You astound me with your powers of deduction.
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
@vinylzone , This is the very first time I have seen a dust cover contibute to feedback. What it does demonstrate is that a tonearm can pass on airbourne vibration that is outside of it's resonance point. It can not be the record vibrating because it is vacuum clamped. Under most conditions the dustcover attenuates sound getting to the tonearm by as much as 15 dB at some frequencies. This can be demonstrated by putting a measurement microphone under the dustcover. This weekend while I am farting around I will do this and take screen shots of the curves generated inside and outside the dust cover which I will put up on my system page. I will use a short sine wave sweep from 10 Hz to 20 kHz. My arguement is dustcovers can improve the performance of your turntable and help to keep your records clean also diminishing stylus wear. Dustcovers are hearing protection for your cartridge. Mark Dohmann related to me in an email that he is working on a dust cover for his Helix turntables to be used during play. When I press down on the dustcover the feedback does stop probably because I am closing the gap at the bottom not because I am damping the dustcover. Closing the gap with felt strips is a more elegant way of solving the problem. |
If the table is subject to vibrations, dust covers could easily exacerbate the problem. Best to avoid vibrations reaching the table always but if not possible and dustcover exacerbates things just take it off when playing. Dust cover vibration is not an issue for me these days personally. My table is rock solid. 💪
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@mapman , What are you using? Even if a table is "rock solid" attaching a dustcover directly to the part of the turntable that holds the platter and tonearm would be more prone to causing trouble. Historically, the tunetables that came with dust covers were suspended such as the Thorens TD 125, The Linn LP 12 and the Sota Sapphire. They were isolated from their dust covers. I favor this type of construction. It is not as flashy as some designs but isolating the platter and tonearm from the rest of the environment has significant functional and sonic advantages. Mass loaded turntables can work very well if located on a very sturdy rack on a concrete floor. I do know of people putting turntables like the Kuzma Stabi XL DC on MinusK platforms. |
@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.
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- 146 posts total