Dustcover Blues

@lewm  I'm pretty good at keeping the dust out of the house. The workshop is indeed a listening room......of a different sort. It is powered by 4 Mirage speakers and an old Adcom amp that refuses to die. The music is shipped down to the shop by a Sonos connect system. Anything playing on the main system can be sent down. It will also stream Qobuz from my phone. Can't work without music. 

@lewm Keep up that attitude Lew:-)

@lewm And that is why Howard Johnson's made 28 flavors. Best Hot Dogs ever.

The idea behind a suspension Lew is to isolate the cartridge from all environmental mechanical vibration including anything coming from it's own plinth including the dust cover. I can turn the volume all the way up and tap all over that dust cover and you can not hear a peep. If I give it too hard a lateral blow the table will skip because I provided an impulse below the suspension's cut off frequency. Anything above will be filtered out. Now in your case, you have a slate plinth and no hinges.  We are both old men, but as you previously related you are 10 years older which makes you REALLY old. Any really old man trying to put an unhinged dustcover on a running turntable is a recipe for disaster. It is not such a great idea for just a plain old man never mind a young man with an essential tremor.

@rauliruegas Pardon for the late reply. I stumbled over an old post of yours. Forgot to push the "LAST" button.

@rauliruegas Glass is a bad material to use for a dust cover as it can ring. Have you ever made a wine glass sing? If it is thick enough to prevent ringing it will be very heavy and difficult to manage even if hinged. 

The dust cover I use now is 1/8" thick Lexan which like wood is self damping, it does not ring. It rests on a thick felt gasket and is sealed all the way around. You can see the felt gasket and the dust cover on my virtual system page. I can drop the dustcover during play and it won't affect play in any way, you can't even hear it even if the volume is maxed out. 

@mahler123 Avoiding the dust cover is old audiophile dogma, the fear being that the dust cover resonates and affects sound quality. In some cases this might happen. Most people like yourself can not hear a difference. Dust covers that sit directly on the chassis that the plater and tonearm sit on might cause trouble. If they are not hinged taking them on and off during play can be dangerous. Ideally a dust cover should be hinged to the plinth of a suspended turntable. The dust cover does not have direct contact to the important chassis, it is isolated. A good example are the Heritage and Cosmos Sota tables. Using a dust cover that is hinged and isolated poses no danger and actually improves sound quality by attenuating sound around the cartridge by up to 10 dB at some frequencies. I have actually measured this. 

Dust is the enemy of records and fine mechanical devices. I will never use a turntable without an isolated dust cover in my system. I will never use a turntable that is not properly isolated by a tuned suspension.  

@audioquest4life , you tossed the old hinge hardware? Sota uses some really nice hinges with adjustable friction. I'm sure Donna would sell you a pair. You can mention my name if you like. Having to lift the cover off al the time is a recipe for disaster.

@lewm , Thanks Lew. I'm not sure about "dogged determination." Setting up the jig is the hardest part. It takes a lot of test cutting to get the tolerances right, but once that is done you can knock these out all day long with minimal effort. Now hand cut dovetails is dogged determination. But, this is not a $10,000 coffee table and I'm not exactly getting paid for it. After all, it is just a turntable:)

@audioquest4life , Glue always makes that claim:) 

Yes, what I did when I was young is quite comical looking back on it. Learning in the school of hard knocks. But, experience is always the best teacher and after 55 years of it you usually gain enough to have some idea what you are doing. The big leap was when I started out with DSP in 1995. Seeing is believing and when you start measuring the lights come on. 

Rock On, Mike

@rauliruegas , I AM!

@dover , I think YouTube might break:)

@lewm , @rauliruegas ​​@atmasphere The skirt is finished and installed. It is lined with felt which makes a good seal. It works perfectly. I can no longer induce any feedback with the dust cover closed. I can sleep at night now:)

I

Dover, The next arm I get will be a Schroder LT. I am already looking into mounting it on a modified Sota base since I plan on making a new one. As long as the suspension can handle the greater weight I should be able to make it fit.

I am in total agreement on the Linn having owned two many years ago. They do seem to have everyone hooked on the upgrade mania. Good business.

