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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
atmasphere's avatar
atmasphere
11,807 posts
 

I don’t know whether the SS SG phono stages account for this but I’d be surprised if they don’t incorporate some corrective filtering in the 500 to 2000Hz region.

@lewm The last time I heard one, it had no eq. The claim was that it didn't need it- which is misleading. And sure enough, the LP I used for reference (which I recorded) didn't sound right. I heard months later that EQ was being offered, but apparently that is incorrect??

The thing is, a strain gauge has really high output; some of them I've seen can drive an auxiliary level input. The parts needed to execute the EQ are inexpensive and small, and could be installed in a very small box that is part of the interconnect cable, since it can be done entirely with passive components.  So it puzzles me why this isn't done.

 It has to happen in conjunction with transduction of the cartridge signal.

Why is that?

lewm
13,046 posts
 

That statement was probably incorrect. Apologies to Mijo. But I still don’t think he should attempt it before acquiring a thorough understanding of RIAA. 

mijostyn's avatar
mijostyn OP
8,570 posts
 

Hold on Guys! I have not bought one yet. I'm just going to listen. I am equally enthralled by low impedance MCs and transimpedance phono stages. Unfortunately, the price of admission is going up in leaps and bounds. 

I see what the problem is with strain gauges unequalized. Most rooms create more error but that should not be an excuse. 

On the other hand I am still greatly enjoying The Voice on my ARC PH3 SE. They match up well and SS guys would never know it was a tube phono stage except maybe rauliruegas who is just claire voyant and really has no idea what he is listening too:)))

I bought a large piece of Macassar Ebony which should be big enough to do the whole plinth. Should I do dovetails or finger joints? What do you think would sound better?

lewm
13,046 posts
 
Direct drive. Cover off.
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rauliruegas
13,060 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.

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