AUDIOCLINIC (Dec. 1989)

Happy Holidays!

It's that time again! I can't believe another year has gone. (I've seen a number of them do that in the years I have written "Audioclinic.")

I hope you get the right toys this holiday season. I sure have, for my toys drift in over the whole year. With all of it, my main interest is to relax and listen to music. I try to avoid spending most of my time attempting to locate the "best" products. I also try to avoid spending time tweaking things. Doing too much of that spoils the real fun.

I can't help but reflect, too, on the matter of obtaining discs or cassettes just for the sake of sonic quality. Well, I do have some which show off my system. Mostly, however, I have recordings of artists I like, including many who passed on long before the CD, the cassette, or even the LP. I'll never hear these people sound the way they must have during their recording sessions. Still, their interpretations make their work stand out above many of today's sonic wonders.

Yeah, we'll still see if we can get just a bit more from our "goodies." I think, though, that we should resolve to do more listening to the music than to the "sound" of our recordings. It won't hurt to attend live concerts, even if the performances are less than stellar. If nothing else, it will help to support the arts. Also, it will give us the right slant on what music really sounds like.

Malfunctioning Hafler Circuits

Q. My stereo system recently developed a problem that I have been unable to pin down. For a couple of years, I have had a pair of extra speakers hooked up via the Hafler passive surround circuit. When listening to stereo signals, the added ambience was beautiful. Until about two weeks ago, these speakers were silent when I played monophonic sources, as surround speakers should be. Then, with no rewiring and with no other obvious symptoms, the Hafler-wired speakers suddenly began reproducing signals (at reduced level) when only mono, in phase signals were present. When listening to stereo sources, I still believe I am hearing just the stereo, out-of-phase information. Any ideas as to what the cause of this problem may be?

-Larry Craven, Raleigh, N.C.

A. It sounds to me as though one channel of your audio system now has somewhat more gain than the other one. In other words, there is an imbalance between channels, and the speakers therefore "think" that a monophonic signal isn't mono.

You should, if my guess is right, be able to correct this condition by a slight adjustment of the balance control. This is not a cure, but it will demonstrate if my diagnosis is correct.

Next comes the hard part: Isolating the stage which has changed its characteristics and caused the problem. If you have a separate amp and preamp, or a receiver or integrated amp with preamp-out and amp-in jacks, you must first determine whether the imbalance is in the preamplifier or the power amplifier section of your system.

Check the preamplifier by feeding a monophonic source into both channels. Turn up the volume to a level which is suitable for measuring the output voltage of the preamplifier; the output voltage on one channel should be the same as for the opposite channel.

Check other inputs to be sure that the problem does not lie with just one input circuit. If you find an imbalance on your first try, I still suggest that you check other inputs to be sure the problem is constant, regardless of the inputs used.

Check the channel balance between channels of the power amplifier in a similar manner. Be sure not to feed too much signal into the power amp. In the name of thoroughness, disconnect the Hafler circuit. If you have a couple of dummy loads, use these in place of the speakers. If the amplifier shows proper balance and the preamplifier also shows proper channel balance, then it may be that there is a problem with one of your loudspeakers (perhaps in its crossover) which is affecting channel balance.

DynamicRange of CDs

Q. The theoretical dynamic range of a 16-bit digital CD system is said to be about 96 dB. I can't make my math come out to this number. If 2 to the 16th power equals 65,536, then the loudest signal is 65,536 times stronger than the softest, non-zero signal. Because log to the base 10 of 65,536 equals 4.8, the ratio of the loudest to the softest signal (the definition of dynamic range) is 4.8 bels, or 48 decibels. Where does this factor-of-2 discrepancy come from?

-Joseph J. Ferrier; Brooklyn, N.Y.

A. If each bit added to a digital signal doubled the signal's power, you'd be quite correct: Each doubling of signal power represents a gain of 3 dB. However, each additional bit doubles the signal's voltage, which is considered a gain of 6 dB. To express ratios in decibels, one multiplies the log of a power ratio by 10, as shown by your formula, but one multiplies the log of a voltage ratio by 20. Sooner or later, almost everybody mixes this up.

Playback Problems With Two "Identical" CDs

Q. My friend and I purchased identical CDs. My copy plays fine on my player. My friend's copy does not play properly on his machine, which is a different make and model. His player's "Error" indicator flashes constantly.

There comes a point where the playback speed changes, and then the disc plays fine. Repeated attempts to play the disc produce the same results in the same place. My disc plays fine on my machine and on his. His "bad" CD also plays fine on my machine.

Even when "repaired," the bad CD plays poorly on my friend's machine. A quick fix was to swap discs, but it surely doesn't answer the question of how two identical CDs could behave as I have described.

-William Hoehl, Hasbrouck Heights, N.J.

A. Most of the time, I have to think that when a disc plays fine on one machine, it's fine. However, in your case, we probably have a machine which is relatively intolerant of CD defects or perhaps a machine which is simply in need of repair. I have to believe that your original disc was freer of defects than was the second disc. The defective one may be a bit warped, or it may have picked up fingerprints or dust. Any or all of these conditions could affect the performance of a player which is just "hanging in there."

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(Source: Audio magazine, Dec. 1989, JOSEPH GIOVANELLI)

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