AUDIOCLINIC (Q and A) (Feb. 1978)

Basic Audio

Q. I am a 16-year-old girl whose father is a "high-fi nut." My problem is that I don't understand what is happening when I put a record on the turntable and put the "needle" on it. With my interest in music, I decided that it's about time I understood what goes on in the mechanics of it. I asked my father about it, but I got a long confusing explanation in another language. --Name withheld by request.

A. When I was a kid and heard someone reading the news on radio station WOR, I clearly remember going behind the radio to look for the person doing the talking, but he was nowhere to be found. That started my present career in sound.

With the phonograph cartridge, most modern cartridges employ magnetic principles which convert the motion of the stylus into electrical impulses, a tiny voltage. We now have a tiny amount of what we call "signal"--a tiny electrical voltage. In order for it to become useful, it must be made larger, and it is the job of the amplifier to build these tiny impulses into larger ones. But, unless something else is done with it, we cannot hear this large signal any more than we could hear the tiny signal produced by the phonograph cartridge. The final conversion of this signal into the sound we hear is done by the loudspeaker.

Again, most speakers employ a magnet and a coil, just as the cartridge does.

We now have the opposite operation from that which we had with the cartridge, as we change the electricity into motion of the loudspeaker cone.

This signal is connected to the two wires which form the beginning and the end of the continuous coil of wire, and we have set up the relationship between the coil and the magnet which results in the coil being alternately attracted to and then pushed away from the magnet. The motion of the cone makes the air around it move because the cone pushes against the air. This moving air strikes your ear drums and you hear the sound as music.

The signal from a tape machine starts out as a small signal, just as from a phonograph cartridge. The tape itself is a series of magnets which have varying amounts of magnetism that move past a coil in the tape head as the tape is pulled through the machine. Remember that a magnet moving in the vicinity of a coil of wire will cause a signal, or voltage, to be produced in the coil of wire. Once this voltage exists, the signal is amplified and fed to the speaker just as it was with the phonograph cartridge.

The FM tuner is, in many ways, similar. The waves coming from the broadcast station can be thought of as magnets moving past your antenna.

Your antenna is nothing more than a coil of wire which happens to be in a straight line, rather than being wound in a circle. The moving magnetic waves cause a small voltage to be produced in the antenna. This voltage is then amplified and ultimately you hear the sound produced by this process. Again, this explanation is very much oversimplified.

Reversed Stereo Perspective

Q. I have noticed that the majority of my symphonic records seem to have the channels reversed. 1 can tell this because the violins are on the right and the cellos are on the left, the opposite from what one perceives viewing an orchestra in concert. All the connections in my audio chain are connected as labeled. Are the records recorded in mirror image, or are the jacks of my turntable mislabeled?

-Andrew P. Guzie, Boise, Id.

A. It appears that somewhere along the line you have some channel connections reversed, most likely at the cartridge itself, rather than elsewhere.

If your stereo perspective is normal when listening to FM, you can be assured that the amplifier, preamplifier, and loudspeakers are properly wired. I suggest that you recheck all connections, as it may just be some piece of equipment is mislabeled. With all of the add-on devices available, channel reversals are very possible unless extreme care is taken at each step of the installation process.

Turntable Construction

Q. Why do a lot of record changers which use ceramic cartridges have two pole motors?

-Robert Watson, Dover, Del.

A. Two-pole motors are cheaper to construct than four-pole motors. They can be used in changers that employ ceramic cartridges because these cartridges are not susceptible to hum pickup as magnetic cartridges are since two-pole motors radiate a tremendous amount of hum voltage.

Power & Tone Controls

Q. I connected a power meter to my receiver's speaker terminals as per the instructions. Why does the meter show an increase in the power whenever I add either bass or treble? I thought that the volume control determines the output power?

-Aaron Holley, Pacifica, Cal.

A. The tone controls either add or subtract bass and treble, consequently the tone control is really a volume control that affects only a portion of the audio spectrum, rather than the whole, like the volume control. Any added signal, whether supplied by the tone control or the volume control, means that the amplifier must supply more power. In short, anything that alters the nature of the signal feeding your speakers will change the amount of power your amplifier provides, and this will be shown on your power meter.

If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 401 North Broad Street, Philadelphia, Pa. 19108. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Source: Audio magazine, Feb. 1978, JOSEPH GIOVANELLI)

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