@dover , I believe you are analyzing the problem incorrectly dover. Increasing the effective mass would make things worse by giving the the tonearm more inertia. The bass gets the suspension bouncing at 24 Hz. Life not being perfect, the tonearm has more inertia at the cartridge end and it is bouncing also enough to generate a 24 Hz signal that keeps things going. If the suspension does not bounce there is no problem regardless of volume. I can peg it no problem. I'm quite sure this would happen with any tonearm mounted as it is just along for the ride.

The bass response is pretty much the same across the shelf. It does not increase appreciably in the corner because the side wall is only 1 foot deep. It is not much of a corner from an acoustic standpoint, not at all like the corner of a room. This was one of the first things I measured.

@lewm , and I was just about to buy one of those Oswald Mills Audio K3's. Actually, it is the only way you can get one of those arms. I plan on tossing the turntable and sticking that arm on an AR XA just for spite.

@rauliruegas , I hope you realize Raul that I was joking. That's what the smiles were supposed to convey. 

I am here because I still play records just like everyone here. We all enjoy music in our own way. You have no idea what my listening capabilities are and I yours. I have no idea what you are listening too and you I. It make more sense on these forums to talk about the way things work, much more concrete. We both dislike unipivot arms for the exact same reason. They do not function the way the ideal tonearm should. We both have probably used and listened to examples we did not like. 

@dover , Are you saying the Schroder is causing my feedback problem? If so that  would fly in the face of the obvious. If I lift the dust cover or close off the bottom the feedback stops. I did nothing to the arm. It is perfectly capable of playing at outrageous volumes without any difficulty at all. It is tuned right where I like it. If you are saying it is just a bad arm I and many others will disagree with you. Thanks for your input. 

@lewm , I think that is an argument for a different day. 

Hold on Guys! I have not bought one yet. I'm just going to listen. I am equally enthralled by low impedance MCs and transimpedance phono stages. Unfortunately, the price of admission is going up in leaps and bounds. 

I see what the problem is with strain gauges unequalized. Most rooms create more error but that should not be an excuse. 

On the other hand I am still greatly enjoying The Voice on my ARC PH3 SE. They match up well and SS guys would never know it was a tube phono stage except maybe rauliruegas who is just claire voyant and really has no idea what he is listening too:)))

I bought a large piece of Macassar Ebony which should be big enough to do the whole plinth. Should I do dovetails or finger joints? What do you think would sound better?

@lewm , It is a good question I plan to ask Mr Ledermann. I have not seen him mention this. I can do it digitally although replicating the phase shift Ralph talks about won't be easy. 

@rauliruegas , I viewed Channel D's web site last night. The Lino C 3.0 is now out and it has both MM and Voltage Mode MC capabilities among circuit improvements. It is also up to $6000 if you option it up. It has also lost the simplicity I like. Makes the Loco look better to me. 

@atmasphere , Have you heard the cartridge? Theoretically it should out track any other cartridge but his MI cartridges track anything I can throw at them. I can correct frequency response problems but I won't get involved if there are not significant advantages in the design.

@rauliruegas , I have no experience with the Lino C. I have an ARC PH3 SE which I really like for high output cartridges. It is a simple unit with just one switch in the signal path. It is a hybrid tube unit with a FET front end. It really does not have enough gain for cartridges with output < 0.5 mv. It is 30 some odd years old and still in perfect working order but it is time to move up. I do not like complex phono stages. It is too easy to damage the low level signal coming from a LOMC. The Lino C is a minimalist unit which is balanced throughout which I really like. Loading adjustments are not required and it is battery operated, another big advantage. I can use the ARC for high output cartridges which I prefer to MCs with certain music. The other candidate is the Sutherland Phono Loco.

@millercarbon , Correct MC, when the gauge is stressed it changes resistance which modulates the supplied voltage which is subtracted afterwards leaving only the signal. Messing with the power supply might not be a good thing. If the voltage subtracted is not exactly equal to the voltage added you will get a dc offset. If your amps go down to DC you could have fried woofer for dinner. If run through a preamp this might filter the DC out.

