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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
Post removed 
lewm
13,114 posts
 

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

vinylzone
288 posts
 

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.

lewm
13,114 posts
 

Nature abhors a vacuum, I always say.

mijostyn's avatar
mijostyn OP
8,570 posts
 

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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