AUDIOCLINIC (Jan. 1988)

Dust and Humidifiers

Q. I have an ultrasonic humidifier which deposits a coating of fine, white dust onto my stereo and TV set. I understand that this dust is from minerals, primarily calcium carbonate, found in hard water. Will this' dust damage my electronic equipment? How can I eliminate it?

-Jose E. Gonzalez, San Juan, P.R.

A. There is no way that dust settling onto equipment can do it any good. On heat-sinks, dust impairs heat transfer.

On contacts, it makes controls erratic and noisy. If it gets into bearings, wear can become significant.

The simplest scheme is to cover the equipment with plastic (or other material) when it is not in use. A somewhat more elaborate plan would be to filter the mineral solids from the water before you put it into your humidifier's tank. Many simple filters are available; they attach to the faucet from which you fill the tank. There are also more complex water-treatment systems using chemicals as well as filtration.

These units must be interposed be tween the water line and the faucet, which involves some plumbing work.

Alternatively, you could use distilled water. Considering that a humidifier uses approximately two gallons of water per day, this will not be an inexpensive solution. Of course, you can buy quite a bit of distilled water for the price of an elaborate water-treatment system.

Be sure to follow directions as to how best to clean all of your humidifier's internal surfaces which come into contact with the water. If those surfaces are free of mineral deposits and other dirt, they can't add to the dust being passed into the air. Cleanliness will also help keep the humidifier producing moisture at peak efficiency.

CDs from Older Recordings

Q. I like music which was originally recorded in the 1960s and early '70s.

A great deal of this music is being issued on CDs. Would it be worthwhile for me to obtain the CDs of these performances? Is the sound going to be better than on records and cassettes?

-Mike Haller, Lincoln, Nebr.

A. Phonograph records will have more background noise than was pre sent on the original master tape, especially if the records have been played many times. Cassettes will also contain more noise than the original master tapes. True, master tapes from 15 to 20 years ago will have signal-to-noise ratios poorer than what a CD is capable of reproducing. Still, on CD you will be able to hear your music free from much of the noise you have become accustomed to hearing.

Add to this the fact that cassettes can jam or fail in other ways, and that LPs can be scratched, and you can see that the CD has another advantage too: It will last virtually forever if properly cared for. What's more, CDs are certainly easier to store than phonograph records.

Hi-Fi VCRs and Noise Reduction

Q. Why don't Hi-Fi VCRs have either Dolby or dbx noise reduction? What is the "hi-fi" all about? Is this just a delayed channel?

-Darrell C. Fong, Franklin Park, Ill.

A. Actually, the Dolby system is used with some Hi-Fi VCRs. It is, how ever, not used on the Hi-Fi tracks; if it is present, it is used on the standard, longitudinal tracks, and probably just on those few Hi-Fi VCRs whose longitudinal tracks are in stereo. This is be cause noise is more of a factor with these tracks in stereo than in mono; in stereo, the available track width must be shared by two channels rather than devoted to one monophonic track. (I don't know why the Dolby NR system was chosen rather than dbx.) The Hi-Fi channels do not need additional noise reduction because their scheme of en coding/decoding FM subcarriers results in a very good signal-to-noise ratio, perhaps a figure as great as 80 to 85 dB.

The Hi-Fi system is not just a gimmick, nor does it use delay. Using de lay to create reverb can be effective for some applications, but it is certainly not a means of obtaining the faithful reproduction that is implied by the term "hi-fi."

Loudspeaker Equalizers

Q. I have read that loudspeaker makers sometimes use equalizers as a part of their designs. It is my under standing that equalizers are nothing more than boosters, so why are they used?

-Andy Johnson; Chicago, III.

A. Equalizers are definitely used by some speaker manufacturers. Al though many loudspeakers can cover a wide range of audio frequencies, they may not be "flat" over that range.

Thus, the speaker designer makes up an equalizer circuit whose function is to boost or cut certain frequencies to make that loudspeaker produce a flat response over as wide a range as possible. This equalizer is usually pack aged as a separate chassis, which is connected either in the system's tape loop or between the preamplifier's out put and the power amplifier's input. For reasons of electrical efficiency, it is better to provide the compensation in this way rather than to include it within the loudspeaker as a part of its cross over network.

The equalizer is more than a booster. As stated, it may lower some frequencies while boosting others. It does, however, try to make all frequencies coming from the speaker equal; presumably, this is why it is known as an equalizer.

You are doubtless familiar with tone controls and perhaps with graphic equalizers. In either type of circuit, the user can adjust the sound to suit his taste. The equalizers used by loud speaker makers cannot be adjusted. If the listener needs an adjustable equalizer, it can readily be added to a system, even when the speaker system's own equalizer is present.

Escaping Bass

Q. I have a problem of bass location in my system. Specifically, the low bass (up to perhaps 55 Hz) does not "come out" in the room where the speakers are. Instead, it appears in the hallway, outside my neighbor's door.

The problem does not appear to be one of poor bass response; there is enough response at 30 Hz to rattle the light fixtures. This phenomenon is quite irritating--I did not build my loud speakers to entertain the roaches in the walls!

