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. 

128x128mijostyn

@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.

Dear @mijostyn  : Good, I applaud your attitude to achieve the best sound that puts you nearer to the recording and nearer to the live MUSIC.

 

I'm sure that your new subs will be an improvement as the new plinth however anything you do can't  carry to full success till you change the item that proccess the cartridge signal and that's is the Phonolinepreamp.

 

No matters what the unit you own rigth now is way inferior of what you are looking for..

 

Yes, it's up to you.

 

R.

Dear Raul, That’s a very interesting article. The author makes a good point that using the term "fast" when describing a woofer is specious, because the "speed" required to reproduce very low frequencies accurately is within reach of any well designed woofer. But then he goes on to say: "There are reasons to use lighter, lower-mass woofer cones. They just happen to be different reasons than the ones you’ve read in print. Smaller woofers don’t make faster bass, but they do reproduce higher frequencies than larger woofers can reproduce, and this is all important when it comes to speaker design. You want the midrange driver and the woofer to integrate with sublime symmetry, with perfection and with nary a single problematic interaction throughout their overlap zone. This is why you want smaller, lighter, "faster" woofer cones -- not because they lead to faster bass. That overlap zone is so amazingly critical to your perception of bass speed that there is little or no tolerance for error." Note also that your expert does allow for the idea that some woofers are "faster" than others; he has only re-defined the term in a sense with which I do not disagree. By the way, no one "told me" anything about this. My conclusions are based on real world experiences that I had maybe 35-40 years ago when I was playing around with woofers to supplement ESLs that I then owned. So, I am expressing my personal independently arrived at opinion.

 

So perhaps I can be faulted for my choice of the word "fast", but what your authority wrote above is what I had in mind. Now if Mijostyn is using an 80db/octave crossover, on both high and low pass filters, then perhaps the capacity of his woofers in his system to produce higher frequencies is moot. On the other hand, all of his audio is passing through the digital domain afforded by his digital filter. That does not appeal to me.

 

Mijostyn, Along the same line of reasoning, you wrote, "Lastly, there is no such thing as a "fast subwoofer" when a woofer is not fast enough it’s high frequencies roll off." I hope you see the internal contradiction there. If there is no such thing as a fast woofer, then there is no such thing as a woofer that is not fast enough. But to both you and Raul, I would concede that I was guilty of sloppy semantics. When I say "fast woofer", I am thinking of the woofer and its enclosure as a whole. And I did not make that clear. You could put a small, i.e, "fast", woofer in an enclosure that limited its speed by virtue of what happens to the back wave, and it wouldn’t sound so fast, which we can define here as able to integrate well with an ESL panel. Like I responded to Raul, perhaps with an 80db/octave slope you need not worry about the capacity of your woofer enclosures to deal with frequencies above your crossover chosen point.

@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 :-) 

 

Dear @mijostyn  : I hope you don't pull the trigger on that kind of SG cartridge because make no sense to play LP recorded with the RIAA eq. curve through a dedicated phono stage with no inverse RIAA eq...

 

Just saying,

R.