Audioclinic (Mar. 1979)

Noise in Phono Stage

Q. I have read product reviews in audio publications concerning a certain preamplifier which states that there is virtually no noise in the phono stage. I own this preamplifier and there is an audible hiss when this preamp and its associated power amplifier are operated with their gain controls at maximum. Why is this?

-Frank H. Abelson, Brookeville, Md.

A. Any piece of electronic equipment has some noise output. All that matters in the final analysis is that this noise must be well below the lowest program level you expect to encounter. The fact that, with all gain controls set at maximum, you do hear some noise is completely unimportant un less circumstances are such that this is your normal listening arrangement.

Such a circumstance might result from having a cartridge with too little out put for the preamp, which can occur with some moving-coil cartridges.

It may be that when the controls of the power amp are open all the way, some of the noise you hear is actually generated from stages of the preamp which are in the circuit after the volume control. This is readily checked by turning the preamp's volume controls down to minimum and noting if the amount of noise is substantially the same as when the controls were fully advanced.

Phono Cartridge Output

Q. Why is it that the more expensive phono cartridges put out less signal voltage than the cheaper models and, thus, need more amplifier volume to sound as loud?

- Stephen Jones, Bayside, N.Y.

A. To make an excellent cartridge re quires a low stylus mass ... the better the transparency, the less stylus mass is required. To compensate for this, dimensions in other parts of the cartridge must be held to within very close tolerances. With all of this, how ever, the output for a low stylus mass cartridge will be lower than for one having a higher stylus mass. One of the electrical reasons for this has to do with the efficiency of the magnetic circuits.

Added Tracking Force

Q. Why is extra tracking force used when a stylus brush is added to the cartridge?

- Michael Terr, W. Orange, N.J.

A. Today's tonearms are balanced for the particular cartridge being used.

This tracking force represents the en tire downward thrust of the tonearm upon the record, which is concentrated on the tiny tip of the stylus. If we add a brush to clean the grooves of the record before the stylus encounters them, we still have the same tracking force now applied to the tip of the stylus and the record cleaning brush both. This means that the stylus no longer has the total tracking force, so more weight will have to be added to the tonearm in order for the stylus to gain the required tracking force.

Eliminating Midrange

Q. I have a reel-to-reel tape recorder and an auxiliary amplifier, and I want to use this amplifier solely for effects.

The signal going into each channel to be split into specific segments: Channel one to cover from 30 Hz to 400 or 500 Hz, and channel two to cover from 5 to 16 kHz. The purpose of all this is the elimination of the midrange so that the auxiliary amp will reproduce only the music portion of the tape, eliminating the voices of the singers.

- Ronald M. Podrazik, Lodi, N.J.

A. To accomplish the removal of the middle frequencies, you will need a graphic equalizer or a crossover with separate adjustments for each of the two channels. The output of the tape recorder will be fed into the two in puts of the graphic equalizer. The output of channel one should be set to boost the low frequencies, and the output of the other equalizer channel should be set to boost all the high frequencies. The mid and upper frequencies of channel one should be set to their greatest "cut" positions, as should the low and mid frequencies of the equalizer.

Music covers a wide frequency spectrum, including the same frequencies as the human voice. Therefore, if you remove the voice frequencies from a tape, you will also remove the musical background, except for the bass and treble.

If you wish to remove the voice and leave only the musical background, this can be done by making a mono mix of the two stereo channels, but with one of the channels out of phase.

The out-of-phase condition means that any sound placed directly in the center--on both channels--will disappear. Another item which will also disappear is the bass. You can, however, boost the bass on one channel without having the voice reappear, al though you might have to touch up balance between the two channels.

Speaker Phasing

Q. How can the phasing of speakers be checked in a bi-amped system? I have several records which contain phasing bands, and according to some of these records, my system is in phase, while with others, not exactly. I wonder if either the low frequency or high frequency sections could be in phase and the other sections out of phase?

- Claude Cook, Mullens, W.Va.

A. The phasing of speakers on a bi-amped system is checked just as you would check the phase relationship between a speaker system employing a passive crossover network. A signal within each speaker's frequency range is fed into each pair of speakers with the wires of one of the speakers reversed. You then note whether the signal increases or decreases in volume ... if the volume increases they are now in proper phase and the wires are now left in their new position. However, if the volume decreases, then the wires must be reversed again. Repeat for each pair of speakers.

The phasing bands on most test records, at least the ones with which I am familiar, are designed to show whether a pair of speaker systems are properly phased. This could not be done with the aid of most test records because the maker of such a record could not know the proper crossover point for your system. What is needed is either a mono record with spot frequencies or a signal generator because it can be set to produce the exact frequency desired for your particular system.

Chirping Receiver

Q. My receiver constantly emits a chirping sound. This noise doesn't come from the speakers, but the receiver itself. It isn't a loud noise, but clearly audible and very disgusting considering the price I paid. What can I do?

-William H. Chapman, Detroit, Mich.

A. Chances are that the sound emanating from your receiver is caused by the vibration of the core lamination in the power transformer. Sometimes, not always, this can be eliminated by tightening the mounting screws of the transformer. There may also be some additional screws which hold the transformer itself together, and these should also be tightened.

It may be necessary to replace the present power transformer. If one is not available, then it may be possible to take the present unit apart and dip the innards in hot pitch. When the pitch hardens the laminations will no longer vibrate. In any case, the noise does not mean that the equipment is in any danger of malfunctioning.

Tuning Meter Drift

Q. My tuner and receiver both have center-channel tuning meters which drift when they get warm. They stop drifting after about a half hour, but there is a significant difference in dial location between center tuning when hot and cold. What is the correct center-tune position? Are the meters more accurate when warm or when cold? How should I tune these units for best reception and lowest distortion?

-Robert A. Ward, Cleveland Heights, O.

A. The meters on your tuner and receiver do not drift off correct center tuning, the actual tuning changes. The meters simply reflect this fact so that you will know that you have to retune the equipment. If the sound quality does not appear to change from the time the tuner is turned on until it is completely warmed up, you can probably ignore the meter altogether.

Once you know the direction of drift, you can mistune the equipment slightly when first turning it on, and allowing it to tune itself as the warm up proceeds.

(Source: Audio magazine, Mar. 1979; Joseph Giovanelli )

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