Audioclinic (March 1977)

To Leave On, or Not to Leave On

Q. A few of my friends, who work at a local FM station, tell me how I am causing the downfall of my stereo equipment by turning it off when not listening to it. At the radio station all equipment, except motors, is constantly on. Keeping the equipment on, they say, will prevent power surges and prolong the life of my equipment. What do you suggest for a home system?

-Larry M. Bauer, Dayton, Ohio.

A. I don't think it makes much difference in terms of a home entertainment system whether the equipment is left turned on, or whether it is turned on when it is to be used. With tube equipment, especially when it was to be used frequently, it was a good idea to leave it turned on, but with solid-state equipment, I don't think this is necessary.

It is true there are power surges which occur when a piece of equipment is turned on, and this takes place because the filter capacitors must' charge up and the charging current is initially very high. However, if the capacitors and diodes are of good quality, these surges will be easily be handled and will not degrade the life expectancy of the equipment.

As you can see, if the broadcast equipment was repeatedly turned off and on the power surges might be a factor in the deterioration of the equipment, not to mention the physical wear and tear on the switches themselves. But under average home equipment conditions, these factors are not problems.

Power Failure and Equipment Protection

Q. Suppose a fuse blows or a circuit breaker trips shutting off power to the stage during a rock concert. Does it matter if all the equipment is left turned on until power is restored? Or is it better to immediately turn off all equipment, wait until power is restored, and then turn on the equipment, one unit at a time?

-Thomas Young, Thomaston, Conn.

A. I prefer to turn off all equipment when a fuse or circuit breaker shuts down the equipment as this avoids unnecessary line voltage transients.

Although nothing will happen most of the time, there is always that one chance that a high-voltage spike can damage some components. So play it safe and turn all the equipment off.

Then turn it on, one component at a time, once the power has been restored.

50 and 60 Hz Strobe References

Q. As is commonly known, all a.c. power line circuits are designated as 60 Hertz. I have noticed, however, that some turntables include stroboscopic reference markings for 50 Hertz. What exactly is 60 Hertz in a.c. power line circuits? What is the purpose of the 50-Hz reference markings?

-Chris H. Jones, Boone, N.C.

A. In order to transmit electric power efficiently over great distances, alternating current is used. In this country, the number of complete alternations per second has been standardized at 60 per second, or 60 Hz.

Having an accurate standard power line frequency is important because some devices, such as turntable motors and electric clocks, are sensitive to power line alternation frequency. If that frequency changes, the speed of rotation changes. (With d.c. servo motors and the like, some turntables are not frequency dependent.) Another reason why a standard power line transmission frequency is important is that this enables us to use that frequency to determine the correctness of turntable speed. Because of the constantly varying voltage on a power line, an electric lamp, especially fluorescent or neon, will flicker at the speed of the power line alternations. This fact is taken advantage of by the strobe markings found on many turntables. The spacing between any two successive dots is such that, when the table is rotating at the correct speed, the speed at which dots pass by an observer exactly corresponds to the flicker rate of the power line. Because of this fact, the dots appear to be standing still when the table is running at its correct speed, and appear to move when the speed is not quite accurate. The apparent motion is because the dot travels either a little more or a little less than the actual distance between two dots during the period between flashes of the lamp.

While it is universally recognized that alternating current offers the most efficient means of power transmission, the frequency of alternations used has not been agreed upon from one nation to the next. The United States and Canada have standardized their power line transmission rate at 60 Hz; much of the rest of the world has standardized on a power transmission frequency of 50 Hz.

Therefore, for a strobe card to be useful at 50 Hz, the spacing between successive dots must be greater than it is for 60 Hz, to correspond with the slower flicker rate of the 50 Hz power standards. You can see the differences in dot spacing when examining a strobe card having both sets of speed references.

If a turntable is to be sold worldwide, it is convenient for it to have both 50 and 60 Hz speed checks, so that no changes need be made when supplying the components for one country as opposed to what is required in another.

Additional Phono Cable

Q. I am rearranging my components and I find that I need three more feet to connect my auto turntable to the preamp. Can standard cables which come with the 'table, or, to the existing low-capacitance cables which come with the 'table.

(These are presumably for CD-4 applications.) The standard cable to which I am referring are the usual Switchcraft type as used between tape deck and preamp, or from tuner to preamp, etc.

Is it necessary to acquire the exact same capacitance cables for attachment as are now on the auto turntable?

-J. Paul Browne, Garden Grove, Calif.

A. If you are not using a CD-4 demodulator and cartridge, chances are there will be no degradation of performance with the extra cable. However, because the extra length does raise the possibility of hum pick-up, be careful as to how you route this extra cable. Keep it away from a.c. lines and well clear of speaker lines. In the latter instance, the problem is not one of hum but rather possible oscillation at high volume levels, especially where the treble is advanced.

If you are using a CD-4 system, then you probably will get into trouble by adding standard cables. In fact, even the addition of more low-capacitance cable might reduce the carrier signal to a level where the demodulator would not work properly.

High and Low Impedance Microphones

Q. I would like an explanation of the differences between low- and high-impedance mikes. What are their physical differences, and why the use of one as opposed to the use of the other?

-Thomas Young, Thomaston, Conn.

A. The impedance in low-impedance mikes ranges from 50 Ohms up to about 600 Ohms, while the high-impedance mikes go from about 25,000 Ohms up to several meg-Ohms.

High-impedance mikes suffer from degraded performance with their high frequencies drastically rolled off by any reasonable length of microphone cable, a 20-foot cable might have response down 6 dB at 10 kHz. A low-impedance mike may be used with hundreds of feet of cable with essentially no loss below 20 kHz.

The low-impedance mikes overcome this problem, and the higher quality public address systems were always designed to accommodate low-impedance microphones. As all solid-state input circuits tend to be of low impedance, these mikes are the best choice, even for the less expensive equipment.

While the outward physical appearances between the high- and low-impedance mikes are similar, a complete description of the various types would be too lengthy to go into in this column. Generally, however, ceramic and crystal mikes are of the high-impedance type, while the dynamic, ribbon, and electrostatic ones are usually of the low-impedance design.

At the present, the dynamic mike is the most popular. These employ a small diaphragm and a voice coil, similar to a dynamic loudspeaker, moving in an intense permanent magnetic field. Sound waves striking the surface of the diaphragm cause the coil to be moved in the magnetic field, thus generating a voltage proportional to the sound pressure at the surface of the diaphragm.

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, March 1977; Joseph Giovanelli)

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