Audioclinic (March 1985)

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Bias Is as Bias Does

Q. What is bias?

-Tom Wick, Huntington Station, N.Y.

A. Bias usually refers to a voltage or current used to shift or control a device's mode of operation. For example, in tape recording, a high-frequency bias current applied to the tape ensures that audio signals will be recorded in as linear a manner as possible, with minimum distortion. In amplifiers, it controls the operating portion of the input/output curve, again preventing severe distortion. The exact meaning of the word, and how the bias is applied, varies with the type of circuit under consideration; the two definitions given here are, however, the ones most commonly quoted in audio.

Loudspeakers and Video Monitors

Q. I plan to purchase a video monitor and incorporate it into my audio system. I have seen references to "shielded" speakers for use with video equipment. From this, I assume there must be an interference problem. If I use my loudspeakers near a video monitor, what problems will I encounter, if any? If nearness is a problem, is the need for distance between speaker and monitor a linear or exponential function? Is there an economically and sonically feasible way to shield my speaker system?

-Rowland M. Hill, Alexandria, Va.

A. Strong magnetic fields can deflect the electron beams in TV picture tubes. This normally causes only slight distortion of the picture details, but on color sets, where a slight mis-deflection moves the beam to a different colored spot, the result can be disastrous color-smearing.

How far the speaker and the picture tube (CRT) must be separated to avoid this depends mainly on the strength of the speaker's radiated magnetic field.

A distance of about 2 feet from the neck of the picture tube is usually about enough; if not, field intensity falls off as the square of the distance (doubling the speaker's distance from the tube neck reduces its field at the neck to one-fourth its original value). I do not believe it's possible (or, at least, practical) to shield your speakers yourself. On the other hand, try it and see-it may not even be necessary in your case. The only thing to fear is magnetizing your picture tube beyond the ability of your set's built-in degaussing coils to neutralize it when you next turn the set on; if that happens, you can buy more powerful degaussing coils at good TV parts stores, or even build your own.

Audible Sidebands

Q. Let us suppose that we have an AM broadcast system with perfect transmitters, a perfect receiver having square-topped i.f. characteristics but no automatic volume control (AVC), and perfect reception, too. Our perfect receiver has a 9-kHz passband or i.f. selectivity. We tune to an unmodulated carrier at exactly 1,500 kHz, and there is another station at 1,490 kHz, sending a pure, 7-kHz sine-wave signal.

The upper sideband of that second station is, therefore, 1,497 kHz, well within our receiver's i.f. passband.

Under these conditions, what will I hear? Will I hear the 7-kHz tone? If I do, and the 1,500-kHz station goes off the air completely, will I stop hearing the 7 kHz transmitted by the station at 1,490?

-Russel E. Worthy, North Adams, Mass.

A. You will hear the 7-kHz signal.

You will also hear a 3-kHz difference tone. Just how loudly you hear the 3 kHz will depend upon the relative strengths of the two carriers and, of course, the presence of modulation, which would tend to override this beat tone.

If 1,500 goes off the air, the 3-kHz signal will disappear, but the 7-kHz signal will still be heard because it falls within your i.f. passband.

"Rabbit Ear" Antennas

Q. I was originally using the folded dipole which was supplied with my receiver. Not happy with the performance of this device, I next tried a CB antenna. It didn't work well, either. I am now using a pair of "rabbit ears" with 300-ohm connections to the receiver.

This works rather well. Why don't more people use these antennas? Are there any drawbacks that I am not seeing or, rather, hearing?

-Mark Lemelin, Kensington, Conn.

A. Using a CB antenna did not work well because it was designed for much lower frequencies. A properly designed antenna must resonate within the band it is designed to cover. The CB antenna works for its application because it is tuned to 27 MHz (approximately). The folded dipole works because it is cut to a length which falls within the band of frequencies from 88 to 108 MHz. This is also true of the "rabbit-ear" antenna you now use.

Here, however, the antenna can be more critically adjusted for proper length, taking into account changes in resonant frequency resulting from proximity to surrounding objects.

It well may be that many people use rabbit-ear antennas, but there are no statistics to tell us this. These antennas are more expensive to supply than the 300-ohm, folded dipoles supplied with tuners and receivers. The folded dipole can be tucked out of sight, whereas the rabbit-ear antenna cannot be.

This is certainly a drawback to its being supplied with FM equipment.

Binaural Recording

Q. What is binaural recording? What are the relative weaknesses and strengths of this recording technique compared to stereo recording?

-Eugen Spralja, San Pedro, Cal.

A. Binaural recording employs two microphones only. These microphones are placed close together, separated by the distance between a pair of human ears, perhaps even set into a dummy head-ears and all.

Though such recordings can be enjoyed when heard through loudspeakers, they are meant specifically for listening through headphones. With headphones, each ear hears only the sounds picked up by the microphone in the corresponding channel. The mike is placed where those ears would be if the listener took the place of the recording dummy. The result can be exceptionally realistic.

In normal listening, however, the sound produced by one loudspeaker blends to some extent with that produced by the other. Normal stereo recordings, which are meant for speaker listening, have extra separation to partially compensate for this-which is why they sound exaggerated when heard through headphones.