Standing outside my neighbor's door, drums and other bass signals

are very physical. I would really appreciate any information about getting the bass into my room, where it belongs.

Structural changes are impossible be cause the walls are concrete.

-Steve Geist; DeKalb, Ill.

A. Your bass problem has to do with what are known as standing waves.

Without going into detail, this has to do with the wavelength of the sound being reproduced. The wavelengths of bass tones are often larger than the room in which the loudspeakers are housed. In addition, there are resonances in corridors much like those of organ pipes.

Should the hallway have a resonant frequency equal to that of the bass frequency being produced by your system, this sound will be very pronounced in the hall.

Probably your best bet is to close the door of your listening room and then walk around the room while listening to your system. You will probably find that the bass peaks when you stand at a particular point in the room.

Perhaps you can locate your loud speakers in such a way that you can then place your favorite listening chair at that point where the bass peaks and where you can obtain good stereo imaging.

Recommending an Isolation Transformer

In response to a reader's inquiry about TV hum (June 1987), you mentioned the requirements for an r.f. isolation transformer. I have experienced a similar problem myself when interfacing cable TV to professional audio equipment in recording studios. In those installations, great care was taken to assure a clean, isolated ground, which was then rendered useless upon connection of cable TV.

There is an r.f. transformer available from Gemini Industries, 215 Entin Rd., Clinton, N.J. 07014. (Fortunately, this transformer is stocked at my local hardware store, but I have never seen it anywhere else-including cable-TV suppliers.) This device provides galvanic isolation to break ground loops, and it also provides a degree of fault-current protection from lightning and other disasters. Considering the in vestment most of us have in our gear, ground-fault isolation is certainly worth while.

-Wayne Kirkwood, Dallas, Tex.

Distortion and Overheating

Q. My receiver puts out 40 watts of power per channel, and my loud speakers are rated at 40 watts each.

When I originally set up the system, I could only turn up the volume control to "4" (on a scale from 0 to 10) before a harsh, pounding sound could be heard. The receiver also overheated very quickly. Recently I moved the stereo to a different location. In doing so, I had to rewire everything. When I turned the system on, none of the previous problems occurred; I could turn up the volume without distortion or overheating. To make room for additional gear, I had to move the system once again, which meant rewiring loudspeaker cables and the various other cables and ground wires. Now my system is as first described: Lots of distortion and overheating. Figuring that the problem had to do with loudspeaker wiring, I attached, detached, and reattached all wires again, but the problems remained. What is going on here?

-Steve Vandenberg, Kentwood, Mich.

A. The most likely cause of your problem is a short between speaker terminals, either on the loudspeakers themselves or on the amplifier. Perhaps a single loose strand of wire has "bridged" the terminals. Disconnect the wires once again, checking to see that no fine wire strands remain to short out the terminals. Examine your cables to make sure that all strands are tightly twisted-no stragglers, please-and then reattach the wires.

You also must be sure to keep phonograph cables well clear of the loudspeaker cables. In fact, avoid running any signals parallel to the loudspeaker cables. The reason for avoiding cable proximity is that, despite the shielding of the low-level audio signal lines, they still have a tendency to pick up magnetic fields from the speaker cables.

These signals will produce oscillations, which could be the cause of both your problems.

Defining Distortions

Q. What is the difference between THD and IM?

-Thomas Wayne Caldwell, FPO N.Y.

A. To gain an understanding of total harmonic distortion (THD), assume that we are feeding a pure sine-wave signal into the input of the circuit which is being tested. One would hope that this sine wave would be reproduced in the output of this circuit exactly as it appeared at the input, except, per haps, for a change in amplitude. In practice, however, it won't be an exact copy; the wave shape will change slightly. A departure from a true sine wave indicates the presence of harmonics--even multiples--of the original sine wave's frequency. If the original frequency is 250 Hz, the first harmonic is 250 Hz (because the first harmonic is always the starting frequency). The second harmonic is 500 Hz, the third harmonic is 750 Hz, etc.

These harmonics will tend to decrease in amplitude as the harmonic number increases. In some circuits, the odd harmonics will be emphasized more than the even ones.

We talk about "total harmonic distortion" because our usual measurements don't discriminate as to the nature of these harmonics. Rather, the measurement simply expresses all of the harmonics of a given frequency in terms of a percentage of the fundamental (the starting frequency, or first harmonic).

Intermodulation (IM) distortion is completely different. Here we are talking about what takes place when two different frequencies are simultaneously fed into the input of a circuit. What we expect to see in the output of this circuit are these same two frequencies. But in addition to the original two frequencies, two "beat" frequencies are generated, one equal to the sum of the original two frequencies and the other equal to the difference between them. We now have four frequencies which can recombine to form all kinds of beats. These additional frequencies will be much lower in amplitude than the first two beat frequencies. In fact, even the first two will have a much lower amplitude than the two input frequencies with which we started.

When we measure IM distortion, we extract all of the added frequencies and express them as a percentage of the two frequencies with which we started.

As with THD, the percentage of IM distortion will be different for different levels of signal input to the circuit being tested.

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

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