Audioclinic (Q and A) (Mar. 1970)

by JOSEPH GIOVANELLI

Product Selection

Q. We are thinking seriously of investing in stereo components consisting of receiver, tape deck with speakers to enable its use as a separate portable unit, turntable and two stereo speakers for the moment. Ultimately, we would like to extend the system to at least two more rooms in the house. For this reason, we would like enough power to operate six speakers. Although I presume that most of the time only two of these speakers would operate at once, probably some of the time four would operate. Only on rare occasions would six speakers be operated simultaneously.

I must admit that both my wife and I are confused by the vast array of equipment available. We feel that we should seek the advice of someone who has no equipment to sell and could thereby give us an unbiased opinion. Would you be kind enough to recommend the best combination of components by brand names and model numbers?

-Ernest A. Bottke, Jr.; Howell, Mich.

A. I regret to say that I do not deal with product selection and evaluation.

This is a subjective matter. The ear is a peculiar device. What I like and would, therefore, tend to recommend can well be something which you do not like at all. Therefore, you would not think much of me after that. Therefore, I do not and will not get into the area of product selection.

When looking for a product, check the features it offers because some components may offer features which you may never use.

When it finally comes down to it, there is only one way to judge the performance of a product, especially speaker systems.

You must listen to the equipment. This can take some doing if you are not near a good high fidelity showroom which features the various pieces of gear you are considering.

The kinds of questions I answer are those of a more general nature, i.e., your question about connecting six speakers to one amplifier.

You need to buy 16-ohm speakers. Remember that when all six speakers are operated simultaneously, you will have three speakers connected in parallel across each channel. This will decrease the combined speaker impedance to around five ohms, which is still within the safe impedance-tolerance limits of your amplifier, whatever brand it may be.

If the speakers you choose had impedances lower than 16 ohms, three speakers in parallel would produce an impedance which is lower than 4 ohms.

Four ohms is the impedance below which it is unsafe to operate most amplifiers. The result of this is likely to be damage to the output stage of the amplifier.

It is not easy to make a selection from among a welter of equipment, but at least your choice of speakers is now narrowed to those having impedances of 16 ohms.

As for your selection of an amplifier, make sure that it will deliver at least 50 rms watts per channel. This is the only way to be reasonably sure that you will have sufficient power fed to all six speakers, even when really loud listening is contemplated.

Test Equipment for Servicing

Q. I have a shop in which I have been performing repairs on black and white TV sets, radios, tape recorders, and so on. I have not, however, taken on the high-quality audio equipment.

If you are at liberty to do so, I would appreciate your recommendations regarding a practical array of test equipment which can be used to service high-quality audio equipment.

My thinking is that I would use a triggered sweep 'scope and a good sine/ square-wave generator in lieu of exotic distortion analyzers.

-Carl C. O'Neal, Bellevue, Nebraska.

A. Before buying test equipment you must first be sure you have an understanding of general electronics and an understanding of what goes on under the cover of the various pieces of audio gear.

I know you already have basic knowledge because you are successfully doing various kinds of service work. I made the previous statement, however, for the sake of completeness. There is no substitute for knowledge. A piece of test equipment can disclose a number of things, but the "sharp" technician can often find them without using a single piece of equipment, just because he recognizes certain problems and uses common sense in terms of how to localize the area in the equipment in which the trouble lies.

As for test gear, a good VTVM is virtually a necessity. You are right about the possibility of using a square- and sine-wave generator. It is always a handy source of signal and an aid in checking frequency response and square-wave performance.

I do not believe that a 'scope will show distortion as well as will a good distortion analyzer. You do not have to be elaborate with the kind of distortion measuring gear you buy either. Good results can be obtained at very modest prices.

I suggest that you consider both a harmonic and an intermodulation distortion analyzer.

Of course, the 'scope does help in identifying the character of square-waves, but it is useful in aligning FM tuners and it also can be used as a voltmeter.

If you plan to repair stereo tuners and receivers, you will need a stereo generator. There are some which are comparatively inexpensive. While they are quite useful, they are not as accurate as a really good one, made for topnotch performance.

In addition to this specialized piece of equipment, you will need a marker and sweep generator to perform basic r.f. alignment. You probably already own this gear, used in connection with your television service work. You should have a capacitance checker and a capacitance bridge.

In your tool collection you doubtless already have various soldering aids which are useful for removing components from printed circuit boards. Various types of pliers, too, are useful, such as diagonal cutters, long-nose, and right-angle long nose. Fingertip wrenches, socket wrenches, and Allen and Bristo wrenches also come in handy from time to time.

A little while ago I mentioned the necessity for a good VTVM. Typically this piece of gear incorporates an ohmmeter together with a.c. and d.c. voltage capabilities. However, such a meter usually has a low-voltage scale of 1.5 volts full scale. This is not sufficient for all applications. You also should have an a.f. VTVM. This meter reads audio frequencies down to perhaps 10 millivolts full scale or in some cases to even 1 millivolt full scale. This is useful in measuring low signal levels which are encountered in the early stages of audio equipment, especially solid-state gear. This meter must have a wide frequency response, for it is likely to be called on to do such jobs as enable you to set prescribed bias levels on cassette decks, make frequency response checks of low level stages, and so on. When you have the foregoing test equipment, your shop will be adequately equipped.

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If you have a problem or question on audio, write to Mr. Joseph Giovanelli at AUDIO, 134 North Thirteenth Street, Philadelphia, Pa. 19107. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Audio magazine, Mar. 1970)

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