Dustcover Blues

@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.

Golly! I am so glad to be instructed by you after only 50 years of messing about with audio.  First, I wouldn't argue for a moment that my SL experience would be improved if I were to add SUBwoofers to the Sound Lab system.  (In other words, woofers that come into play at below 50-60Hz and never above those frequencies.) You are absolutely right about that. I already explained to you privately that I have made a choice not to do it, partly because the system is in our living room, which is already too dominated by audio equipment.  But if I did do it, I would never add a woofer that comes in at 120Hz; there is too much real music at that frequency and down below to 40-50Hz. Your 12-inch electromagnetic woofers, impressive and loud though they may be, can never match the speed and articulation of an ESL at 100Hz.  But that's just my opinion based on some prior experience.  The only woofer I would even think of pairing with an ESL at those frequencies would be one based on a transmission line enclosure using a much faster woofer than average.  With the Beveridge system (in my basement), I am not constrained by decor, and the 2SWs do require woofer supplementation; that's how they were originally marketed.  There I do use KEF B139 woofers in a large and ungainly TL cabinet that I built myself decades ago.  But even there, the crossover, as determined by Beveridge and not me, is at 80Hz.  Likewise, the slope of the hi-pass filter is Beveridge, 18db/octave.  Consequently, I use an outboard electronic crossover for the woofers that has a complementary 18db/octave slope. If I had my druthers, I would use the Linkwitz-Riley slope of 24db, because that maintains phase between the high an low frequency reproducers. The woofer x-over has controls for level and frequency, but I keep it around 80Hz.  Finally, there is no free lunch in audio. Every electronic x-over I have ever heard has a "sound".  I've never heard a digital x-over, but I am sure it has a sound too, not to mention the extensive amount of digital room correction that you have chosen to incorporate.  I admit to being almost too pure a purist, but I like to avoid x-overs, both passive and electronic, if it makes sense. This is all fine. The goal is to satisfy onesself.  I would never be so bold as to sit here and tell you what you are hearing over there. Perhaps you should adopt the same approach.  Sorry for the very long digression, but you raised my hackles.  I didn't know I had hackles.

Helmholtz Resonator.  Every definition I can find, including the one on Wikipedia, states in one way or another that you need a closed container with a large hole at one end and a small outlet at the end of a narrow neck at the other end.  Apparently Helmholtz built several of different sizes to demonstrate how size of the enclosed volume of air, neck length, and aperture area all determine the frequency heard at the small outlet. Based on what you wrote, it seemed to me that your problem persists after you seal the dust cover to the plinth surface.  Thus there is no pathway for air to go in or out. So, you cannot have a "Helmholtz Resonator".  For sure, you do have a resonance problem, just not the one envisioned by Dr Helmholtz.  Where have I gone wrong in this reasoning?  I think you may have the forme fruste of "dust cover blues", as you so aptly put it in the first place.  In other words, you have an extreme and unusual problem that nevertheless falls under the category of why some of us eschew the use of a dust cover when playing LPs.

@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

Whether its Helmholz or not isn't important. Resonance is.

This isn't the first time I've seen this. Since most high end machines aren't meant to be used with the dust cover down (if they even have one) the dust cover may well not have been included in the design of the turntable insofar as its resonance control methods are concerned.

IOW its the luck of the draw that it works or not.

If you simply grab the dust cover with both hands and try to squeeze it can you make the oscillation stop?

@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?