gregr@tekig1.UUCP (Greg Rogers) (12/15/83)
Read on for a) A statement of an old problem b) A reply to the Acoustic Feedback Problem c) A different but related problem that almost all turntable users suffer from. New to the net, but old to engineers. As I have stated and explained previously in this newsgroup, playing a turntable with the cover DOWN has long been known and well documented, as UNACCEPTABLE except under carefully controlled environmental conditions that are unlikely to occur in a home situation. The reason for this is the buildup of static charge on the dust cover that will attract the tone arm and reduce the tracking force. Since the amount of charge will vary with humidity, temperature, clothing of user, and many additional factors, it is not a predictable level and hence cannot be compensated for in any general sense. By the way, the charge buildup is not usually caused by foolishly rubbing off the dustcover (although that would do it), but rather by transfer from a human body when the cover is lifted and then replaced. It's possible that your problem may be caused by this reduction in tracking force making the tonearm-cartridge more susceptible to minute movements of the turntable/tonearm/furniture system from the acoustically coupled or mechanical coupled vibrations. There are however any number of other reasons that could be responsible for your problem. In any case, if you must persist in this unadvisable practice of closing the dustcover then you should determine whether the feedback is a result of mechanical or acoustical coupling. Try placing the turntable temporarily on foam rubber to isolate it from whatever furniture supports it. You should also place the speakers on foam rubber as well. This is simple and will identify if the problem is mechanical coupling. It usually is NOT in cases of severe feedback. More likely (and harder to fix) the problem is acoustical coupling. In this case you will have to reduce the level of sound at the turntable, at the frequency, which the feedback is induced. You might test this theory by moving the turntable away from the speaker (to reduce the direct transmission) or away from the walls or cabinetry (to reduce reflected sound) or simply putting a temporary barrier around the turntable. Any of these actions may improve or make worse the problem. This should indicate you have found the cause. The solution would then be to find a way to reduce the acoustic level at the turntable permanantly. You may have concluded at this point that the proper solution is to leave the dustcover open (you stated this solved your problem) which I have of course recommended above. HOWEVER, if you are very concerned about getting the very best sound from your system (this is TRUE audiophile stuff from here on {as opposed to subjective I can hear what you can't audiophile stuff}), then you probably still have a problem. Long before the acoustical or mechanical coupling reaches levels that cause severe feedback, it begins to alter the sound by superimposing itself on the electrical signal produced be the cartridge itself. This is a frequency modified version of the original signal (frequency modified by the path by which it returns, and the coupling mechanism at the turntable) as well as one or more time delays caused by either an acoustical path or a mechanical path or both. Hence the electrical signal produced by the cartridge and the subsequent sound of the system is modified by earlier sounds etc. This effect is simple to measure, hence easy to document, and the only thing left is to argue about how audible it is. It is likely that this effect is one of the prime factors which determine why different turntables sound differently. It is rather easy to measure a turntables susceptability to the problem. You might also begin to imagine how a small degree of this effect might enrich the sound, add a bit of spaciousness to the sound, or under some conditions "improve" the sound, but always at the expense of true accurate reproduction of the recorded signal. Of course you might also be able to understand that a little too much of this will result in a noticeable reduction in sound quality in many respects (clarity, imaging, ad infinitium). I will leave it to your imagination to conclude in which category "some audiophile" turntables can be measured as belonging in, while other turntables of far less reputation but better engineered are measured as belonging in. The solution to this problem from a practical standpoint involves either a better turntable/tonearm design and/or a reduction in the mechanical and acoustical coupling discussed above to levels far below those required to create the howling type feedback that started this discussion. In this case the mechanical coupling problem is usually always significant. I'm sure that all this discussion will be controversial (this newsgroups been kind of boring lately anyway) but I find it much more interesting discussing the sound of measureable effects, then trying to explain why some people "claim" to hear effects that can't be measured. The obvious advantage of digital CD discs which suffer from neither of these problems is important to ponder. It helps explain several interesting "complaints?!" against CD's (or are they virtues!?). So flame on subjective "audiophiles"! Greg Rogers Tektronix