AUDIO QUESTIONS and ANSWERS --Advice on readers' technical problems. LARRY KLEIN
Live Speaker Demo
I was told about a live speaker-comparison test done in a local hi-fi store.
Someone talks into a microphone in another room and the salesman switches between speakers to demonstrate which speakers are more true to the original voice. This seems like a good test to me. How do you feel about it?
V. FICARA
New York, N.Y.
A. British Broadcasting Corporation, for many years used a somewhat similar test for evaluating speakers. They would set up one of their announcers and the speaker system under test out in the middle of a field and compare his live voice against the speaker's reproduction of it. An open field was used to avoid the acoustic effects of an enclosed environment-which brings us to the matter in question.
First of all, for the test to work it is obvious that you must get a "perfect" recording of the "test" voice. But what is "perfect"? Voices are heard slightly differently in different acoustic environments because the actual frequency/energy content of a sound in an in door environment varies with the positions of both the source and the listener. Because of such room effects, the test you describe has limited validity below 400 Hz or so in discriminating among reasonably good speakers.
In addition, a voice, no matter how basso profundo, is not going to be able to disclose anything about a speaker's very low-bass response-or lack of it. Nor will a coloratura soprano hitting her highest note reveal much about a speaker's ability to deliver the "shimmer" of a wire-brushed cymbal. In short, the live-voice test will probably enable you to separate the good from the absolutely rotten among loudspeakers, but I don't think it is definitive enough to enable you to make any finer distinctions.
Orphan Turntable?
Q. The manufacturers of my turntable ceased all advertising shortly after I bought it. The unit in question is a PE 4060, tested in the October 1973 issue of your magazine, which I used as a reference in buying it in late 1975. So what has happened to PE/ Impro?
Did they go out of business or some such disaster? The unit is working fine now, but what will happen when it needs parts or servicing? I really don't want to be told I have an "orphan." I went through enough trouble with my previous "orphan"--a Studebaker.
GARY STEPHENSON; Bellingham, Wash.
A. Fear not. The PE factories were bought (adopted?) by Dual and their production facilities are now turning out new Dual machines. The Dual warranty stations are fully equipped with parts and know-how to handle any problems with PE equipment.
"Defective Discs" Defined
Q. I would appreciate a technical or professional definition as to what constitutes a defective record. As a record purchaser I have been getting many discs that I consider defective.
STEVEN AMATO; Portland, Ore.
A. Amato, your question has invited me into an area where even courageous angels tread very lightly, but here goes. As a starting point, the Recording Industry Association of America (RIAA) has dimensional standards (detailed in Bulletin No. E4) that establish the physical parameters of about a dozen factors ranging from the diameter of the disc to the "contour" of the recorded groove. There is a range of tolerances given for all the parameters, and one might imagine that if a particular disc exceeds this range it can be considered defective. However, the RIAA states that the standards are "to facilitate equipment design and assure interchange ability. They are not intended to indicate or imply quality or performance levels." Since it seems we can't get a professional ruling, let's approach the question pragmatically. Would it be valid to say that any disc that won't perform mechanically on your record player is defective? (By "perform mechanically" I mean the stylus is able to stay in the groove, the disc's run-out groove trips the changer mechanism at the right time, the disc drops properly, and so forth.) There's no easy answer here either. Depending on the nature of the problem, a very cheap brute-force player might play a given record successfully, while an expensive light-touch player might not. This sort of thing can occur, for example, when the resonance produced by the tonearm mass interacting with the cartridge compliance is at a frequency close to the record's warp frequency.
So it seems that discs can be "defective" on some players but not on others. This is why attempts to return "defective" discs frequently meet with a less than fully cooperative attitude from the seller. The record retailer really has no way of telling whether the complaint--whatever it is-has its source in the record. The problem may be in the record player, or it may arise from an unfortunate interaction of the two. For the dealer, the fact that the record will play successfully on his store machine is usually sufficient evidence to categorize the complaining customer as some kind of a crazy.
Up to now we've been talking about gross defects--groove jumping; sticking, and the like. How about the occasional tick or pop caused by surface dirt or bubbles in the vinyl? Some listeners are apparently willing to endure a disc's sounding like hail on a cold tin roof; others become enraged if a tick or a pop is heard at, say, 15-minute intervals. To the latter group, I can only suggest that they convert to a tape system, for they will otherwise find no happiness in this world.