I do not think it is weak bass but rather exaggerated highs supposedly a hangover from the RIAA curve. Several reviews have noticed this. I can correct for it so it is no worry for me. Weak bass I won't tolerate. If I can't feel the bass drum beat that would kill it for me.

@rauliruegas , you did not make a suggestion for a phono stage or comment on the Lino C. I'll be taking 5 records I know very well down with me. I should be able to pick up any major problem depending on the system Mr Ledermann is using. I won't spend that kind of money if I am not completely comfortable with it. I am also very captivated by the transimpedance approach to amplifying a moving coil cartridge. The only problem is that it limits you to very low impedance cartridges. I have no problem with this as there are many excellent low impedance cartridges. I read uniformly good things about the MySonicLab cartridges then there is Lyra and Ortofon. The new Verismo looks very interesting. The My Sonic Ultra Eminant Ex with an impedance of 0.6 ohms is perfect for a transimpedance stage. 

@rauliruegas , what would you suggest?

On December 3rd I am traveling to New York City and on the way we are stopping at Soundsmith to audition the strain gauge cartridge. If I decide to go with that it comes with a dedicated phono stage. For a moving coil phonostage I was looking at the Channel D Lino C, the transimpedance unit. I would probably try a My Sonic Lab cartridge to go with it. 

@lewm , Actually, right now I am using 48 dB/oct slopes with a cut off of 100 Hz. Yes, smaller drivers are capable of higher freuquencies.  The "speed" of a woofer is indicted by it's frequency response as I said before. Otherwise a subwoofer's sound is not dictated by speed as long as you stay away from it's limits. 12" woofers generally are good to 1000 Hz and we go nowhere near that. With the right crossover and time correction you can match subwoofers to any loudspeaker. 

I can get you over your digital phobia in a heart beat. Who knows. Maybe next time you wander towards Vermont :-) 

@rauliruegas , I know exactly what is going on. The two chambers above the subchassis are resonating at 24 Hz. This causes the subchassis to bounce at 24 Hz. This is picked up by the cartridge completing the feedback loop. The sub chassis is a solid 1" thick aluminum plate that extends to within 3/32" of all sides of the plinth. Your middle ear has a vent tube to release pressure. It is called the Eustacean Tube. When it gets block your hearing gets damped by up to 10 dB at some frequencies because pressure in the middle ear will not allow the ear drum to move as well. On a hunch I decided to block my turntable's Eustacean Slot with the skirt you see on my system page effectively giving it a hearing problem. It was very easy to test. The first track on Soundgarden's Badmotorfinger gets the feedback going throughout the entire song. With the skirt in place there is none. It is a bit hard to miss. When a 24 Hz feedback loop gets going the entire house shakes.

Right now the skirt works perfectly I just have to dress it up a bit. Yes, I can and will build a new plinth that won't do this when I run across that special plank of wood. If I am going to put in all that effort it has to be very special.

@lewm , It can work either way. I think the majority of people building subwoofers come up with a design concept, then choose an appropriate driver and finally given the parameters of the specific driver tweak the dimensions of the enclosure to suit. 

If you look at the picture of my system on the floor you will see a woofer to the outside of each panel then in the center those two boxes on the floor are subwoofers. They are pointed at eachother. These woofers have 200 lb sealed enclosures with a Q of 7.6. 

The new design is a decagon cylinder with 1.5 to 2" side walls (they vary). There will be a 12" driver mounted in each end. They will also be sealed. There is no reason to resort to ports when you have advanced "room control" which is really speaker control. With enough power you can make a subwoofer do just about anything. I would have liked to use larger drivers but space will not allow.

The tonality of a system is debatable and we all have our own preferences. What is not is image and detail. DSP allows you the adjust the frequency response of the channels individually so that they are exactly the same. This gives the best image and with it detail. Every audiophile that has heard it goes out and gets a processor and I am not kidding, every single one. 

