Audioclinic (Nov. 1979)

Differences in Receiver Selectivity

Q. What advantage would a receiver with 75-dB selectivity have over one which has 60-dB selectivity?

- Lowell Knapp, Charleston, W. Va.

A. If you are in an area where strong FM signals are present and wish to receive some other signals which are considerably weaker, and if these signals are located on what are known as "alternate channels," the receiver with 75-dB selectivity will be better able to receive the desired signal (with less interference from the nearby stronger signals) than would the receiver having only 60-dB alternate-channel selectivity.

If you are not interested in receiving relatively weak signals or if the signals to which you wish to listen are not located on the dial close to stronger signals, then it won't make much difference whether you have a lot or a little alternate-channel selectivity.

Stereo vs. Mono

Q. t would like your comments about the listening arrangements of two of my friends. The first has two rather small speakers, standing vertically, about two in. apart. They are toed in, so that lines from their centers would cross about a foot in front of the speakers. My friend sits six ft. from these speakers when listening, often way to one side.

The second friend has two medium-size speakers, also standing vertically, and also about two in. apart, but his are directed parallel. He sits about 14 ft. from them. I would like to know how much stereo effect each of them would get and how important it may be for them to have bought two speakers.

The preceding question was based on the assumption that stereo has some purpose in general, especially for my two friends, and I would like to question that assumption. I have had a high-fidelity system for some 17 years.

I have never felt the need to "modernize" to stereo. Of course, I have heard friends' stereo equipment, not to mention quad, and I am really unimpressed. My reaction has always been that if I were sitting near the front of the concert hall, I would expect the sound of the strings to come from the left because I see them there. Likewise, I would expect the sound of the percussion to come from the right, because I see them there. But at home what am I supposed to do--imagine the position of the instruments? I really could not care less where the instruments must be placed on a stage or in a pit. In fact, I wouldn't be surprised to learn that some composers had occasionally wished that the instruments in an orchestra could have been blended by being altogether in one little spot and had been frustrated by the impossibility of doing so. After all, in chamber music they do not space the instruments far apart just so the audience will get a stereo effect. Therefore, I would like to know if stereo is just a rip-off to sell twice as many speakers.

-Thomas D. Stowe, New York, N.Y.

A. In my opinion, with two-in. separation of two speakers, your friends may as well listen to mono discs or tapes. I cannot imagine how either one can hear any stereo at all, sitting so far from the speakers with such close spacing.

While these listening arrangements are quite far from what most use, listening is a subjective experience. If your friends have experimented and have come to the conclusion that their special setups are the best for them, that's fine.

Stereo serves as a means of localizing sound sources, and man was de signed with two ears so that this localization is possible. Knowing which direction a sound is coming from could mean the difference between life and death in some situations.

Many composers, early and late, have experimented widely with stereo to get the most from live performances. Gabrielli and Ives are two notable examples. Antiphonal church music was designed to maximize the stereo effect.

If you believe that stereo is not useful for your enhanced listening enjoyment, fine. Good sound has been produced in mono for many years. Stereo discs can be played very well with monophonic equipment. The mono phonic listener is considered during the processing of stereo discs, especially those in the popular music field.

For me, when stereo came along, it made an incredible difference to MY listening enjoyment. This was before I was seriously involved in audio. There fore, I don't believe I had an "axe to grind" in favor of people spending more money on speakers and special amplifiers, cartridges, and the like.

Because of stereo and its problems, cartridges advanced tremendously in performance. The results of this advance are that mono discs sound better today than when played on cartridges made when the discs were first released. There is a more transparent quality to many of these discs, especially at their inner diameters.

If it was true that stereo is actually a rip-off, eventually the public would have stopped buying it. But stereo has been with us in one form or another for more than 25 years. I judge from this that most people have found audible benefits from this kind of sound.

Equipment Without Tone Controls or Filters

Q. What is the purpose of building preamplifiers without tone controls and filter circuitry?

-Gustavo Saetta, Buenos Aires, Argentina

A. There are those who believe that an amplifier should ONLY have a flat frequency response, and filters and tone controls do affect such a flat response. Many others are concerned with maintaining the lowest possible amount of phase shift. Both tone controls and filters will introduce phase shift.

There are occasional deficiencies in program sources, with loudspeakers or with room acoustics, which can often be helped by the use of tone controls.

Excess record scratch or low-frequency rumble can be reduced by the use of filters.

Some designers overcome these obstacles by designing tone controls which can, in one way or another, be completely switched out of the circuit.

This will provide the flat response and low phase shift which some people desire.

If an individual feels so strongly about all of this that he will never use tone controls and filters, a preamplifier can be constructed for less money by not including these devices.

Power Boosters

Q. I have an antenna preamplifier with a remote power supply, in addition to a broadband VHF distributor. Will I be over-amplifying the signal and burn out either my FM or my TV set if I connect these to my equipment?

-George B. Kritchen, Fairfield, Penna.

A. I do not believe the use of these will burn out either of your receivers. However, it is possible that, if you are located near strong stations, your receivers may be overloaded which will manifest itself in the form of the station appearing at various points along the dial or as background to other stations being received. If both of these items do contain amplifiers, then you stand this chance of over load.

