.

 

@tvad , Yes, you are quite correct. The very best tables came with dust covers and for a time even the less expensive tables came with them. Then manufacturers learned from Transcriptors (I think) that if you made your turntables cool looking enough you could sell a lot of them even if they are lousy tables. I should know as I fell for that. Soon they became so odd shaped there was no way you could attach a dust cover. But, visual stimuli dramatically improves the hearing of a large population of people. This whole mess ends at the Techdas Zero, without a question the coolest looking turntable on the planet and technically it is very impressive. Personally, I would rather spend that money rescuing geto kids from a toxic environent and getting them someplace where kids can have fun being kids like a summer camp. 

put the dust cover in the recyclable basket for pickup

Leaving aside our differences regarding to use or not to use a dust cover, I am thinking about your hypothesis that right now your turntable/cover are together acting according to the principle of a Helmholtz Resonator.  The idea does "resonate", but so far as I can tell from my reading, a Helmholtz Resonator requires air (or whatever fluid is resonating) to be put into motion.  Energy has to be put into the system for that to happen.  In your conception, what is the source of energy?  I would offer an alternative hypothesis, for good or ill. Could it be that energy being emitted by the cartridge body, the same energy that is heard as "cartridge noise" is bouncing around under the cover which is by chance tuned to resonate at a low bass frequency, which is why it is excited only when you play certain LPs with low bass frequencies and at a certain SPL above some threshold that causes the whole thing to get excited and re-enforce itself via positive feedback?  That is what I meant when I remarked that you are experiencing a problem that I associate with dust covers per se.  I put this out there for discussion, not to antagonize or criticize. In my own experience, when I played LPs with dust cover down, inevitably the dust cover itself could be felt to be vibrating, I always thought sympathetically with cartridge vibrations.

The dust cover is there to prevent dust build up on the equipment, not the record. Remove it when playing a record.  

Curious problem. I hear no difference in sound whether the dust cover is up or down on my TD 166 mk2 Thoreans. 

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.

Partial retraction.  OK, I see that if it were the movement of air in the room, caused by your woofers, that was inducing the Helmholtz effect, then shutting down the woofers would possibly reduce the SPLs sufficiently to ameliorate the Helmholtz resonance of your dust cover/TT.  I guess I just find it hard to believe that your subwoofers blow around enough air to do that, but I have not been there, as you have.  Still, I would take a look at your room frequency response correction devices at the suspect frequencies.

Your microphone test for proving the utility of a dust cover.  Isn't that highly dependent upon the room, the speakers, and the proximity of the TT to the speaker?  In other words, you could prove it for yourself and your system, but not universally for all situations. I'd move my turntables before employing a dust cover to shield the tonearm/cartridge from SPLs.  In addition, what do we know about the importance of the effect you observe, in terms of ultimate fidelity?

Dear @mijostyn : I don't know which kind of material you are using in the dust cover.  Years ago when I was using two Denon TTs 80/75 each one was mounted in an Onyx 50kg base and the other marble of more or less same kgs. and I was using with dust covers made it of tigthness glass really thick and I can't remember any trouble down there. Dust cover material is critical.

 

I don't use any more.

 

R.

 

R.