AUDIOCLINIC (Nov. 1990)

Ribbon Tweeters

Q. I would like to know the advantages and disadvantages of using ribbon tweeters as opposed to dome or cone tweeters.

-Name withheld.

A. A ribbon tweeter has a tremendously transparent sound quality; I have yet to hear a cone or dome tweeter work as well. I have not checked it, but it seems to me that the ribbon tweeter would tend to beam the highs, whereas the dome tweeter would disperse them.

Ribbon tweeters are expensive to produce, and those that I have seen have not been acoustically efficient.

Thus, if connected as add-ons to existing tweeters, this lack of efficiency could make it difficult or impossible to balance against the other drivers in the system.

Most ribbon tweeters are best at reproducing the higher frequencies.

Thus, if they were used to replace more conventional tweeters in an existing system, the crossover networks would no longer cross over at the correct frequency. In that situation, you would probably have to continue using the existing tweeters, adding a second crossover network at perhaps 8 kHz, where the ribbon would take over. This network would roll the existing tweeters off, avoiding response peaks caused by duplication of frequencies in the vicinity of the crossover point. It would also ensure that the ribbon tweeters would not be forced to handle frequencies too low for them.

Direct Coupling and Loudspeaker Demagnetization

Q. A power amplifier owner's manual I've read mentions several disadvantages of direct-coupled amp designs, saying that any d.c. offset in the amplifier's output will slowly demagnetize the woofer magnets. I noticed that when I switched my speaker selector on, the woofer cones moved forward very slightly, so I measured the voltage across the amplifier's output terminals: It was about 150 mV. Is the manual I've read correct? If so, what will be the long-term effect of this d.c. level on the speakers?

-Bill Prunkl, Glen Burnie, Md.

A. I really doubt that 150 mV will be sufficient to damage the speakers. Also, the effect of d.c. offset depends on its polarity. While one polarity can weaken the woofer's magnets, the opposite polarity will tend to magnetize the woofers rather than demagnetize them.

Some amplifiers have internal adjustments that can be used to null out the offset voltage. Check with your amplifier's manufacturer or a qualified technician to see if your amp's offset can be reduced or eliminated by this or other means.

Videocassette Life

Q. How many times, within reason, can I expect to be able to re-record a videocassette before the tape becomes unusable?

-Stephen Goodwin, Stony Brook, N.Y.

A. There's no definite answer. The number of recording and playback cycles to which a tape can be subjected depends on its quality and on the cleanliness and condition of the VCR's drum and tape path. I have one cassette I've been using for years, and which I have doubtless run at least 100 times. I do find that there are significant dropouts at its start, but if I wind past the first 10 feet of tape, quality is fine again. I suspect that the greater wear at the start is occasioned by cueing before playing and/or copying portions of the tape.

Auto Radio Background Noise

Q. I have a 1984 Corvette. When playing a tape, the system sounds fine. The radio, however, has background static. I have had the radio in for repair twice, tried a new antenna, and have added a signal booster and a filter. The background noise is still there. Less expensive radios in our other cars don't have this problem. Any advice or suggestions you can offer would be gratefully appreciated.

-D. J. Batka, Roswell, Ga.

A. I have very little to go on regarding the background noise you mentioned. Is this sound heard only when listening to AM? Is the sound present regardless of whether the engine is on or off?

Given the information you presented, I have no way of knowing if the problem is in your radio, the car's ignition system, or in the antenna transmission line. You mentioned changing antennas. Did you check the transmission line (the cable that connects the antenna to the radio) to see if it is "open" or shorted? If this cable is damaged, the best antenna in the world will be ineffective. Have you listened to your radio while playing it in the service shop? You would at least know that it worked there.

(Editor's Note: A Corvette's plastic body provides less shielding than a conventional car's metal one, so radio interference is a common problem. Check with other Corvette owners in your area to see which local stereo installers have coped successfully with this situation. -I.B.)

Note:

Whether or not I use a particular letter in "Audioclinic," every letter is answered. This one-on-one correspondence with readers is one of the things that makes Audio the unique publication that it is.

I enjoy answering all of your questions. It is not possible for me to inspect a system while attempting to work through each problem. I suppose you can say that each answer is really a guess. But this isn't "Twenty Questions"--I get only one shot at the answer. To make my guesses more accurate, I need all the help you can give me, right from the start. So when writing, try to give me as much information about your problem as you can. Sometimes the thing you don't think is important is just what I need to answer you correctly. I'd rather that you send me too much information than too little.

It would also be helpful if you would let me know whether or not we can publish your name in connection with your question. In this regard, I never do publish a question without letting the reader know that in advance. If my correspondence doesn't mention using your question, you can assume that it won't appear in print. You can be sure, however, that the fact that I don't select a given question does not mean I take less care when answering you than I do when answering a question that will be published.

-J.G.

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

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