You may not be able to check for overload without installing the amplifier and distributor properly. This way you can, and should, check for added noise and distortion. Use an indoor antenna of some kind connected to each receiver id turn. Then note the strength of each signal received, the number of such signals, and the freedom from background noise. Do not move the antenna; connect the ampli fier and distribution device as indicated, and make similar checks. If the signal strength is improved and you hear more signals, free from annoying back ground noise, the reception of your system will have been improved. If overload is present under these conditions, you will know that it will be far worse with an outdoor antenna.

Audio With Co-Axial Cable

Q. I want to make connecting cords for my stereo audio lines, cut to exact length for the sake of neatness and to avoid the problems which can arise from having excess cable coiled in the vicinity of a.c. lines. I would like to use RG-58/U co-axial cable from the turner and tape deck to the preamplifier, and from the turntable to the preamplifier. The longest run will be about 24 in. In addition, I want to run about 25 ft. of coax to carry the signal from the preamplifier gain output to an Advent SoundSpace Control which is located at my listening position. Is there any reason why I should not use this type of cable?

-David E. Illig, New York, N.Y.

A. You can use RG-58/U cable, but I see no benefit from doing so. It is not flexible; the center conductor is solid wire. If the cable is handled very much, this conductor can break, probably at one of the connectors. Why not use ordinary audio cable? You will not have any problems, even with the 25-ft. run you mentioned. If you en counter any serious interference problems, then consider special cables.

Speaker Shrillness With "Dynaco" Four-Channel

Q. I own the best stereo system that there is, the best of everything. Recently I bought two extra speakers to be used with the Dynaco quadraphonic sound circuit and installed them in the rear of my listening room. These speakers spoiled the total sound, introducing painful, shrill distortion. I could not believe they could be so bad. I took one to my basement where I have a TV set and fed sound from this set via a 60-watt amplifier. The sound was clean, with good definition, and without any trace of shrillness.

In my stereo system I use solid-state amplifiers. The two new speakers are each eight ohms, connected in series and between the "hot" terminals of the amplifier. What causes a speaker to produce such shrill sound when apparently everything concerned is in good order?

-Name withheld.

A. From what I can see, the sound produced by the rear speakers is quite loud with respect to that from the front speakers. Regardless of what kind of audio quality these rear speakers can produce, you should not really be aware of them for good quad sound.

I suggest that you place a variable resistor in series with the rear speakers so that you can adjust their relative level of sound for proper balance. This variable resistor will also provide better conditions for your amplifier. If, indeed, the amplifier is too heavily loaded by the rear speakers, the load will be relieved by reducing the power supplied to them.

If you ever need to check out a loudspeaker as you did in this instance, do not use audio from a TV set. TV sound is poor most of the time, usually rolled off at the high end. (It is improving, however, partly because of some rather interesting new transmission techniques.) Phonograph records with which you are thoroughly familiar are your best sources for checking a speaker system.

Perhaps it should be added that the manner in which you have connected your speakers means that the rear-channel speakers will produce the difference signal between the left and right channels of a conventional stereo signal. Because of the nature of phonograph records, this difference signal is the vertical component of the modulation impressed on the grooves. Any tracing distortion is exaggerated when one listens to the vertical component of stereo recordings. The problem is more apparent when the rear-channel speakers are too loud.

Turnover Controls

Q. I recently bought a receiver which has two "turnover" switches. The function is puzzling to me. The frequencies are: bass, 400 and 200 Hz; treble, 2.5 kHz and 5 kHz. When I put the bass turnover switch to 200 Hz, the bass does not sound as deep or obvious as at 400 Hz, which makes no sense to me.

- Peter A. Kass, Milwaukee, Wisc.

A. The bass tone control affects low frequencies; the treble control affects higher frequencies. Once bass boost starts, the amount of bass increases with decreasing frequency. With treble boost, the amount increases with increasing frequency. The turn over controls are means by which you can choose between two "starting points" for bass and two for treble.

Let's examine the bass. If we assume a starting point of 200 Hz, and if we are boosting bass, let us say that we have 8 dB of bass boost by the time the frequency descends to 50 Hz. Let us now assume a starting point of 400 Hz. Remember that bass increases with decreasing frequency below this starting point. Because we started at a higher frequency, by the time we get down to 50 Hz we may have 14 dB of bass boost. Therefore, you will hear more bass with the control set to its 400 Hz position than you will when it is set to its 200-Hz setting.

Sound from a Phonograph Stylus

Q. I was standing near my turntable with its dustcover raised and heard music coming from the stylus. It seemed as though there was a small speaker at the end of the stylus. I cleaned it, doubly cleaned the record, and still heard this music. What is causing this?

-Robert D. Delorme, West Warwick, R.I.

A. In any phonograph system you should expect to hear some sound coming from the stylus/cartridge/shell combination. The stylus does contain some mass, and this mass is moving. If there is a moving mass, it must come into contact with surrounding air, some of which will be pushed. In this manner sound is generated from any vibrating object, including a phono graph stylus. Added to this is the fact that the tonearm shell is a small resonator, and thus it provides a means of slightly amplifying the sound produced by the stylus.

(adapted from Audio magazine, Nov. 1979; Joseph Giovanelli )

Also see:

Picking at the Congress (Dec. 1979)

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