In your first paragraph you describe the very reasons why I never use a dust cover. Also, I think you might be kidding yourself about the benefits, but I wouldn't argue that point. What you're observing is nothing special or out of the ordinary in terms of the problem with a dust cover. It's more or less the argument I posed to you long ago.
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.
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@lewm , As I just mentioned I have been using a dust cover continuously since 1967. This is the very first time I have had any issue wih a dust cover and it is a fluke which I will correct. It also happens at volumes which most people can not dream of acheiving at 24 Hz including you. Very few people would ever notice it. Perhaps I should not have mentioned it as I knew the anti dust cover crowd would have a field day. But, it is an interesting phenomenon which should be easy to correct and it merits discusiion. I have ordered some self adhesive felt cloth that I will cut into strips and position all around the periphery of the plinth under the edge of the dust cover. This will be relatively attractive and will stop the Helmholtz effect without harming the dust cover which is very attractive as it is. Otherwise the turntable sounds wonderful and I am very pleased with the Soundsmith Voice cartridge. Don't listen to me. My hearing sucks. |
@mijostyn thanks for sharing your experiences and process. Keeping records clean from the start is certainly easier and cheaper than buying an ultrasonic record cleaner! |
@mrklas , Yes, if the dustcover is hinged to the plinth of an unsuspended turntable moving the dustcover will make the tonearm skip. This is one reason so many manufacturers do not offer a usable dust cover and many do not use one. Trying to put an unhinged dustciver over a turntable in operation is a PITA and asking for it. The way around this problem is to get or build a dust cover that is larger than the turntable and hinge it to the cabinet the turntable sits on. To keep it up use a prop just like with some car hoods. If you were really handy you might be able to adapt a gas filled strut to work. If the cabinet is as heavy as it should be using the dust cover with reasonable care will niot make the turntable skip. This is just one of the advantages having a suspended turntable. You can do just about anything to the plinth and it won't bother playback. |
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