AUDIOCLINIC (Nov. 1987)

Attenuation of Sound

Q. What is attenuation, especially regarding sound?

-Glenn McGregor; Freeport, N.Y.

A. In electronics and acoustics, attenuation means a reduction in a signal's strength. The main cause of sound (or acoustic) attenuation is absorption-chiefly by the surfaces and furnishings in a room, but also, slightly, by passage through air.

Where there are no reflective or absorptive surfaces, sound level varies inversely with the square of the distance between the source and the hearer. For example, if you get twice as far from the sound source, you'll hear one-fourth the sound level heard at your original position. In this case, the total sound energy has not diminished much, but it is spread over a wider area.

Getting the Most From an AM Tuner

Q. I am looking for a good AM tuner. Basically, what I want is good sensitivity, selectivity, and frequency response. In most combination AM/FM tuners I have seen, the sensitivity and selectivity of the AM sections are so poor that they barely pick up the local stations. Frequency response has also been poor, rolling off very quickly above 3 kHz. I believe that such a tuner, in addition to good selectivity and sensitivity, should have provisions for connecting an external antenna.

Does such an AM tuner exist? If not, what can I do in order to receive AM as well as possible?

-Eric Nelson, Landisville, Pa.

A. In AM tuners, unfortunately, selectivity and wide frequency response are mutually exclusive. Narrowing the bandwidth of the i.f. stages would improve rejection of unwanted signals, but it would also reduce high-frequency response. With conventional i.f. transformers, the audio roll-off must begin at some relatively low frequency, such as 2 kHz, to ensure adequate selectivity. With ceramic i.f. filters, which have a steeper slope than trans formers, the roll-off frequency can be raised to perhaps 5 kHz and still avoid adjacent-channel interference. Some tuners (even in car stereos) have automatic or manual i.f. bandwidth adjustments, allowing wide-band listening when appropriate and narrow-band reception when necessary.

Please understand that the side-bands of undesired adjacent-channel signals will often extend into the pass-band of the tuner's i.f. filters. If these signals are strong enough, you will hear some odd "chatter" or "splatter" in the background of the station you are listening to. These effects are stronger during evening hours; if your tuner has adjustable bandwidth, it is well to use its narrower setting at night, sacrificing fidelity for the sake of uncluttered sound.

You mentioned sensitivity as a consideration when purchasing an AM tuner. It is really not as much a factor as you might think. The noise heard in the AM broadcast band is determined more by atmospheric and man-made noise than it is by the internal noise of the tuner.

If you live in a quiet area (one free from man-made radio-frequency interference) and you wish as good sensitivity and selectivity as you can find, I suggest you check the features of the various shortwave, general-coverage receivers available. I have seen some which have wide-band and narrow-band i.f. settings, just what you might enjoy. You may not be interested in shortwave, but these receivers are of ten the best overall receivers for AM.

Who knows? You might even enjoy shortwave listening-although that surely must be done with narrow i.f. settings.

Along with the sensitivity and selectivity these receivers provide, they are often very good with regard to another important factor: Image rejection. You mentioned external antennas. When these are used, signals are strong enough that image and other spurious responses definitely become a factor in less well-designed tuners.

Noise from Amps and Preamps

Q. Which component is the source of noise in a music system, the power amplifier or the preamplifier?

-Won seok Lee, Tarrytown, N.Y.

A. Both the amplifier and preamp, like all electronic equipment, produce noise. However, the preamplifier's noise is more likely to be audible from the loudspeakers. In part this is be cause preamplifiers, having more gain, have more noise; in part it is because noise from the preamplifier is further amplified (along with the signal) by the power amp. In any case, the amount of noise produced by the overall system (exclusive of tape hiss or surface noise on phonograph records) should be virtually inaudible at normal volume set tings unless you place your ear against one of the loudspeakers.

Range Expanders and CDs

Q. Does a dynamic range expander offer any enhancement or effect on Compact Discs? Is the dynamic range of CDs of such high magnitude that a sound processor would be superfluous?

-Name withheld

A. I see no value in using a dynamic range expander with Compact Discs unless you know that the dynamics on a given disc are restricted. After all, the CD is capable or reproducing about as wide a dynamic range as one encounters when listening to live music. Any further expansion would not be high fidelity reproduction. Worse, the loudest passages would certainly be heard at ear-damaging volume, unless the softest passages were too low to be perceived.

Noises in Headphones

Q. When I turn on my amplifier, I hear through my headphones a buzzing noise which rises in intensity until it reaches a maximum. Then it drops off, followed by a hissing sound which in creases in volume until a steady level is reached. The hiss continues for the entire time the equipment is on. When I turn off the amplifier, the noise slowly decreases to inaudibility. This noise is heard only with headphones. The position of the input selector is of no consequence here. When inserting the head phones into their jack, I notice (with the amplifier turned off) that there is intermittent static as the plug slides into the jack. Does this mean I have a faulty headphone jack?

-Name withheld

A. You will be glad to know that you have no problem with your headphone jack. There is always some background hiss in electronic equipment.

