AUDIOCLINIC (Mar. 1988)

Which Switch?

Q. I have found a need to switch loudspeaker polarity under some conditions. How does one go about selecting toggle switches of the proper cur rent ratings? Are there any dangers or shortcomings involved in inserting switches in the relatively high-current wiring in a loudspeaker line?

-David M. Schneider; Marion, Iowa

A. Any reasonable toggle switch will be suitable for your application. Most of the switches used for a.c. power switching can handle 10 amperes, and this should be fine. If need be, you can certainly use an industrial-grade switch capable of handling 20 amperes or more. You can check all of this with Ohm's Law power formulas; the power which you should consider is that which will be supplied to your speakers.

I have done quite a bit of polarity switching at high volume and have not had problems. Even so, I recommend that you keep the volume low during switching, to avoid gradual arcing and damage to the contacts.

As for disadvantages, I suppose there are those who will be concerned about some loss in damping to the woofer cone. This is about the only drawback I can think of, assuming that we are talking about solid-state equipment. Tube equipment might be dam aged because of high voltages developed within the output transformers during switching.

Outdoor Antenna Troubles

Q. My favorite FM stereo stations are located some 50 miles from me. In an attempt to dispose of a small amount of noise, I have installed an FM antenna atop the chimney on my two-story house. The signals, however, are ac companied by even more noise when using the outdoor antenna than when using the indoor antenna. I have tried rotating the outdoor antenna, as well as using a shielded lead-in-with no improvement.

I am puzzled by this because it runs counter to what I might reasonably expect. I would, therefore, appreciate any advice you may offer.

-Name withheld, Arlington, Va.

A. If there is a signal-strength meter on your tuner, check it to see if you have more signal when the external antenna is used than when using the indoor antenna. There should be a marked increase in meter readings with the outdoor antenna. If there is no such increase, check the antenna connections very carefully. There may be cracked insulators associated with the antenna which would render it useless.

Check, too, to be sure that you have installed the various pieces of the antenna properly.

Assuming that you do get the expected increase in signal with the out door antenna, it may be that some of the stations you are listening to are broadcasting SCA signals, and that your tuner is sensitive to this back ground music service. It may require either an SCA filter or perhaps an alignment of the existing filter. SCA can produce noise when it is combined with the stereo information present on the same FM station.

Differences in Power Mains

Q. I have not seen any articles concerning differences between a.c. volt age and frequency in different parts of the world. How do these differences affect various components? I have heard that only components having motors (turntables, cassette decks, CD players, etc.) are affected, while those without motors (amplifiers, tuners, and the like) are not. I know that one can change voltage by means of suitable transformers, but if the frequency is important as well, can one similarly change frequencies as needed in different countries?

-G. Papadorfos, New York, N.Y.

A. If you travel in countries other than the United States, Canada, and a few others whose power mains supply 117-V, 60-Hz a.c., you need to take the local power system into account for any mains-powered devices you take along.

If the device in question is designed to operate only at 117 V a.c., running it at 220 V will destroy it. If the device is designed to operate at 220 V and it is run at 117 V, damage will usually not be done, but the device won't run correctly, in any event. As you pointed out, there are transformers designed to change the mains voltage to whatever the device needs. Such transformers are interposed between the mains and the device.

Some motors are affected by the frequency of the a.c. voltage supplied to them. Frequency directly determines the speed of such motors' rotation, especially if they are of the hysteresis--synchronous or induction type. However, I know of no means by which you can easily convert the mains frequency to suit the needs of your equipment.

You may need to change pulleys or other mechanical parts to accommodate the different motor speeds which will result from the change in line frequencies. Check with the manufacturer of your motorized equipment as to the availability of such parts.

In today's equipment, we are seeing more of a different sort of motor, one that operates from d.c. (which has no frequency). The power mains does supply a.c. to the equipment, but by the time the voltage reaches the motor, it has been converted to d.c. Motors of this kind are electronically servo-controlled.

Purely electronic components and' motorized components with electronic speed control employ transformers.

Such transformers must be operated from a.c. so that voltage can be produced in their secondary windings.

The frequency of the a.c. is not critical except where there is insufficient iron in the transformer core. The amount of iron affects the inductance of the winding and, as a consequence, the reactance. In order to save a long explanation, let me just say that if the reactance is really low (as it will be at lower frequencies), the transformer will run hotter than it will if its reactance is higher. If the transformer runs hot when operated at 60 Hz, it may overheat and burn out when operated at 50 Hz where its reactance has dropped.

Electrical systems which operate at 220 V a.c. usually also operate at a frequency of 50 Hz, and manufacturers of 220-V equipment must take this into account. As a result, when a piece of equipment has been fitted with a power transformer that can be adjusted for either 117 V or 220 V, it will have no problem with 50-Hz power mains.

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(Source: Audio magazine, Mar. 1988, JOSEPH GIOVANELLI)

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