Audioclinic (Q and A) (Jun. 1974)

A Reader's Comment About Phonograph Hum

Editor's Note--Some time ago, March, 1972 to be exact, I answered a reader's question with the hope of curing the hum in his phonograph. I do not know whether my suggestions were helpful. In any event, my answer prompted another reader to share his experience with all of us:

If your correspondent has (1) an older Garrard (such as a Lab-80) or (2) a Shure cartridge (such as an M91 or M93), he, himself, may be causing the hum problem because of improper installation of the cartridge.

The Lab-80 has a metal cartridge holder. The Shure cartridges have grounding straps attached to their cases and to the RG lugs on the cartridge. In the case of the combination of the Shure cartridges and the Lab-80, and worn lugs on the shell plugs, you can have one heck of a lot of hum without half trying. The cure is to pull the ground strap out of the cartridge (which it is designed to do). I almost went "bananas trying to find the ground loop in my system after I changed over to an M91E from an M-44E (which does not have a metal case); of course, I knew what I was doing and did not bother to read the directions first. I found out that other folks did the same thing at the store I work for because this same problem showed up with other turntables and cartridges, where metal cartridges were involved.

For what it is worth . . .

-Thomas N. Ronayne, Detroit, Michigan

Centering a Woofer on its Baffle

Q. One of my associates and I occasionally indulge ourselves in the construction of bookshelf-size speaker en closures using both acoustic suspension and non-acoustic suspension loudspeakers.

The enclosures are all essentially of the infinite baffle type-no bass reflex de signs, etc. One of us claims that placing the woofer off-center on the front panel will give better results (such as bass response) than when placing it directly in the center. The other claims that placing the woofer in the exact center would be acoustically more desirable.

Barring changes in dimensions, total air volume or any other variable, who is correct as to the placement of the woofer? What would be the criteria for such placement?

-Ken Clarke, Columbus, Ohio

A. I never center a woofer on a baffle board, neither vertically nor horizon tally. If a woofer is centered, the baffle board is free to vibrate in two equal modes, adding to coloration. These modes tend to reinforce each other.

By not centering the woofer, the baffle will still try to vibrate, but the modes of vibration will be different on each side of the woofer, thus considerably weakening the mechanical resonances which may be present.

There is no substitute for a heavy baffle board. Its weight will keep the amplitude of vibrations low and thus keep the sound cleaner than it would otherwise be, regardless of the placement of the woofer.

Scratch Filters

Q. Would you know of any company that markets a scratch filter for record surface noise?

-Ronald R. Kostecky, Ramsey, New Jersey

A. I suggest that you inquire about such a unit by writing to the large mail order houses such as Allied and Lafayette. Noise is most perceptible to the ear in the range of about 3,000 to 5,000 Hz. Hence, one of the new breed of audio equalizers (quite expensive), which can shape frequency response over the span of as little as an octave or even one-third octave, could be of great help. Some audio preamps have quite effective scratch filters, and you might inquire of your audio dealer which are the best in this respect.

(Audio magazine, Jun. 1974)

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