20hz
doubtful
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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- 146 posts total
@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. |
@mijostyn Looking forward to new thread comparing different brands of diving weights atop the dust cover. Being Friday, I am really hoping to hear @tubebuffer 's thoughts on the question. As a last resort, you could limit listening to panflute recordings until you land a solution. Hang in there! Cheers, Spencer |
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- 146 posts total