The hiss may be too low in volume to hear through your loudspeakers. With sensitive phones coupled tightly to your ears, hiss and other noises are often more dramatic than with loud speakers.

There is usually an attenuation net work between the true amplifier output and the headphone jack. By adjusting the values of the components in this network, greater signal attenuation to the headphones is possible, and the hiss will be virtually removed. (I cannot offer any specific guidelines here; you will have to obtain a schematic of your particular equipment before making any changes.) To compensate for the reduced signal, you will notice that you will have to turn up the volume slightly higher than you are used to.

In lieu of these changes, using less sensitive headphones would also help.

It is quite usual to hear various crackling sounds as a pair of phones is plugged in. For whatever the reason, if there is a difference in voltage be tween the sleeve of the phone plug and either the tip or the ring, that difference (however small) will be sufficient to produce sound in the phones. If the noise is really loud, something is wrong. Perhaps the output stage is out of balance or there is a leaky coupling capacitor. If the crackling is very light, forget it.

Imbalance in a New Light

I read with a great deal of interest Arthur Stoddard's question, in the January 1987 "Audioclinic," concerning his channel imbalance problem. I have had the same problem for a number of years, and I believe I have identified a cause that you did not mention. I suggest that he check out two very vital components of his audio system: His own two ears.

About five years ago, I began to notice that I had to adjust the balance control to favor the right channel in order to obtain what was, for me, a balanced image. At first I attributed this to a misaligned balance control on a kit-built preamplifier. When the phenomenon persisted through changes in sound equipment and three changes of listening rooms, I began to suspect that the problem might be an artifact of my hearing rather than an equipment problem. The phenomenon was also present while listening via headphones (which effectively ruled out room acoustics as the cause).

I have not consulted a hearing specialist, but, after experimentation, I have concluded that I have suffered a slight but noticeable hearing loss in my right ear. I am satisfied that this ex plains the channel imbalance problem--at least in my own case.

-Allen R. Meals, Houston, Tex.

Connecting Phones to a Preamp

Q. I bought a preamplifier which lacks a headphone output. Nevertheless, I still wish to listen to FM and other sources via headphones. Is it possible to connect headphones to the output of the preamplifier? I could install a headphone jack and volume control.

My preamplifier has a 2-V output level; my headphones' impedance is 600 ohms.

-Dan Welton, Shelton, Conn.

A. I am uncertain as to whether you can obtain satisfactory sound by connecting headphones to the output of your preamplifier. The preamp could well be designed for a load impedance higher than 600 ohms. Even where the nominal preamplifier output impedance is low (perhaps 100 ohms), the size of the preamp's coupling capacitors could limit the amount of bass sent to the phones. You can try it, of course.

If low frequencies are not as prominent as you know they should be with your phones, you might increase the value of the coupling capacitors.

If you get the desired results from a temporary hookup, you might then consider installing a headphone jack. I do not believe you need to install a separate volume control; the existing control should operate very well.

Distortion Near Record's End

Q. I have been having problems with my turntable. As the end of most records approaches, I hear more and more distortion. I have adjusted tracking force and anti-skating to what appear to be the correct settings. Could these settings be incorrectly calibrated?

-O'Neal Douglas; Ruston, La.

A. It is possible that your anti-skating force has not been set properly, but it seems to me that it would have to be way off the mark to produce the results you have described.

In order to get this force at least close to the correct setting, do the following: Obtain a blank disc from a re cording studio. If this is not practical, use the blank side of an Eva-Tone Soundsheet (one of which was bound into our March 1987 issue). Place it on the turntable, blank side up, and "play" the disc. Adjust the anti-skating control to a position where the tonearm drifts neither inward nor outward, no matter where the arm is placed on the disc. If there is no setting at which drift is absent, set the anti-skating adjustment so that the drift will be outward on some parts of the disc and inward on others.

This will be a good compromise rather than a perfect adjustment, because it does not take dynamic forces such as groove friction into account. Still, if your previous adjustment was nowhere close, you will have better sound re production under these conditions than before.

I believe, however, that the distortion will still be present. Distortion can be the result of a badly worn or damaged stylus or can come from too low a tracking force. You might wish to check tracking force with an accessory force gauge rather than relying on the one included with your table. Of course, if you did not balance the arm initially, such built-in tracking force gauges can give erroneous readings.

I can recall working on a turntable which exhibited the same symptoms as you have described. It was a matter of tracking force, all right, but there was a subtle aspect to it. Many turntables (such as the one I was working on) are equipped with a cueing lever which raises and lowers the tonearm.

This turntable's lifter was just slightly bent. Thus, as the arm approached the inner grooves, the tonearm was lifted slightly-not enough to raise it from the surface, but enough to reduce tracking force to the point where distortion be came serious. To add insult to injury, the lifter also added drag, which would further increase distortion as well as increasing wear on both the stylus and the record-groove walls.

If your tonearm's stylus overhang is adjustable, perhaps this adjustment should be checked. If the error is sufficiently large, inner-groove distortion is possible. Also make sure that the cartridge is straight in the headshell.

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(Source: Audio magazine, Nov. 1987, JOSEPH GIOVANELLI)

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