[abernardi]

So my re-immersion into vinyl is coming along slowly but coming along nevertheless... I put the turntable (Micro-Seiki B21, AT - OC9/II, Graham Robin tonearm) on 3 stillpoint cones, on a 3" butcher block on 4 herbie's feet.  This helped tremendously in tightening up the mids and lows.  However, I have suddenly encountered some kind of low frequency feedback and a ground buzz.  At first I thought I was getting a 60Hz ground hum, but it wasn't constant, it rose quickly in amplitude, it was feedback.  It started when I added back my DAC and preamp into the system.  So I took them out again and the buzz went away, but the feedback remained.  I have since added them back and was more careful not to cross the power lines with the interconnects and I think that was the buzz problem.
I assumed it was 60Hz, but maybe it's lower.  I remember sweeping my room once and am aware there's a pretty good standing wave at around 35Hz, but I'm not sure the feedback was that low.  It may have been
Anyway, tonight I tried closing the dust cover on the turntable and that cured the feedback problem.  But I've heard it's not good to play with the dust cover on.  I've heard two reasons for that, static electricity can alter the tracking force or the dust cover can rattle or cause a resonance problem.  Which one is it, anyone?

I'm assuming this is an example of airborne resonance?  Can anyone advise me on this?  Do I need to move the turntable?  Or, should I put some kind of weight on the shell and then re adjust the tracking?  Is it a tonearm thing?  THANKS!!!

[Quiet Earth]

I'd like to make a suggestion on the low frequency feedback part of this.

Remove the Stillpoints to see if the low frequency feedback problem goes away. Put the turntable directly on the butcherblock with the Herbies footers under the block. Or remove the butcher block and the Stillpoints, and try the Herbies footers directly under the turntable. Experiment a little bit, but see if you can get the feedback part of your problem to go away by removing the stillpoints.  I really like stillpoints footers, but I found that I could induce a low frequency feedback with one or two different records when my turntable platform was resting on Stillpoints. Now I have my turntable resting on its platform with the platform supported by Herbies footers and no low frequency feedback problems so far. It sounded better (clearer, tighter) with the Stillpoints under the platform, but there is no feedback with the Herbies footers in that location.

It's a trade off. Experiment a little bit.

I think having a dust cover attached to the table complicates matters, but I have been dust coverless for many years now so I don't feel qualified to comment on the dust cover. I do own a dust cover, but it comes off of the table when the record goes on. It is not permanently attached.  Maybe you could remove the dust cover just to see if it improves your situation.

[abernardi]

yes, I suspect the stillpoints too.  And come to think of it, that dust cover when open is hovering over the turntable at a 45 degree angle, it's an open box, but that would be too small to amplify a tone that low, no? 

[*Scotty*]

If the cover is vibrating due to airborne low bass frequencies when it is open, it will contribute to and possibly cause the feedback problem you are experiencing. You report that the problem does not occur when the cover is closed. When the cover is closed it is no longer freely suspended and it is probably damped enough when closed that magnitude of any vibration is below the threshold necessary to cause audible feedback.
 I would play my music with the cover closed and rock on.
Scotty

[abernardi]

So I wonder where I heard that it was a bad idea to play with the cover closed?  Thanks!!