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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. 

128x128mijostyn
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
13,046 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.

dover
2,243 posts
 

@mijostyn

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.

Actually you can build it yourself, it’s an old audio amateur design, here is an example

Its all a parody - I call the arm "a bridge to nowhere", nowhere being the TT - its a con.

Its similar to the Linn Sondek - comes from the land that gave us haggis, well the Linn like the Haggis is a bag of bits that nobody wants.

 

dover
2,243 posts
 

@mijostyn

@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.

The feedback is being amplified by the cartridge, which is mounted on the arm, which is mounted on the TT subchassis.

The closed lid changes the environment, but the arm/cartridge is what is getting excited at a particular frequency.

I would have tried a couple of things -

Bypass the suspension and see if any changes occur.

Increase the arm effective mass by 2g

You might find the problem disappears.

Also I agree with @atmasphere - locating the TT in a corner or niche is the worst possible location for the sort of issues you have.

 

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