halle1@houxz.UUCP (J.HALLE) (12/12/83)
I have a fairly severe acoustic feedback problem that is causing me to blow fuses daily. Can someone help? I have a Kenwood KD-7X turntable and a Signet TK1Ea cartridge. Tracking and anti-skate are at 1.5 g. Almost every time I put the dust cover down I get feedback, usually quite bad. It even happens if the loudness contour is off (the usual position). I don't seem to have mechanical feedback problems, and I don't seem to have problems with the dust cover up. Can anyone help? I really don't want to leave the cover up all the time I'm playing. (I still can return it if necessary.) Thanks.
sleat@aat.UUCP (12/18/83)
I have a hypothesis regarding the nature of your feedback path.
I once borrowed a turntable on which I observed the following behavior:
It had a hinged plastic dust cover, like most. While playing a record
with the dust cover down, I chanced to gently wipe some of the dust off
of the top of it with my sleeve. I was rather startled when the tone arm
lifted entirely off the disc and banged into the dust cover! (I was
also elated at having made a neat discovery, and disgusted with the
turntable.) It was immedately obvious what the problem was, and almost
as immediately obvious what the implications were. The static charge
created by rubbing the plastic with the cloth of my sleeve was easily
enough to create a gram or so of force between the cartridge shell and
the cover. The obvious implications were that if I could generate enough
force to entirely lift the sucker off the record with one wipe of my
sleeve, far smaller disturbances could still play havoc with tracking forces.
You state that you only get the feedback with the cover down. My hypothesis
is that the cover is acting like a diaphragm, being vibrated by the airborne
sound. These vibrations are then coupled electrostatically to the tone arm.
If you wanted to test my hypothesis, you could tape some aluminum foil to the
inside of the cover and ground it to the tone arm (via the cartridge wire
shield). If I am correct, this should eliminate the problem.
I should note that it is possible to have static charges on plastic which
last for a very long time, particularly in dry weather, so the problem
doesn't go away if you simply avoid wiping the cover while playing records.
Since discovering this phenomenon, I've been very suspicious of plastic
dust covers. I should think that any high-end turntable should use a
conductive metal coating on the inside of the cover. It's pretty silly
to go to great lengths to accurately control tracking force, etc., then
have it messed up by something so simple (though admittedly perhaps so
simple as to be non-obvious).
In retrospect, I remember that I had borrowed the turntable to make a tape
copy of a disc, and make the mistake of talking near it (loudly, but not
by any means shouting) while making the copy. Sure enough, on the quiet
passages, noises which sound very much like garbled talking can be heard.
I had also observed that the turntable base had very poor mechanical
isolation from the table, so I attributed the coupling to that path.
Now I'm inclined to think that it might have been through the cover.
It's things like this, much more than the details of ultimate performance,
that make me want an optical disc player. This whole mechanical reproduction
system gives me the heebie-jeebies. (Yes, I know, speakers & microphones
(and eardrums) are mechanical, but the less the better.)
Michael Sleator
Ann Arbor Terminals
{cbosgd | cosivax | mb2c | psu-cs | uofm-cv}!aat!sleat