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.
Actually you can build it yourself, it’s an old audio amateur design, here is an example Its all a parody - I call the arm "a bridge to nowhere", nowhere being the TT - its a con. Its similar to the Linn Sondek - comes from the land that gave us haggis, well the Linn like the Haggis is a bag of bits that nobody wants.
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The feedback is being amplified by the cartridge, which is mounted on the arm, which is mounted on the TT subchassis. The closed lid changes the environment, but the arm/cartridge is what is getting excited at a particular frequency. I would have tried a couple of things - Bypass the suspension and see if any changes occur. Increase the arm effective mass by 2g You might find the problem disappears. Also I agree with @atmasphere - locating the TT in a corner or niche is the worst possible location for the sort of issues you have.
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@dover , I believe you are analyzing the problem incorrectly dover. Increasing the effective mass would make things worse by giving the the tonearm more inertia. The bass gets the suspension bouncing at 24 Hz. Life not being perfect, the tonearm has more inertia at the cartridge end and it is bouncing also enough to generate a 24 Hz signal that keeps things going. If the suspension does not bounce there is no problem regardless of volume. I can peg it no problem. I'm quite sure this would happen with any tonearm mounted as it is just along for the ride. The bass response is pretty much the same across the shelf. It does not increase appreciably in the corner because the side wall is only 1 foot deep. It is not much of a corner from an acoustic standpoint, not at all like the corner of a room. This was one of the first things I measured. |
Dover, The next arm I get will be a Schroder LT. I am already looking into mounting it on a modified Sota base since I plan on making a new one. As long as the suspension can handle the greater weight I should be able to make it fit. I am in total agreement on the Linn having owned two many years ago. They do seem to have everyone hooked on the upgrade mania. Good business. |
@lewm , @rauliruegas @atmasphere The skirt is finished and installed. It is lined with felt which makes a good seal. It works perfectly. I can no longer induce any feedback with the dust cover closed. I can sleep at night now:) I |