The advantages of binaural recording are that the setup of equipment is simple and the recording is made in the same way that a listener would perceive the sound from the location of the microphones. However, there are disadvantages. If the performers do not maintain good internal balance among themselves, the playback of the recording will reflect this. It is interesting to note that, when the listener is present during the recording, he is not seriously troubled nor aware of the poor balance. He will, however, often be annoyed by it when he hears the sound from loudspeakers or, to a lesser extent, through headphones. In addition, the need for headphones can restrict the listener's movements and activities while listening. Binaural recordings also lack the extra clarity that multi-miked recording techniques can produce. Multi-miking's effects are rather clinical, but some people do prefer them.

Bi-amp Troubleshooting

Q. I have a serious problem with my biamplified sound system. Its high end became very faint, starting about a week ago, and I have gone to great lengths to find the answer to this sudden problem. I checked the fuses for all speakers, the connections from my preamplifier to the crossover, the connections from the crossover network to the amplifiers, and the connections from my amplifiers to my speakers. Everything is connected correctly. Please advise me of something else to try.

-Ron Truesdale, Greensboro, N.C.

A. First, check the settings of any "level set" controls on either the power amplifier or the crossover network. If their settings have been changed, this could account for all of your difficulties.

The balance of more than one sound system has been disturbed by some unsuspected, unsupervised dusting of the equipment cabinet.

Next, disconnect the tweeters from their power amplifier and connect a pair of tweeters known to be working.

Operate the system in the normal way and see if any sound is heard from these speakers. If you hear highs in abundance (assuming your experimental speakers are high enough in efficiency), you will know that something is wrong with the tweeters. Perhaps a transient blew them out. If no sound is heard or if the sound is still faint, you should go on to the next experiment.

Disconnect the power amplifier from the high-frequency output of the crossover network and substitute another amplifier, known to be operating correctly. (The amplifier could be the one normally used to drive the woofers. The woofers do not need to operate during these tests.) If sound is now heard from the tweeters, you will then know that there is a problem with your original power amplifier.

If, after all of this, you still do not have enough signal in the tweeters, it is possible that the crossover network is defective. You would then need to have it serviced.

Differences in Speaker Loudness

Q. Why would the loudness be noticeably different between two pairs of speakers? I use two pairs simultaneously and find that Pair A is much louder than Pair B.

-Mike Kendall, Gilman, Ill.

A. The reason that one set of loudspeakers produces more volume than another has to do with their relative efficiencies.

The amount of acoustical power output relative to the amount of electrical power input is the measure of a speaker's efficiency. However, even when two pairs of speakers have the same efficiency, one may sound louder than the other. This can be caused by a lack of smoothness, especially in the lower treble; our ears are easily fooled into believing that such peaky sound is louder than smooth sound of equal acoustical power.

Speaker sensitivity ratings don't tell the whole story, either. They measure output on-axis, not total radiated power. A speaker that can produce a given sound output off-axis as well as on-axis will sound louder, in most listening situations, than one producing that output only on-axis.

Eliminating Stereo from Records

Q. The method of connecting speakers between the two "hot" leads of a power amplifier to eliminate the monophonic signal from a stereophonic record has been widely reported. This procedure causes the ambience of a record to be heard.

I'd like to know how to achieve the opposite effect: The elimination of all stereo signals, leaving only the pure mono content. My reason for this is to "clean up" the sound on my old, monophonic 45-rpm records. When listening to these records on my stereo system (in the stereo mode), it appears that all the wear and scratchiness is in one channel or the other but is never in the mono image. If such a connection is possible, it should cancel out all the "wear noise" and leave only a pure mono signal, making the records sound like new.

-Thomas E. Dimock, Ventura, Cal.

A. Assuming your preamp does not have a "Mono," "L + R" or "A + B" switch position (which would do exactly what you seek), you can obtain a fully monophonic output from a phono cartridge by strapping its two "hot" terminals together, and also by strapping the two ground terminals together. The cartridge can then be connected to the phono input. There may be some mismatching of load requirements with this arrangement, but the sonic effects won't be significant.

Strain-gauge cartridges cannot be strapped in this manner. If you have such a cartridge, the outputs from its preamplifier must be fed into a mixer and the levels of each mixer input adjusted for proper monophonic balance.

Or you could sum the preamp's outputs, using a Y-connector, and feed that sum through another Y-connector into your deck's stereo inputs.

Although it is true that vertical information will be reduced or eliminated by the foregoing arrangements, noisy discs are still likely to sound noisy. You will discover that there is enough horizontal component in the noise that your old discs will never sound brand new; all you can hope for is some improvement.

Sometimes a slightly better result can be obtained by leaving the cartridge wired for stereo and using a mixer as described for the strain-gauge cartridge. Signal can be extracted from the jacks normally assigned to feeding signal into a tape recorder. The mixer can be adjusted for best noise cancellation.

(adapted from Audio magazine, Mar. 1985; JOSEPH GIOVANELLI)

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