Lastly, there is no such thing as a "fast subwoofer" when a woofer is not fast enough it's high frequencies roll off. Usually, the cone breaks up first.  Even 18" drivers can make it to 500 Hz. I think what people really mean by this is muddy vs well defined. Mud is coming primarily from the enclosure. Then there is the amp's ability to control the driver. Some amps are good at it, others not so hot. You need a powerful amp with a very low impedance output stage. Transmission lines are a way of effectively doubling the size of the driver at certain frequencies. They are very difficult to make and require a lot of trial and error tuning. It is much easier to use large drivers or a multitude of small ones.

@wyoboy , several issues. The first is that the cabinet is built into an alcove so it is definitely not moving. The second is that I designed the room so that it has no back wall. It is broken up between the kitchen and dinning room, the nearest solid wall being about 75 feet away. I also use line arrays which limit dispersion so as to minimize room effects. There is some modal behavior but it is very weak in comparison to the usual situation. The bass at both side walls is very much determined by the boundry effect. In order to get the turntable to a balanced frequency response environment I would have to put it out in the middle of the room. 

@rauliruegas , I owned Velodynes for perhaps a decade in the 90's and I got them working tolerably well but I was using an outboard crossover.  IMHO the major problem with subwoofers is not the cone material but the enclosure and positioning of the drivers. Any good subwoofer driver 12" and up is capable of beating 0.5% distortion it not over driven. Everything else comes down to enclosure resonance and shaking. For commercial subs Your Velodynes are not bad but (and I really mean this politely) they are not near as good as what I am using now never mind. what I have in store. I aim to build the most accurate subwoofers possible with current drivers and I will do it. (or go broke trying)  As for measuring. How do you know what and how I am measuring except the tools I have mentioned in these threads? The only part of a HiFi system that is not measurable is the human mind. Everything else is easily measured with the right tools. The bane of speaker design has for decades been the room they are placed in and even that is measurable. Any problem you identify can be fixed. If you can't identify it it becomes a mystery and something you "can't measure."  

As for my dustcover problem. That has been fixed by sealing off the space under the turntable with a skirt. I can now bury the volume with the dustciver down and it will not feed back. I put a picture of the solution up on my system page. The final solution is a new plinth which I will get around to when I find that special plank.

@atmasphere , I can do that easily. I'll give it a go tomorrow. 

@limomangus , I'm not quite sure what you are saying. You have had a number of turntables and you have used them with the dust cover on and off and do not have any problems with either?

@rwwear , I already use one, a digital one! It is 3 dB down at 18 Hz and rolls off at 80 dB/oct. The way I run my subwoofers if I did not use this type of rumble filter the cones would wind up across the room with the first glitch:-) My problem is occuring just above the cutoff frequency. I could program the filter up higher but that would start interfering with the sound. The solution is in controling the resonance problem and there is a solution to this. I just have to figure it out. 

@rauliruegas , The subs I am using now do have paper cones. The ones I am thinking of using in the new subs have aluminum cones which I am not crazy about because they are easy to damage but I am space limited and I need a driver that will operate in a 1 cubic foot enclosure, These will. Subs you do not hear as much as feel even though I run mine up to 100 Hz. People shy away from running subs up high because everyone is used to using just low pass filters and not full two way crossovers. Plus, they have no ability to match them in time with the main speakers. They use very low low pass filters so they can not hear how bad the match up really is. If you try to cross this way at 100 Hz it becomes a real mess.

@lewm , thank you for the compliment Lew. Right now if I am going to listen to rock loud I keep the dust cover open. This weekend I am going to make a skirt to seal off the bottom of the turntable. Hopefully, it will work. Moving the turntable to the center or back of the room would certainly work as the bass is much lower in those locations. I doubt moving the turntable along the shelf it is on would do anything as the bass response is pretty much the same all along that shelf. As you may notice the turntable is right above the phonostage. There are holes drilled in the granite in strategic places to lead the wires through keeping AC wires away from signal wires. Moving the phonostage is not a nightmare but close. I can get to all the wiring by removing the records and equipment but it was originally put in place before the granite went down when it was much easier to deal with.

The turntable can handle the location fine except for this one fluke. The cabinet is not the problem. It vibrates very little if at all. The only direct connection between the cabinet and the wall is the trim around the upper cabinet and there are triple 2 X 6 studs under it. It is also not just planted on concrete. It is right on top of the foundation which goes down 12 feet to footing. 

