@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.
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
@tvad , Yes, you are quite correct. The very best tables came with dust covers and for a time even the less expensive tables came with them. Then manufacturers learned from Transcriptors (I think) that if you made your turntables cool looking enough you could sell a lot of them even if they are lousy tables. I should know as I fell for that. Soon they became so odd shaped there was no way you could attach a dust cover. But, visual stimuli dramatically improves the hearing of a large population of people. This whole mess ends at the Techdas Zero, without a question the coolest looking turntable on the planet and technically it is very impressive. Personally, I would rather spend that money rescuing geto kids from a toxic environent and getting them someplace where kids can have fun being kids like a summer camp. |
Leaving aside our differences regarding to use or not to use a dust cover, I am thinking about your hypothesis that right now your turntable/cover are together acting according to the principle of a Helmholtz Resonator. The idea does "resonate", but so far as I can tell from my reading, a Helmholtz Resonator requires air (or whatever fluid is resonating) to be put into motion. Energy has to be put into the system for that to happen. In your conception, what is the source of energy? I would offer an alternative hypothesis, for good or ill. Could it be that energy being emitted by the cartridge body, the same energy that is heard as "cartridge noise" is bouncing around under the cover which is by chance tuned to resonate at a low bass frequency, which is why it is excited only when you play certain LPs with low bass frequencies and at a certain SPL above some threshold that causes the whole thing to get excited and re-enforce itself via positive feedback? That is what I meant when I remarked that you are experiencing a problem that I associate with dust covers per se. I put this out there for discussion, not to antagonize or criticize. In my own experience, when I played LPs with dust cover down, inevitably the dust cover itself could be felt to be vibrating, I always thought sympathetically with cartridge vibrations. |
- 146 posts total