Audioclinic (Q and A) (Apr. 1974)

by Joseph Giovanelli

Sparking When Connecting an FM Antenna

Q. When connecting the twin lead of my outdoor antenna to the tuner, I noticed a spark when I made the connections.

By using a VTVM, I measured the voltage at my twin lead's terminals: zero volts. However, when I connected only the probe to the twin lead I got a reading of 4 V a.c. By reversing the 120 V a.c. wall plug polarity of the VTVM, I got zero volt with both the probe and the ground clip connected. Again, using the probe only to either side of the twin lead I obtained a reading of 30 V a.c.

The mast of the antenna is embedded in the earth at the side of the house. The twin lead runs perpendicularly to a telephone line and several cables of house wiring but is no closer than 6 feet from the twin lead. The twin lead is twisted to avoid pickup. My main concern is the sparking when I connect this twin lead.

-Serge Zamora; San Diego, California

A. I think your antenna problem has to do with the fact that your mast is grounded, as it should be. The way some antennas are designed means that at low frequencies (in this case, 60 Hz) the antenna is grounded, too.

If your tuner's chassis is connected to the a.c. via of bypass capacitors, and if the wall plug is inserted in the right polarity, the chassis will be just a bit "hot" with respect to ground. If the input of your tuner is connected to ground, possibly by a center-tapped, balanced input coil, then the two input terminals to which the twin lead would be connected, would also be "hot" with respect to ground.

Reversing the wall plug would eliminate this problem.

If you connect your voltmeter between your twin lead and chassis ground, you will see quite a bit of voltage. This is the same voltage which was present when you obtained the sparking.

Stray voltages are all around us, and the measurements you have described have to do with them. They are picked up capacitively by your twin lead. They are much like the voltage which you obtain merely by touching the "hot" probe of your a.c. voltmeter.

It is the difference of potential between the twin lead and the input of your tuner which accounts for the sparking. By grounding the chassis of your tuner, this voltage will be eliminated.

Phonograph Channel Separation

Q. I took my turntable to a local sound dealer who has a large electronic service department of good reputation. I asked to have it cleaned, lubricated and to correct its main problem. The signals from channel 1 and from channel 2 were not properly separated in my earphones.

His repairs corrected the problem so that it is not normally noticeable except when using a test record. The right channel still is heard in the left earphone just slightly and vice versa.

Should not one earphone be completely silent with such a test or is there some bleeding of one channel into the other?

-Nelson C. Vogel; Clearwater, Florida

A. No cartridge will produce perfect separation between channels. This is the reason why the data sheets which accompany a cartridge when it is purchased state the amount of separation between channels at various frequencies.

These separation figures are expressed in dB. If separation between channels was perfect, there would be an infinite number of dB separation. If only one channel was recorded on a disc, only one speaker or one headphone would be producing sound, assuming that we did have this infinite separation between channels. Because there is really a finite separation between channels, however, there will be some leakage.

Thus, when only one channel is re corded, signals will be heard in that channel, with some signal also heard in the channel having no actual recording.

The undesired, leakage signal will be lower than the desired signal in an amount equal to the separation between channels AT THE FREQUENCY OR FREQUENCIES OF THE RECORDING. To make things just a bit worse, the discs, themselves, are not perfect.

They have a limited amount of separation between channels, thus adding to the leakage you hear. It is a fortunate circumstance that we need only a few dB of separation in order to produce good stereo. Discs and cartridges in combination have far more separation than this "few dB."

(Audio magazine, Apr. 1974)

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