If the skirt works I will make a new plinth cover and bring it down to within 1/8". Just enough room to be able to level the platter. If the skirt does not work then I will just have to keep the dust cover up when playing at volume which is not all that often, I'd wind up divorced. 

@atmasphere , Thanx for your input. The cabinet is not really a "stand." It is a very large built in cabinet weighing hundreds if not over 1000 lbs with the granite and records installed. Look at the picture. The only thing that will affect it is ground rumble from something like a dump truck traveling down the street. Having said this, the idea behind an isolated turnable is that it should perform well regardless of what it is planted on. In this case only frequencies below 3 Hz should be able to get to it. But, I have a situation where airborn vibration, sound is able to shake the sub chassis at 24 Hz creating a positive feedback loop. The dust cover is certainly part of the resonating system because when I open it the feedback stops. As you suggest I am sure the acoustic situation around the turntable is in part responsible for getting the sound pressure levels at 24 Hz high enough to get things started. However, with the dustcover up the table sounds great and I can thump on it with the record playing, volume wide open and you won't here a thing. I can also throw myself against the cabinet and you won't hear a thing. Try that with any stand!  Yes, I have now demonstrated this to non beleivers on several occasions. I have the bruises to prove it.

Lets look just at the facts that we have so far. With very loud volumes using program with high output at low frequency's the subchassis shakes at 24 Hz. If I raise the dust cover the shaking stops immediatedly. If I press down hard on the dust cover it extinguishes over a period of a few seconds. There are two cavities above the sub chassis separated by plate that has a hole the size of the tonearm board in it along with the 1/8" space around the platter. I think it is safe to assume that air is resonating in and out of this structure shaking the sub chassis. My best guess is closing off the space under the turntable will stop the resonance by preventing pressure (or vacuum) from building up under the sub chassis. I forgot to mention that the only structure I can feel vibrating is the sub chassis and I would guess that it is shaking up and down at lease 1/32" 

@rauliruegas , I owned two 12" Veldyne subs for years until the foam surrounds disintegrated. I also had the main amps rolled off at 6dB/oct. Once I had digital bass management I was set free to design whatever I wanted for subwoofers. Distortion in subwooder drivers is dependent on the motor design and the size of the excursions the subwoofer is taking. The Velodyne uses a sensor on the cone in a negative feedback loop to help control the driver at long excursions. It allows you to get more clean volume out of a smaller driver. However another path to low distortion is, do not let the driver take long excursions. You can do this by increasing the size of the driver or by going to multiple drivers. It is all about the surface area of cone you have working for you. The design of my current subwoofers was based on the thought that using four 12" subs with very heavy, ultra stiff enclosures with the mass focused in line with the driver would lead to a low distortion subwoofer line array that would match well with line array main speakers. The subwoofers are made with Corian laminated to MDF with a layer of glass microspheres in epoxy between the two. Each one weights 200 Lb. They turned out to be excellent subwoofers but I still detect some coloration due to cabinet resonance in spite of the insane construction. They also are taking longer excursions than I want at high volume. The new ones are a balanced force design with a 12" driver in both ends of a cylindrical enclosure that has 1.5 to 2" walls. They vary in thickness!  Cylinders are inherently stiff structures. Only a sphere is stiffer. The forces of the opposed drivers will cancel out leaving an encosure that will not shake at all. 8 drivers will decrease excursions decreasing distortion.

1st order crossovers are not steep enough for subwoofer especially when trying to match them to ESLs. To decrease the distortion and increase the headroom of ESLs it is vital to remove from 100 Hz down from them. The subwoofers also have to match the radiation pattern of ESLs to mate well. In this case the ESLs are line sources over their entire range. You have to keep the subs out of the midrange but you want to run them up to 100 Hz which requires a slope of at least 48 dB/oct. That is 8th order! Plus, you have to match the subs the ESLs in time and phase. If you use a turntable you have to add a steep sub sonic filter or your subwoofer drivers will jump out of the enclosures at volume. The only way you can accomplish all of this without "F"ing things up is with digital bass management. 

