AUDIOCLINIC (Jan. 1990)

Light Dimmer Interference

I stumbled across a cure for light dimmer interference recently and wanted to share it with other readers.

In my house, I have installed two strings of recessed lighting, both of which are controlled by dimmers.

These strings are located at right angles to one another, precluding orienting an AM antenna to a null point. I have tried various brands and models of dimmers, all to no avail.

Recently, however, I put an undimmed 100-watt bulb near my turntable to help me see better. With this light on, the horrendous buzz was totally eliminated. (I don't mean when playing records; I mean when I run my AM tuner!) I have no explanation for it, but this cure worked for me in at least this instance.

Keith Richardson, Mason City, Iowa

Loudspeaker Impedance Measurements

I am writing in response to the technique presented in the July 1988 "Audioclinic" regarding measurement of speaker impedance.

Although the reader's question was answered, it is important to point out that the technique presented discusses only the magnitude of the impedance. Equally important to know is the phase of the impedance. The magnitude and the phase together then describe the complex impedance or, equivalently, the "real" and "imaginary" parts of the complex impedance.

One situation where knowledge of the phase will be very important is in determining the amount of power delivered to the load.

Andrew Koranicolas, Cambridge, Mass.

Speaker Protection Problems

Q. My speakers are protected by 2.5-amp fuses capable of handling 350 watts. When I turn up the volume on my receiver about three-quarters of the way, the speaker fuses blow.

Should I use fuses of higher current rating, or must I put up with not being able to use my receiver's full potential?

-Tim Tripp, Osseo, Minn.

A. The amount of power delivered to your loudspeakers depends on the instantaneous loudness of the music being played, the signal level produced by your program source, and, as you say, the position of the volume control knob. There is, however, no relationship between the position of the knob and the absolute amount of power delivered to the speakers.

From what you wrote, I don't know how much power can be supplied by your receiver. Let's assume, for discussion's sake, you've got 400 watts per channel. Under some conditions, full power won't be delivered, even with the volume control turned up fully.

Under other conditions, it's possible that full amplifier power will be delivered to your loudspeaker system with the control set to less than half-scale.

To add to this, we don't know how much power the speakers are protected against by those fuses. Are the fuses of the slow-blow variety? If so, they will handle more power before failing than will fast-blow fuses.

We could apply Ohm's law to all this, but I have found this of little help when determining the amount of protection a given fuse size exerts on a loudspeaker system. This has to do with the complexity of the waveforms involved plus the fact that power demands can be great but perhaps not be sustained.

I certainly would not move to a higher fuse rating until I knew for sure that your speakers are not being driven to their limits with the present fuses. If these fuses were supplied or recommended by the speaker manufacturer, you surely should not increase their current rating.

After all, it is possible that your amplifier can't even provide as much power as your speakers can handle but that, due to overdriving, the amp is clipping. Again, those fuses may be saving you from speaker damage.

Spindle Misalignment in CD Players

Q. I am writing concerning what appears to me to be a problem with several CD players I have examined.

When I removed the top covers, I observed that each player exhibited some degree of rotational inaccuracy similar to a moderately warped and/or off-center LP. The cause of this can easily be seen when examining the spindles. They appear to be misaligned and perhaps a bit sloppy. I asked several technicians about my discovery but received conflicting points of view as to whether or not this is a real problem.

-Robert C. Chase, New Cumberland, Pa.

A. Till now, I have never given any thought to the spindle and its alignment. I examined both of my players and was surprised at the close tolerances in the mechanical assemblies.

One of these players is rather inexpensive; it is, therefore, amazing that a piece of equipment which is so complicated can be so well made.

Any minor rotational errors don't affect the playback of my discs. Tests have been conducted which demonstrate that most players will track properly despite rotational problems caused by the discs or the player.

If an LP is warped, one can hear wow. If the warp is severe, there may be an audible "thump," as the tonearm tends to ride out of the grooves. Because a CD is not analog, its sound quality should not be degraded by small rotational disturbances. When these disturbances are sufficiently great, the output will either quit altogether or "pop in and out." In my experience, at any rate, I can't find any conditions which tend to degrade the audio quality of a CD--however slightly--until the errors are so great that the digital information cannot be recovered.

FM "Bleeding"

Q. I was listening to an album which I am very familiar with, and it seemed distorted. I turned up the volume in an effort to determine the nature and/or cause of this distortion but could not figure it out. In my frustration, I stopped the recording without turning down the volume beforehand, as I usually do. I distinctly heard the FM tuner--the exact station programmed. I switched from LP to both of my tape loops and could still hear the sound, but when I switched to my CD player, I could not hear it.

I have good patch cords from my CD player to the rest of my system. The rest of these patch cords were supplied by the makers of my gear. Because the problem is not present when

I listen to CDs, is it perhaps a good idea for me to replace all of my patch cords? Of course, I can solve the problem by turning off the tuner, but this is not my preferred "way out."

- David C. Samuel, Greensburg, Pa.

A. Your signal "bleeding" won't be solved by purchasing different patch cords. Most preamplifiers have provisions to prevent the very thing we are talking about. They work by placing a short across all inputs not switched into use at any given time. As with any switch contact--and that's all we're dealing with here--oxidation can ruin the effectiveness of the short. Clean the program selector switch with suitable contact cleaner. This simple "cure" may be all that is needed.

The fact that the leakage from your tuner is not heard when you play CDs can be the result of a couple of circumstances. First, it may be that you have your volume control turned lower when playing CDs than you do for other program sources. Obviously, the lower the volume, the less you will hear of the leakage. Second, it may be that the switch contacts associated with your CD player are located further from the tuner's switch contacts than are those of some other program sources. Further, the impedance of the player may be lower than that of your other program sources, thus acting to short out some of this leakage. Finally, it may be that the tuner produces high signal output. It is sometimes hard for some shorting systems to remove all traces of such a high signal level. If the tuner has a level control, turn it down a bit--just far enough to "kill" the leakage but not so far as to force you to turn up the preamp's volume control to an exaggerated setting.

Deterioration of CDs

Q. There has been a lot of talk about "laser rot," the deterioration of picture and sound which occurs on perhaps 10% of videodiscs. It has been stated that this could occur on Compact Discs as well, because CDs are coated with just a thin layer of aluminum.

It is my understanding that the deterioration of videodiscs is caused by a problem in the bonding of the two halves. Because CDs have only one play side and are not made the same way videodiscs are, how can "laser rot" occur?

-Jim Zerr, Glen Burnie, Md.

A. To date, I have not heard of any problems related to the deterioration of CDs for any reason, including the "problem" of the thin coating used to protect the surfaces of these discs. As a result, I don't have an answer as to how such action could take place on Compact Discs.

Audio published the results of some accelerated wear tests in which CDs were submerged in boiling water, left in car trunks during summer days, and more. CD failures involved warping, but no other failures occurred--at least as I understand the test results.

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(Source: Audio magazine, Jan. 1990, JOSEPH GIOVANELLI)

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