By LARRY KLEIN, Technical Editor
Additive S/N
What is the combined ultimate signal-to noise ratio to be expected from a power amplifier that has a 95-dB S/N and a preamplifier with an 80-dB S/N?
RALPH RISELEY; Essex, Ontario
A. The answer to Mr. Riseley's question involves, as we shall see, unexpected complexities. As a simple starting point, how ever, it is safe to say that if one component in the system has a noise figure 6 to 10 dB worse than the others, it will determine the noise figure of the system. In the case cited, the 80-dB S/N of the preamplifier would be the over all noise of the system.
Now for the ifs, ands. and buts. When published specifications are used as the source of the S/N figures, you can be sure that each component is using a different reference level.
Suppose the power amplifier has a 95-dB S/N referred to its full-power output of 100 watts, which is achieved with a 1-volt input signal.
Now, say the preamplifier has a rated maximum output of 6 volts. (Some of the better ones do use rather high output-reference levels, both because the S/N figures become more impressive, and because the preamplifier can actually deliver that much output with low distortion.) However, if the preamplifier's 80-dB S/N is referred to its 6-volt level, this corresponds to a 64.5 dB S/N referred to a 1-volt signal, which is all the power amplifier needs-or can handle without overload. The preamplifier will still be the dominant factor, but the system S/N is really then closer to 65 dB than to 80 dB. (There are additional complications that can further worsen the S/N figure, but we won't go into them here.) In the case of products reviewed by H-H Labs, everything is referred to standard levels, thus permitting valid comparison of sys tem noise levels with combinations of components. All power amplifiers are rated for noise relative to a 10-watt output, regardless of their maximum power capabilities, and all preamplifier tests employ a 1-volt output-level reference. You still have to consider the input sensitivity of a separate power amplifier, but this is also specified as the voltage needed for 10 watts output.
Putting the matter in its proper perspective, it doesn't make sense to become too concerned with a precise determination of S/N for most home listening situations, because items external to your system such as the S/N level in the program material and the back ground (ambient) noise in the room tend to overshadow the noise in your system's electronic components.
If you have a setup that includes, say, a preamplifier, a separate four-channel decoder, and power amplifiers with input-level controls, it pays to experiment with the relative settings of the three sets of controls: you will find that one particular combination of set tings provides the lowest noise level for a given loudness level. Don't be concerned with the setting of your preamp's volume control: obviously, as you turn down the controls on the other components the gain controls on your preamp will have to be set higher. But let the S/N (and possible distortion on peaks) be your guide.
Brand-name Mentions
Q. I find that I generally enjoy your magazine and the technical articles in it, but because you shy away from mentioning brand names many of the articles are not as helpful as they might be. Why do you do that?
DONALD PETERS; Montreal. Quebec
A. We "shy away" from brand names--except, of course, in test reports--for several different reasons. First of all, some readers tend to interpret any mention of a feature or function of a specific brand or model as a sort of between-the-lines comment. positive or negative, when it was not intended to be. Rather than go through the clumsiness of printing a disclaimer for each mention, we prefer simply to omit brand names as not being to the point.
Another part of the problem has to do with the mechanics of writing a feature article or an answer to a question for this column. As a case in point, if in a general article on turn tables we mention that the undamped cueing levers on certain models are clumsy to use.
should we mention the brand names? The difficulty there, from our point of view, is that we may leave out some brand or model that has an undamped cueing lever, or that some model that previously was undamped now is damped-or will be by the time the publication appears. There are so many brands and products in each category and the models change so frequently that the task of being definitive and up-to-date on any characteristic is almost hopeless. We find that it is of much greater value to the readers if. in our general how-to-buy articles, we state what we find to be important positive (or negative) factors among a number of the models we have checked and leave it up to the reader to apply that information in searching out the model (which we may not even be aware of) that tickles his specific fancy.
We appreciate the fact that those readers who must rely on mail order for equipment would like us to cite the tested specs and performance of every unit in a category we discuss, but unfortunately such a task is beyond the means of any testing establishment on this planet. For such readers, the best we can do is suggest careful reading and comparison of the test reports (even though they can never cover all models available) in this and other publications.
Stereo FM Hiss
Q. My FM reception has a constant background hiss on a number of stereo stations. When I turn the function switch to the mono position the hiss disappears. Why does this happen?
J. SHERMAN; Orange, N J .
A. FM signal reaching your tuner is not strong enough to provide an adequate signal-to-noise ratio: But I have no way of knowing whether this problem arises because you have an inadequate antenna, are located in a particularly poor reception area, or have a defective (or simply insensitive) tuner.
Multi-track Recording
Q. I have heard about studio tape recorders that can simultaneously record eight, ten, or more separate tracks on 1/2- or 1-inch tape. When will such recorders and tapes to play on them be available for home use?
CHARLES AVIN; Cleveland. Ohio
A. Probably never. You can get perfectly discrete material with few technical limitations on four-track open-reel tape, but the major technical/aesthetic/psychoacoustic problem still remains: how to record four discrete channels so as provide a reasonable facsimile of the original in home playback. The difficulty is far from solved for only four channels. For six, eight, ten, or twelve?
Forget it! Speaker Rotation
Q. I have heard that it is a good idea to "rotate" speakers like you rotate tires that is. exchange your right speaker for the left and vice versa at regular intervals. I would like to know if you believe in this practice and if you do, please explain why it is necessary.
PHILIP HUNT; Cambridge, Ohio
A. This a new one on me. As far as I know the only parts of your hi-fi sys tem that benefit from rotation are your records and open-reel tapes. For most records. 33 1/3 rpm is about right.
------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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Source: Stereo Review (USA magazine)