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

Showing 3 responses by mapman

@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. 

@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.

 

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. 💪