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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. 

mijostyn
rauliruegas
13,384 posts
 

Dear @mijostyn  " has no idea what he is listening too "

 

That is all my knowledge level I'm and my friends totally sure.

 

To my quality level standards your phono stage is totally out of question even with your Voice..

 

At this time all your statements in this thread tells me that your overall quality MUSIC level targets are way lower that what you  showed in other threads. If this is your reality then you are not a gentleman that really want to help analog lovers in this forum through your posts.

 

Your technical knowledge level is way superior to.

 

R.

dover
2,504 posts
 

@mijostyn 

I'll let you in to a little dirty little secret - its the arm.

mijostyn's avatar
mijostyn OP
8,570 posts
 

@rauliruegas , I hope you realize Raul that I was joking. That's what the smiles were supposed to convey. 

I am here because I still play records just like everyone here. We all enjoy music in our own way. You have no idea what my listening capabilities are and I yours. I have no idea what you are listening too and you I. It make more sense on these forums to talk about the way things work, much more concrete. We both dislike unipivot arms for the exact same reason. They do not function the way the ideal tonearm should. We both have probably used and listened to examples we did not like. 

@dover , Are you saying the Schroder is causing my feedback problem? If so that  would fly in the face of the obvious. If I lift the dust cover or close off the bottom the feedback stops. I did nothing to the arm. It is perfectly capable of playing at outrageous volumes without any difficulty at all. It is tuned right where I like it. If you are saying it is just a bad arm I and many others will disagree with you. Thanks for your input. 

@lewm , I think that is an argument for a different day. 

lewm
14,703 posts
 

Like you, Mike, I was joking. I wouldn’t have a chance trying to change your point of view.

mijostyn's avatar
mijostyn OP
8,570 posts
 

@lewm , and I was just about to buy one of those Oswald Mills Audio K3's. Actually, it is the only way you can get one of those arms. I plan on tossing the turntable and sticking that arm on an AR XA just for spite.

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