Unfortunately, I don't have much else to offer. A record cannot be labeled defective as far as the record companies are concerned except when judged in the context of the rest of the products in the record bins. If it is substantially noisier, more warped, more difficult to track, and so forth, than others of its ilk, I would say the disc is defective. However, I would hate to have to argue the case before a jury consisting solely of record manufacturers and dealers.
Impedance Matching
Q. I wish to precisely match all the impedances (output to input, etc.) in my system to reduce the total output distortion. My system is made up of a variety of makes and models and I believe I am having difficulties.
Are there any products available which can be used for impedance matching?
RHEAL DUMOULIN; Montreal, Ontario, Can.
A. Impedance matching is one of the least understood areas in hi-fi, and people with some training in electrical engineering are frequently more confused about the question than innocent audiophiles. In general, the impedance of two connected devices (output to input) should be matched when the object is to insure maximum transfer of energy. An example of such a need and match would be an FM or TV antenna with a "balun" matching device that converts the antenna's 300-ohm impedance into the 75 ohms required by the coaxial down-lead. At the receiver, another impedance-matching balun reconverts the 75 ohms back into the 300 ohms required by the tuner's antenna-input circuits. Some tuners have internal 75-ohm matching arrangements.
With most audio components, precise output-input-output matches are seldom required, nor are they even desirable. The general rule is that the impedance of the signal source should be low and-the impedance of equipment being driven or fed by the source should be at least ten times that of the source impedance. The object of this apparently gross mismatch is to prevent the equipment being fed from loading down (overloading) the equipment serving as the source. Practical example: the output impedance of a preamplifier should be very low. This will in sure that for complicated electronic reasons the shielded cables plugged into its output jacks will resist hum pickup and won't cause a loss of the higher frequencies. (Both these problems can occur when a high-impedance device-such as a microphone or another component-is feeding a long shielded cable.) Sometimes there are difficulties when the source impedance is not quite low enough at all frequencies (impedance measurements are sometimes made only at a mid frequency) and the relatively low impedance of a load affects performance at the frequencies where the impedance of the source rises. As an example, consider a tube preamplifier (or even some solid-state units) whose output impedance isn't quite low enough to feed a power amplifier with a 10,000-ohm impedance. The result could be an audible rolloff of the frequencies below 50 Hz or so. The same problem could occur when an FM tuner is feeding a preamplifier .whose tuner-input jacks have too low an input impedance.
Phono cartridges have very special impedance-matching requirements. For complicated electrical reasons, the "match" of most magnetic cartridges to their preamplifier's in put impedance is critical and directly influences audible performance in the middle- and high-frequency areas. Although manufacturers have agreed on 50,000 ohms as the proper load to be seen by each cartridge channel for a flat frequency response (regardless of the cartridge's internal impedance), things don't always work out as well as hoped. Not only do the capacitances of the phono leads and preamplifier inputs enter into the circuit, but phono preamp circuits also frequently interact with the coil inductance of the cartridge to produce unpredictable high-frequency response aberrations. In this connection, I suspect that many of the disagreements among audiophiles as to the inherent "openness," transient response, clarity, etc. of various cartridges and preamplifiers, either individually or in .combination, result from random factors traceable to the interrelationships between the two units, rather than being a function of either one singly.
Black Discs
Q. I understand that the color of vinyl is not necessarily black. So why are records always black?
JAMES FRAISER; New York, N.Y.
A. They aren't always. However, as far as I can determine, records are usually black for two reasons: (1) cosmetics, (2) tradition. The carbon black is actually only one of the several additives that are mixed with the vinyl; others are used to aid the disc-pressing or disc-playing process. The fine carbon powder has no lubrication or antistatic properties but is simply there to obscure the bubbles or other visual (but not audible) "defects" be low the grooved surface. And don't bother to ask what other materials are added to the raw vinyl-manufacturers are usually quite secretive about such details.
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Also see:
TAPE TALK--Theoretical and practical tape problems solved. CRAIG STARK