Advice on readers' technical problems.
By LARRY KLEIN, Technical Editor
15-ips Dolby Decks
Q. Is there some technical reason . why I can't find a 15-ips open-reel tape machine with built-in Dolby circuits?
FLOYDE SARASOHN; Queens, N.Y.
A. The "technical reason" was--until recently--that Dolby Laboratories simply refused to allow their licensees to put Dolby-B circuits into large-reel, 15-ips machines. Dolby had two reasons for this: (1) They wished to avoid potential confusion between the professional A-type four-band Dolby system and the consumer-oriented single-band B-type system. (In their view, a 15-ips, 10 1/2-inch machine could be considered a professional rather than a consumer product.) (2) Since the B-type sys tem reduces hiss only, it is not as useful for fast, wide-track professional machines because they are relatively less troubled by high-frequency noise. The four-band Dolby A system provides noise reduction over most of the audio band and hence can cope with low-frequency noise also. In any case, the Dolby people have now relaxed their stand and are willing to allow Dolby B to be used in 10 1/2-inch-reel, 15-ips machines as long as they are clearly consumer-oriented- whatever that means. However, a separate Dolby adapter can be plugged into any machine you like, and this seems to make more sense than buying a deck with built-in Dolby circuits.
Speaker Fusing
Q. I'm about to buy a set of speakers and I'm interested in fusing them, having read a review that suggests such a move. Can you tell me how to go about this? In addition, I also intend to buy an amplifier that has meters calibrating power output and decibels. Just what do decibels indicate in this in stance, and how do I interpret them in terms of output power per channel?
DAN CURRY; Normal, Ill.
A. To respond to your first question, first, it is difficult to select the proper fuse for a speaker system without extensive trial-and-error testing. The problem is this: in a two- or three-way speaker system, one of the drivers may be more susceptible to overdrive damage than the others. Therefore, the current rating of the fuse should be chosen so as to protect the "weakest" unit. To com plicate matters, the impedance of a speaker changes somewhat with frequency, and it is the speaker's impedance that determines the current flow from the amplifier for a given audio signal level. In other words, the amount of current flowing through the various drivers varies with the frequency of the signal. Without knowing what amount of current is "safe" for each driver through out its frequency range, I don't have enough information to help you select the proper fuse. But the manufacturer of your speakers should be able to.
In regard to the significance of the meter readings on a power amplifier, there appears to be no standard. The meaning of the meter reading depends only on what the manufacturer has set it to be, and there's no fixed relationship between the decibel reading on the meter and the amplifier's moment-to-moment wattage output.
Hi-Fi from Scratch
Q. I am a ham radio operator and lately I've become interested in stereo. I have built all sorts of ham equipment "from scratch" and would like to know where I could find plans for tuners and amplifiers that I could build. I'm not talking about commercial kits.
STEPHEN JENKINS; Tampa, Fla.
A. Our sister publication Popular Electronics regularly publishes build-it-yourself projects in the hi-fi area.
Such projects are also found in that magazine's Electronic Experimenter's Hand book. Back issues of both are available from Ziff-Davis Service Division, 595 Broadway, New York, N.Y. 10012.
In addition, there's a publication called Audio Amateur that is directed toward people such as yourself for whom a kit does not represent enough of a challenge. Write to Audio Amateur at P.O. Box 30, Swarthmore, Pa. 19081 for further information. A word to the beginner: unless you have some practical experience in electronics plus the test equipment needed to be able to check out your projects, I suggest that you are always better off building a kit rather than a complex project from scratch.
Energy Crisis
Q .I think that it is absurd, considering the energy crisis, that you people continue to advocate the use of high-power amplifiers. Have you no conscience?
RONALD KRAUSNER, Los Angeles, Calif.
A. I hope Mr. Krausner will be able to contain his indignation long enough to look at a few facts. The high-
power amplifiers, by which I assume is meant 100 watts per channel continuous and above, almost all operate in what is known technically as Class-B condition.
This means that when there is no signal going through the amplifier, very little a.c. line current is being drawn. Conversely, the louder the amplifier plays, the more current it is pulling from the a.c. wall outlet.
With no signal being fed to it, the aver age high-power amplifier draws about 30 or 40 watts of a.c. power, which is not a great deal more than a night light. Only on signal peaks will the amplifier come close to drawing its full power potential, but loud signal peaks are of very short duration-usually much less than a second. Since it is the average signal level that over any given time period is going to determine the amount of line current drawn, the total current consumption of a 40-watt-per-channel amplifier is not a great deal lower (in some designs, it might actually be higher!) than the cur rent drawn by a 140-watt-per-channel amplifier. The reason that some people want "super" power is to be able to re produce those brief peaks without incur ring momentary "clipping" (overload) distortion. I suspect that the electric typewriter that was evidently used to compose Mr. Krausner's letter consumed as much current during the time it was switched on as a high-power amplifier would have if played at reason able levels for the same time period.
Because the number of questions we receive each month is greater than we can reply to individually, only those letters selected for use in this column can be answered. Sorry!
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