@lewm , @rauliruegas @atmasphere The skirt is finished and installed. It is lined with felt which makes a good seal. It works perfectly. I can no longer induce any feedback with the dust cover closed. I can sleep at night now:)
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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.
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@rauliruegas , I AM! @dover , I think YouTube might break:) |
Glad you have things started out and now you are able to "rock out" again and play your music at listening levels that only a few appreciate...I get it. I also like to play loud every once in a while. Well, perhaps, more lately;). The work you did to your system and room to ensure that you can enjoy playing music loudly without problems is what makes this hobby fun. Your subwoofer builds are intriguing to me because customized them to take advantage of using DSP, which you already use for your system. I use SVS (4 each) with the limited DSP they have, but it works. Like you, I have lots of mass for my turntable. It is already 120 plus pounds, sitting on top of a 275 pound Clearaudio MontBlanc stand, on top of bamboo floor with anti-vibration and sound absorbing adhesive (that's what the special glue container says (275 per 5 gallon), on top of concrete. No resonance issues in this system even with 100db plus levels, which is not part of my loud norm. only once in a while.
Back to the main topic. Of course, you realized this symptom was some form of resonance emanating from the dust cover based on increasing the loudness of your music. I experienced that once with my old (still is use for living room) Denon DP62L sitting between a pair of Polk Audio SDA2's being driven by Carver M1.5Ts. This was in the early 80's when I first joined the military and lived in the barracks, so no thought of treating room acoustics. Whenever I played Def Leppard or AC/DC really loud, I would hear that resonant feedback that you mentioned. My trick to eliminate this resonant feedback was to prop the dust cover up so that it was not entirely touching the turntable base, about a 45 to 55 degree lift. I forgot what I used, but I am sure it was some crude and rudimentary fix that only Soldiers would figure out when you wanted to make things 'happen" or accomplish the mission.
Back then I was a novice and knew virtually little about tweaking, only, plug and playing and treating issues if they were gross, bass to boomy,.move speakers away from walls, too little bass, move back to walls, not loud enough, get bigger amps, and bigger speakers, and so on and on it went for years. I have learned a lot since then...hence, my moniker, which has philosophical meaning to me, the journey will never end. We will always learn something new as we grow older or experience new things, in this hobby, other hobbies, or in life. Hmm, now I want to dig out some Rush 2112, Three Blind Mice, and Miles Davis, talk about eclectic tastes in music, LOL. Low level listening sometimes, in the room live feeling most of the time. It is all enjoyable.
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