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

@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.  

Jeez. That’s only two Commandments from Mijo. Do you suppose a dust cover can be isolated from the TT, if it’s firmly attached with hinges to the TT? Especially a spring suspended TT where induced vibrations of the cover are easily transformed into activation of the suspension? Anyway, if you hear no difference with vs without, then do what floats your boat.

@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. 

I apologize to Mijo for being facetious, but I don't like dust covers anywhere near the turntable when I am playing records.  When the TT is not in use, a dust cover is a good idea.

@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.