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.
You can try moving the turntable to a low bass null point in your room. Otherwise, maybe a frame with silk streched over it to keep the dust out while playing the record. ETA: Don't say nothing can pass above 2hz. Springs will vibrate at harmonics just like loudspeaker cones and musical instruments. Just lower in amplitude. |
@vinylzone , The springs are dampened. The sub chassis is a 1" thick aluminum plate. Think SME 30/2 @jasonbourne52 , IMHO that defeats the purpose of a dust cover. Records never have a neutral charge. Just playing the record develops a small amount of static charge. Static always attracts dust like a magnet. Small particles can be sucked right into the groove. Keeping the records covered and discharged are the best ways to maintain clean records. If you are doing a good job you should never have to clean your stylus. (hardly ever) @fuzztone , I have an 80 dB/oct rumble filter 3dB down at 18Hz. The feedback is just above. It does take a vigourous bass line to get it going but the thing about feedback is that it is self perpetuating and it does not require a 20 Hz tone to get it going, like blowing across the top of a bottle. |
