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.
Yes. But it appears you have a standing wave; if you were to break that up the boundary effect might be a bit reduced (that's how it worked out in my room anyway...). From how you've written about this, it appears that if you make just a bit of an improvement the problem would be gone since it only happens with certain cuts at higher volumes. I forgot to ask- what sort of stand are you using for the turntable? |
Look at my system page and you can see where the turntable is. It is on a granite slab sitting on a built in walnt equipment/record cabinet (which I made) which rests on a concrete slab. The uprights are 1 7/16" plywood. All the shelves are braced. The entire affair sits in an alcove in the side wall. On my system page you can also see graphs of the frequency response at the turntable dustcover up and down. There is a definite rise in the bass but no really big peak or trough. Over 1 kHz the response drops 10 dB with the cover down. |
@atmasphere , see above! |
Dear @mijostyn : "" much steeper slope without phase issues. "" I use first order high-pass and the blend between main speakers and the Velodynes are in " heaven ". "" It was digital bass management that finally solved this problem "" not really must be digital IMD goes really low if you cross over 80hz to 100hz and does not needs to be digital. "" I am working on new enclosures and will use 8 instead of 4 drivers which will halve their excursions. "" Look, contrary of what you think my take is not more drivers or more subs. What we and you need is proved bass quality performance developed by each driver and that means low bass driver it self. Velodyne subs are the lower distortions levels in the world. Eacg bass driver in the Velodyne is monitored over 16K times at each second and when the driver distortion goes near the Velodyne limit of distortions impedes that the distortion levels goes up and no matters if the subs are developing 120db SPL. Quality is the name of the game in overall bass management too. I understand that you designed and builded your four subs, well maybe needs not more drivers but a different design but I can’t know for sure. In the mean time enjoy the MUSIC with out dust cover because life is to short and you have not bougth it. R. "" On my system page you can also see graphs of the frequency response at the turntable dustcover up and down. There is a definite rise in the bass but no really big peak or trough. Over 1 kHz the response drops 10 dB with the cover down "" Maybe you are not measuring what you need to measure that can tell you what is going own down the dust cover. You like science then use it in the rigth way.
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@mijostyn Equipment stands are really important. I'm using a Sound Anchors stand with anti-vibration platforms (Ultra Resolution Technologies, long defunct but highly effective; the stand was custom built to accomodate them) for both the preamp and turntable, but I found out 20 years ago that it still had problems- I had to place the whole thing on a set of bearings rather than use its points into the floor. This made a profound difference. Although my 'table (Atma-Sphere 208, which is quite 'dead'; you can thwack the platter as its playing and not hear anything in the speakers) has no suspension, after the bearings were installed I could run the volume up to 110dB (measured) without any ill effects from vibration affecting the turntable in any way. Have you done any measurements to verify the effectiveness of your stand? I say this because I've seen stands from established manufacturers that actually made things worse!
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