AUDIOCLINIC (Dec. 1981)

Tried and True Test Records

Q. I have read different statements to the effect that various test records differ by perhaps a few decibels from what is considered to be really "flat" RIAA frequency response. This situation seems odd to me. If it is also true that there is no real agreement as to which is the more accurate when differences do occur, this seems almost unbelievable.

Should it not be a fairly straightforward procedure for a lab equipped with a scanning electron microscope to mea sure the groove amplitudes at various frequencies (at least laterally cut discrete frequency bands), and thus determine the accuracy of amplitude vs. frequency?

-Jack W. Brent, Seattle, Wash.

A. Good test records should not disagree. I have personally checked two and found them to be in very close agreement, probably within 0.5 dB. These are the CBS Technology STR-120 and the NAB test record. The first is available from CBS Special Services, 3651 Fruitridge Ave., Terre Haute, Ind. 48705, and the second from Gotham Audio, 741 Washington St., New York, N.Y. 10014.

While it is true that a laboratory could use an electron microscope to measure groove amplitudes as you have suggested, correlating them to record diameter vs. wavelength, such measurements are routinely made by specially prepared light sources. Means are provided to accurately measure the reflected light, and this arrangement has proven to be very accurate.

When you delve deeply into this, you will discover that there are inaccuracies far more significant than the possible disagreement between good test records. You will find that, on different days, using the same test record and the same phono cartridge, your results will differ with each test run made. This has to do with temperature vs. compliance of both the vinyl material of which the disc is composed, and that of the cartridge.

When I want to make a frequency run on my cutting system and really want to ensure accuracy, I must check the play back cartridge with a relatively new, fresh test record beforehand in order to monitor performance. The deviation from flat response must be taken into ac count when comparing my results to those obtained when cutting my own test frequencies.

Dim Watted

Q. On the tuner section of my receiver, the lights at one end of the dial are darker than the rest. Will this affect performance?

-Richard Pizzo, Canterbury, Conn.

A. Dim lights are not likely to affect performance. This dimness may be the result of some basic problem associated with the basic layout of the dial and/or placement of the individual lamps. It is possible, too, that one of the lamps is defective, resulting in this condition.

Bad Vibrations

Q. If I tap my finger on either the base or turntable platter when playing re cords, the result of this action can be heard through the speakers. This tap ping can be heard only when the stylus is on the record: Is this normal?

- Jeff Culbertson, Overland Park, Kans.

A. What you are experiencing is closely related to acoustic feedback, except that you are stimulating the phono cartridge through the base br platter by hand rather than by means of the sound output of the speakers. This is normal, in that it is what is supposed to happen, though it is not particularly desirable.

When you tap the base or platter of the turntable, this causes the record to vibrate, which in turn causes the phono cartridge's stylus to move. It is this stylus motion which produces the sound, de sired or undesired.

Further Notes on FM Distant-Signal Reception:

In the July, 1977, installment of this column, you answered my question about "FM Antennas for Reception of Distant Signals." I have additional information on this same subject.

One does not often come across reviews of FM antennas, but here are two references: Consumer Reports, July, 1973, pp. 465-469; Consumer Guide: Stereo Series, Summer, 1974, pp. 6 17.

An organization called the World-Wide TV-FM DX Association (P.O. Box 163, Deerfield, III. 60015) publishes the VHF-UHF Digest, which is a periodical dealing with many aspects of DX; FM Atlas and Station Directory is helpful in locating and identifying FM stations but most important, a little booklet called Beyond Shortwave is very easy to read and would probably interest other Audio readers. The association also gave me a list of their members who probably would be willing to help someone in their area.

The TV-FM DX Association shares your distrust of signal amplifiers but mentions that some DXers build low-noise, tunable, selective amplifiers with high-quality electronic components and obtain excellent results.

The most complete reply from a commercial source was from the Winegard Company ( 3000 Kirkwood St., Burling ton, Iowa 52601) which offered specific recommendations for employing their consumer products in this type of application.

A local MATV/CCTV/video company (6NS Telesystems, Shelburn, Vt.) has lots of information on commercial-grade, low-noise amplifiers and specially cut antennas. They were very willing to offer helpful data and suggestions, although much of this equipment is very expensive.

-Ron McKinnon, Middlebury, Vt.

Even where a signal amplifier exhibits a good S/N ratio, one must take into account the presence of local signals.

These signals will be amplified along with the weaker ones you desire and could overload the amplifier, but if this does not occur, they could be capable of overloading the front-end of the tuner. In either instance, an overload will de crease the sensitivity of the equipment to weak signals; further, it will make the front-end nonlinear. Nonlinearity in r.f. circuits will act the same as nonlinearity in audio circuits: Intermodulation products will be created, taking the form of spurious signals at dial settings on which such signals should not appear. They are capable of blocking out desired signals which should be received at such dial settings.

Tonearmaments

A. After getting a new turntable I have had a persistent problem. When the tonearm is moved over the platter with the turntable running, regardless of whether or not a record is actually playing, I pick up a "popping" and "ticking" noise much like static. If you know of a solution for this problem through shielding, filtering, or any other method, please let me know.

-Eron Lee; Tempe, Ariz.

A. I think the lack of grounding of your tonearm is the problem. Often such grounding is carried through by way of the various pivots. If there is any oxidation or "slop" which can lead to poor electrical connections, grounding will be come intermittent.

It may be that you can relieve this condition by lubricating these pivots with WD40 or a lubricant recommended by the maker of the tonearm.

If this does not cure the problem, I suggest running a flexible wire from the tonearm proper back to a ground point on the turntable base.

There is also the choice of returning the arm to its maker for the appropriate repairs.

Bass Brawl

Q. It is my understanding that sound is the result of air set in motion, and the lower the note to be reproduced, the more air that is needed to be set in motion. If this is true, does it follow that the larger the speaker, the lower the frequency of a note it can produce? Is there more involved than size in determining how low a frequency a speaker can pro duce? Specifically, can two 8-inch woofers move more air (and hence, produce lower notes) than one 12-inch woofer?

-John S. Vizard, West Hartford, Conn.

A. In one way or another, if we are to produce lower and lower frequencies, we must move more and more air if all of these frequencies are to have equal measured or perceived amplitude. We can move that air via loudspeakers in two ways and still have the same amount of bass: By moving a small cone over a great distance or by moving a larger cone over a proportionately smaller distance.

It is certainly true that given an equal distance of cone excursion, the speaker having the larger of the two areas will move the most air. Inasmuch as the two 8-inch speakers have a total combined area greater than one 12-inch speaker, the combination of two 8-inch speakers will move more air than does the 12-inch speaker

Some speakers are designed to have greater cone travel than others. Hence, this fact must be considered when comparing the possible amount of bass response that a given speaker can produce. In addition, you must consider the effects of the enclosure in which the speakers are housed. You can have a speaker of a given size which has a rather small amount of cone travel and produces bass equal to that of a speaker having a large amount of cone travel if the proper enclosure is used, one which makes use of otherwise wasted back-waves.

Speaker design is very exacting and complicated. Even with this brief explanation, you can appreciate the many interrelated problems which must be taken into account.

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(Audio magazine, Dec. 1981, JOSEPH GIOVANELLI)

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