Audioclinic (Jan. 1979)

Electromagnetic Speakers

Q. I recently acquired a very old pair of 12-inch speakers that are well built and in excellent condition. Each speaker has four wires coming from it; two are connected to the voice-coil and two to the magnet. I get some sound by using only the two wires from each voice-coil when my 30-watt amplifier is fully turned up. Is there any way I can use these speakers with my present stereo system?

--Rick Nicholson, Lyons, N.Y.

A. Your speakers are of a type not seen these days. The magnet is an electromagnet energized by the two wires that do not run to the voice-coil.

Such electromagnets were made as either low- or high-resistance types. The low-resistance magnet was designed to be connected in series with the B supply of tube amplifiers. The magnet coil served as a filter choke, as well as serving as the electromagnet for the loudspeaker. If its resistance is from 200 to 400 ohms, you will know that you have one of the series type. The high-resistance magnet was intended to be connected directly across the power supply from plus to minus, and not in series with the load. The resistance of such a magnet could be, perhaps, 10 kilohms.

When these magnets are operating correctly, they should be just lukewarm to the touch. Any really high temperatures, like burning the fingers, means that the voltage supplied must be reduced in order to prevent burning out the coil.

Unless the speakers have a relatively low resonant frequency and really sound good after all the trouble to make them operate properly, the work involved may not be worth the effort.

If there is a manufacturer and model number shown on the speakers, this can be checked out. If the firm is still in business, then data related to it can be supplied. If no such information is available, there is likely to be a series of numbers or letters printed somewhere on the speaker frame. Someone involved in re-coning loudspeakers will have a code book, enabling him to translate the numbers and letters into the name and maker of the speaker, along with the date of manufacture.

This information should enable you to make the speakers work relatively well.

P.A. Line Voltages

Q. Please explain the details of a 70 volt line in a public address system?

- S.A. Elosh jr., Campbell, Ohio.

A. When an amplifier is operating a certain amount of voltage is produced across its output terminals, which will depend on the voltage of the load and on the output power produced by the amplifier. In the so-called 70 volt system, the amplifier is so set up that when it's run near maximum power, the impedance is such that 70 volts is produced across its output terminals.

Often this is accomplished by the use of a transformer. This voltage is used to feed a number of speakers in a paging or public address system. In such systems one does not look to match the impedances between speakers and the amplifier. All that is really required is that there be sufficient power and voltage so that the load, large or small, can draw its required power. Each speaker is not connected directly to the line, but a step-down transformer is connected between the line and the speaker. The impedance of the transformer is much higher than the impedance of the 70 volt line, therefore a number of transformers can be wired across this source.

The transformers also keep the amplifier from overheating and allow for the addition of more speakers later on should this become necessary. Another benefit is the ability to have longer cable runs between the 70 volt source and the transformer as the copper losses in the line will not add up to a significant fraction of the impedance of the transformer. This is probably the major reason why systems of this kind are so popular. For a given cable run, the size of the wire can be much smaller than it would be if the speaker was fed directly by the amplifier.

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

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

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