I hope to have the new subs finished by next Summer. I have to finish the wife's new kitchen on the way. I will show you the results. As you can see from the frequency response curves that I put up the current system has no trouble reaching 20 Hz with power, enough to get my turntable bouncing:-) Gotta love those subwoofers!

@atmasphere , see above!

Look at my system page and you can see where the turntable is. It is on a granite slab sitting on a built in walnt equipment/record cabinet (which I made) which rests on a concrete slab. The uprights are 1 7/16" plywood. All the shelves are braced. The entire affair sits in an alcove in the side wall. 

On my system page you can also see graphs of the frequency response at the turntable dustcover up and down. There is a definite rise in the bass but no really big peak or trough. Over 1 kHz the response drops 10 dB with the cover down. 

Works for me Lew. Obviously, I am having trouble involving the dust cover. It is a first for me and my music habits have not changed. Once I fix the problem the dust cover will be all benefit again:-)

@atmasphere , Actually, because there is no rear wall and I use dipole linear arrays my room is very well behaved. The turntable is adjacent to the listening position. The bass is due to the boundary effect. This happens in almost every room. The table handles this just fine with the dust cover open. I am not putting it in the middle of the room.

I already have four subwoofers soon to be 8. Although they meet the definition of a "swarm" system they are arrayed in a specific pattern for a reason. At any rate the problem is not with the subs. The problem is a resonating turntable and it only happens with a few records cranked to the max. Next weekend I will make that skirt to see if closing off the space under the turntable will stop the resonance. The dustcover is now effectively sealed. Sealing off the bottom will prevent air pressure from having immediate access to the interior of the table.

@atmasphere , The original Sapphire came with a dust cover. It was not an option. Later it became an option, extra charge. This design is not all that much different from the original Sapphire. The early Cosmos had a Corian plinth cover. Later they reverted to wood. As far as I know this problem has never been reported before. If I had to guess it is because few people reach bass output below 30 Hz that would excite the resonance. I have to go almost to max volume to excite it and this is with very boosted bass. Look at the curves I put up on my system page. There are two pictures of the table with a microphone sitting on the platter. "UP" is with the dust cover up (the "UP" curve) and the other with the dust cover down (the "DOWN" curve) You can see that up against the side wall the bass is way up, at least 20 dB from where it is in the center of the room. It may be worse than usual because the table is in an alcove. If you read my posts you'll get a better feeling for what is going on. If I press down hard on the cover the feedback will slowly diminish. If I open the cover it stops immediately. The reason it feeds back is the sub chassis is bouncing at 24 Hz which gets the subs going at 24 Hz which keeps the sub chassis boucing. The sub chassis resonance frequency is 2-3 Hz. The tonearm is 8-10 Hz. 

I have three cavities. The one below the sub chassis, the one above the sub chassis and the one under the dustcover. The last two communicate through the tonearm board hole and the 1/8" space around the platter. The last two communicate through the slot that goes all the way around the sub chassis. It is also about 1/8" 

I am going to build a skirt to go around the plinth to seal off the space under the turntable. I'm hoping that will stop it.  I plan on making a new plinth cover out of a more exotic wood than walnut. Depends on what I find. It is like shopping for vegetables. You buy whatever looks good.

Look at those curves again. The dust cover diminishes higher frequencies up to 10 dB. Except in the bass it is 10 dB quieter under the dustcover. Does this mean anything? Who knows? I can not tell the difference between up or down until the feedback starts. I keep it down to protect the records. Am I actually doing that? Who knows? 

@lewm , thank you Lewm. It is a pleasure instructing you:-)

As I mentioned before I dropped the crossover frquency from 120 to 100 when I got the Sound Labs as they have so much more surface area than my old Acoustats. Given you have even more you could try a little lower maybe 80 Hz but I would never go lower than that. If you have a digital crossover you can cross up higher and still keep the sub out of your midrange with steeper slopes. There is no comparison between digital and analog bass management. They are totally different worlds. If you replaced the crossover in the Beverages with a digital one you could cross up a little higher using a much steeper slope without phase issues. Would be fun to try. 

