Nice finger joints. Dogged determination, for sure.
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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@audioquest4life , you tossed the old hinge hardware? Sota uses some really nice hinges with adjustable friction. I'm sure Donna would sell you a pair. You can mention my name if you like. Having to lift the cover off al the time is a recipe for disaster. @lewm , Thanks Lew. I'm not sure about "dogged determination." Setting up the jig is the hardest part. It takes a lot of test cutting to get the tolerances right, but once that is done you can knock these out all day long with minimal effort. Now hand cut dovetails is dogged determination. But, this is not a $10,000 coffee table and I'm not exactly getting paid for it. After all, it is just a turntable:) |
Having read through this old thread, and as someone who listens infrequently to vinyl, I have the following question: If I don't get an artifact such as the OP in this thread describes, what are the reasons one typicaly doesn't use a dust cover while playing the record? I recently picked up an lp of Julian Bream playing Spanish Guitar Music when I was visiting out of town. It sounds great but for the first time since buying this turntable a few years ago I experimented with cover on/cover off and could detect no difference. I'd always just imbibed the dogma that the cover should be off while playing. I love this pristine and really quiet sounding lp and would like to prevent as much dust as possible and so I will continue to use the cover since I don't hear any difference at all. Am I doing something that will effect the outcome of the election or contribute to Global Warming or committing some other unforgiveable sin? |
@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. |
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