AUDIOCLINIC (Jun. 1988)

Fan Guidelines

Q. I am trying to choose a small a.c. fan to cool components inside a rack-mount cabinet. My main concern is to avoid inducing electrical noise into my system. I have a couple of 3-inch fans (115 V, 50/60 Hz, 5 watts, input-protected) which I obtained as surplus from a disk-drive maker. It seems to me that, because computer peripherals require "clean" power, these fans would be suited to my needs.

-Andrew David; Minneapolis, Minn.

A. By taking a few simple steps, you can make sure there will be no problem using the fans you describe, nor any other small fans. Fan motors do not employ brushes and commutators as they did many years ago. Motors of that kind could introduce noise, especially when listening to AM. The only way I can see how today's fans could introduce electrical interference into an audio system is by way of induced hum, either into tape playback heads or into phonograph cartridges. This should not be a concern, however, as long as the components to be cooled are located far enough from the phono graph or tape recorder so that the fan will be well clear of these sensitive items. If the amplifier (or other equipment employing large power transformers) is located too near the phono graph or recorder, its power transformer will probably cause more hum than the fan. As with all power cords, keep the fan's cord well clear of the phono interconnecting cables.

More About Cable TV Hum

I read with interest the item in the June 1987 "Audioclinic" regarding hum in TV audio. I, too, like reader Pat Yacques, had experienced the problem for more than a year, and the solution--after trying everything-was very simple. One or more components (most likely power amplifiers) in my audio system have three-prong a.c. line cords, with the third prong used as a ground for protection against shock hazards. With the ground from the cable drop (the point of attachment of the cable to the home), audio grounds etc., the addition of this three-prong grounding scheme produces many different possible ground loops within the audio system. A very pronounced 60-Hz hum may result.

Sometimes there is no ground at all, or a very poor one, at the cable drop.

Thus, a voltage is developed between the audio and cable system grounds.

This voltage finds its way into the audio system and is heard as hum. I suggest two solutions.

First, an isolation transformer can be created by using two 75-ohm to 300 ohm baluns; connect them together by tying the 300-ohm leads of one unit to the 300-ohm leads of the other unit, and then feed the main cable to one balun's 75-ohm side. Connect the TV set to the second balun's 75-ohm side.

This circuit isolates the TV ground from the a.c. line ground with no appreciable video signal loss. However, be cause of the lack of r.f. shielding, this system may cause r.f. interference with the desired cable signal.

Also, by removing the audio system a.c. ground, the entire audio system floats above ground but at a level constant within the system, and no hum results. So the second alternative is to use a three-prong to two-prong a.c. adaptor plug in the a.c. line cord of any three-wire, three-prong audio equipment. Do not connect the pigtail to the outlet grounding screw. I use this method with my amps; it isolates the a.c. ground from the system and eliminates any trace of hum. The grounding protection can be obtained by installing a ground fault circuit interrupter (GFCI), which can be found in duplex outlet form at hardware stores. Place this ahead of all a.c. line cords which have been adapted from three- to two prong. This GFCI will provide better and faster shock protection in the event that the protected piece of equipment should develop a problem connected with the a.c. line.

I also suggest that a surge protector, of the type used for computers, be placed in the a.c. line ahead of all audio and TV electronics.

-Andrew J. Megna, Jr., Emmaus, Pa.

I should add just one thing: Be sure that the baluns used employ isolated windings, or the scheme proposed by Mr. Megna will not work.

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(Source: Audio magazine, Jun. 1988, JOSEPH GIOVANELLI)

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