Lew, the speed of a driver will dictate it's upper frequency limit. Most subwoofer drivers, even the 18" ones are capable of reaching 1000 Hz before cone breakup.The electromagnetic forces generated by ESLs are several orders of magnetude weaker than a modern subwoofer motor. It is not a speed or transient response issue. The problem is matching the subwoofer up to a dipole ESL. The very muddy midrange that a sub produces clashes with the incredibly detailed midrange of the ESL. If you turn off the main speakers and just listen to most subwoofers you will clearly hear some midrange leaking through. This is made even worse by timing errors. This is why early on we were keeping crossover points so low, to keep the sub out of the midrange. The trade off was decreasing distortion in the ESL. The higher you cross, the less distortion you get out of the ESL. It was digital bass management that finally solved this problem.

I still have some issues with the subs I have now. I think there is some distortion added by the enclosures and although they will go very loud I think they could do it with less distortion from the drivers themselves. I am working on new enclosures and will use 8 instead of 4 drivers which will halve their excursions. 

There are different forms of helmholtz resonators. With the dust cover closed you have a sealed system with a large opening at the bottom (under the plinth) leading to a constriction at the opening for the tonearm board into the blind cavity under the dustcover. This is complicated by the springloaded platform hanging underneath. Regardless, as you suggest,  I have a resonating system and I need to figure out how to make it a non resonant system. My next approach will be to make a tight fitting skirt that closes off the bottom of the plinth so that air cannot enter from below. I'll use cheap poplar just to see if it works. If it works I will make a new plinth cover using the old one as a jig. 

You definitely have hackles. 

Mike

@tomic601 , very interesting point tomic. Yes, it has the magnetic bearing. But, it is the whole subchassis moving. There is no give that  can feel in the thrust bearing. If it has a resonance it would be way up high. 

@rauliruegas , whenever you get new equipment you can run into teething problems like burning up the high frequency balance control in your brand new $45,000 speakers and this one with the turntable. This is where the hobbiest approach comes into play. What would life be like without the occasional challenge? 

Thinking about time can get very depressing. Keep yourself busy having fun. This is just one way to have fun and a good one at that.

@lewm , my SLs are also 8 feet tall. They are a custon Job because the full width 845's were just too wide for my 16 foot wall with the theater screen. According to Roger West bass performance is the same but I am not so sure about that. If you want to "stabilize" the speakers you have to add weight to the top of the speaker. The bottom is fixed by the floor and the interface. Sound Labs speakers will make bass. I never said they did not. The problem is at higher volumes vigourous bass will distort everything else. It is a phenomenon I have noticed will every single ESL I have owned. Taking the bass out of them cleans things up and I am sure you would notice the improvement if you would try it. Unfortunately, analog crossovers do not work near as well as digital ones. 18 dB/oct is too slow. You will be getting subwoofer up into your midrange where it certainly does not belong. 

I had forgotten when I switched to the Sound Labs I changed the crossover to 100 Hz 48dB/octave. That really does not matter. The drivers being used are specifically subwoofer drivers. There is a marked difference in parameters between subwoofers and woofers given the much higher x max of subwoofer drivers.

I hate to be a stick in the mud Lew but, you have no idea what your system is doing unless you measure it. Throwing tube traps and other room mods in an empirical fashion depending on what you think you are hearing is a sure fire way of screwing things up. I know, it is your system and you are entitle to screw things up any way you want.

The helmholtz resonator theory is in no way shape or form dead. What would you venture is boucing the sub chassis at 24 Hz? That is not it's natural resonance frequency nor that of the tonearm. Something else is resonating at 24 Hz and it certainly is not the granite the turntable is sitting on. Look at the design of the RH Labs subwoofer. The enclosure is described as a helmholtz resonator. At any rate I think I know how to stop it. My brother said to think of sound waves as waves in the ocean. What would happen if the turntable was rhythmically immersed in water? What he was getting at is you have to keep the water (air pressure) from getting into the turntable. 

The easy thing to do here is just keep the cover open during play. Never take the easy way out. It is not sporting and you never learn anything. I do not think there can be any question that a closed dustcover keeps dust off the record. I have also demonstrated that a closed dust cover does attenuate sound by as much as 10 dB (not 15 dB) at certain frequencies. 

