Ok I have played my dust cover up....down with the turntable on.and I have played it off.....i believe good balance of my turntable. Keeps me from ever hearing any strange sounds .The cabinet the turntable is on is well balanced. I hear all of these problems, but I never had any....I have had a number of turn tables.. .Since 1969......
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.
- ...
- 146 posts total
@mijostyn Filter theory says that the greater the filter slope, the greater the hump is prior to cutoff- and the closer to the cutoff frequency. A digital filter might behave differently (some do, some don't) but a simple solution might be to move the cutoff down a few Hz so as to get the turntable resonance out of the filter bump that occurs just above cutoff. That should be easy- worth a try. |
@atmasphere , I can do that easily. I'll give it a go tomorrow. @limomangus , I'm not quite sure what you are saying. You have had a number of turntables and you have used them with the dust cover on and off and do not have any problems with either? |
Dear @mijostyn : If you are using paper cone I think that you have to stay that way ( btw, I cross too at around 100hz. My Velodyne HGS are heavy modified by me because its electronics design: input and crossover are very poor but great all other design/build. ). Stay away of other cone material on subs.
Now, I think that you are not measuring what you need to measure ( I don't know what you need to measure. ) or you are doing not in the rigth way.
In theory everything can be measured " problem " is what to measure and you have to " figure " out about instead to use your time in this thread.
R. |
- 146 posts total