Interesting experience you have. I don’t like my dust cover to be next to my system at all when I listen, I even keep it in different room when it’s long listening. Maybe it’s too much, but I think my cover is very reflective as it pretty big put on-of style and covering whole tt.
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.
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@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.
Why is that? |
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