@secretguy , that is common mythology. It was perpetrated by manufacturers that can't or won't add a dust covers to their tables. It is very much like the tube vs solid state and  Analog vs Digital arguements. Most people with dust covers will tell you that they can not hear any difference dust cover up or down. I do not know for a fact whether or not I can. Once I get this straightened out I'll see if I can set up a blinded AB comparison. My own mentality is to protect my records first and work on the sound second. 

Yes Raul, like a fish tank. You could have one made up here easily. Not sure about the hinges. They would have to be very hardy to hold the weight. My old Transcriptors had a glass plinth and cover. I only kept it for a few months. It was awful. I got an Oracle after that and it was almost as bad. After, I went back to the Linn LP 12 until I found the Sota Sapphire. Breath of fresh air.

@tomic601 , Donna really does the business end. Christan is the one to talk to about the technical aspects. I have not brought it up with him yet but, I will as soon as I have it figured out. I am making a"skirt" to go around the bottom of the plinth closing off the cavity under the turntable. Switching cartridges does nothing.

Raul, I think you are right. The material a dust cover is made of is important. It can't be flimsey but it can't weight too much either or hinges won't be able to hold it open. The Sota cover is much heavier than it use to be. It is made of butte joined 1/8" Laxan with all the edges chamfered. It is acutally quite a nice cover. There is a lot more going on here besides the cover. Read on!

 @lewm , Your speakers should be able to go down a few Hz lower than mine because they are 4" wider. You do not listen to the music I do. As I remember you are into old jazz and singers? If you want to know how your system would handle what I listen too sometimes get a copy of Soundgarden's Badmotorfinger and play the 1st cut at 100 dB peaks, 95 dB is probaly enough. Measure it with a meter. I do not think you will like what you hear. You'll be OK at 85 dB. That kind of bass is going to distort the heck out of everything. The voice will actually modulate with the bass. Like you I have had one ESL after another since 1978. I set up one pair of Beverages in a big system in Miami. ESLs will make bass but they will not like doing it. You listen to relatively polite music. Going down to 40 Hz is probably enough but even in jazz the bass drum can go down lower. Even on very old recordings from the early 60s the bass drums can go quite low. With modern recording, drum and synthesizers there is a lot of information below 40 Hz.

I use 4 12" subwoofers each one with an X-max of over 2 cm. They are in a linear array so acoustically they function as one driver with useful output down to 10 Hz. Each driver gets 2000 watts. I cross over to them at 100 Hz and use an 8th orde slope or 48 dB/oct.  Look at those frequency response curves I put up on my system page. That is the frequency response of my system and room at the left lateral side wall with the microphone sitting on the plater. The response at the listening position is much flatter with the bass up 5 dB at 20 Hz and the treble down 3 dB at 20 kHz. The bass and treble curves are intensional. I programmed the system to do that because I like it that way. At the wall it is very bass heavy which certainly does not help but I am now stuck with that position so, I have to work with it.

Now for the juicey part. I put felt strips under the dust cover to seal it when closed and..... the feedback got worse. I hate when that happens:-(   But, lift the cover and it goes right away. I do not know what made me do it but I put two fingers under the turntable where you can feel the underside of the subchassis and it was boucing at 24 Hz! Lift the dust cover and it stops. Put it down and the subchassis starts bouncing at 24 Hz. This is way above the chassis's resonance frequency. Somehow I have created a 24 kHz musical instrument. When I am listening loud it will have to be with the cover up until I figure this one out. There are three spaces. One under the dust cover, one above the subchassis and below the plinth the one below the subchassis and above the granite the table sits on. There is an open port at the tonearm board between chambers 1 and two. Also, I do not have to be playing the turntable. If I play a CD with a heavy bassline the subchassis will start bouncing to the music. There is no feedback path so it stops immediately. Lift the dust cover and it all stops. My brother has a PhD in aquadic acoustics. His response was to tell me to think about sound waves like water waves. "Now you can figure it out for yourself."  Wondeful. 

@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.  

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.

@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. 

@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. 

@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. 

@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.

@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. 

@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.

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. 

@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.