AUDIOCLINIC (Q and A) (Dec. 1974)

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Speaker Impedance Ratings

Q. I own a pair of JansZen Z-600 speakers which I drive with a Dynaco PAS-3 and Stereo 70 combination. This system has been just fine during the entire eight years that I have had the system.

I met another owner of the identical speaker model at a recent party.

He left me with a vexing problem; it is more in my head than in my ears (though there is a connection, yes?). To be brief, he flatly stated that, despite the manufacturer's claims, the Z-600 is not an 8-ohm speaker.

My friend measured it as having a 4 ohm impedance. (He made this measurement with his ohmmeter.) Needless to say, I was startled by this information. On the following day, I borrowed an ohmmeter from a neighbor. I disconnected the speakers and measured them. Much to my surprise and chagrin, the speakers were 4.8 and 4.6 ohms respectively.

Is there something else that I am missing? I have no knowledge in the technical area of audio, so 1 am sure there must be a simple explanation here. I really cannot believe that the manufacturer would deceive the consumer in this way. It just does not make sense.

If it were so, is there any possibility of damage to either of the speakers or to the amplifier because of an incorrect output connection?

-David Kraft, Jackson Hts., NY

A. The impedance of a speaker is not measured with an ohmmeter.

You can measure d.c. resistance with an ohmmeter, however, and this is what you have done. D.c. resistance, however, is not the same as impedance which takes into account inductance as well as d.c. resistance. (Inductance is present where a.c. is involved, and tends to oppose the flow of such current, just as resistance opposes the flow of current, both d.c. and a.c.) Unless you have access to a variable resistor, an audio generator, and a meter capable of properly reading a.c. voltage, in addition to the ohmmeter, you cannot measure impedance under home conditions.

Even if your speakers were four ohms rather than eight, no harm could come to your amplifier, even with wrong connections. Perhaps you would obtain somewhat less power than you would if the proper impedance tap was selected, but this would only mean that your speakers would be driven less hard than otherwise. Therefore, no damage could occur to the speakers.

While it is sometimes impressive for an amplifier to have the highest possible numbers related to power output, there is no advantage at all for a manufacturer to mislead the consumer-by a false indication of his speaker's impedance. I think we can assume that the speakers are properly manufactured at their rated impedance.

Cueing with Headphones

Q. If I am playing one phonograph, how can I listen on a set of headphones or whatever, so that I can cue another record on the idle turntable and without the sound of the cueing coming through the main loudspeakers?

-James Golden, West Roxbury, MA

A. The problem of cueing with headphones can be solved by the use of one of the small, headphone amplifiers which are readily available. Means must be provided so that it can be switched between one and the other of your phono equalizer outputs. In other words, you must have a means for paralleling the mixer and the equalizer with the headphone amplifier. The switch will connect the headphone to either of the pre-amplifier/ mixer connections, but not both at one time. When a disc is playing for the dancers, you would turn down the other mixer controls associated with the second table. You could then cue up the disc on that table without its being heard in the room.

If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 134 North Thirteenth Street, Philadelphia, Pa. 19107. All letters are answered. Please enclose a stamped, self-addressed envelope.

(Audio magazine, Dec. 1974, JOSEPH GIOVANELLI)

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