AUDIOCLINIC (Q and A) (Apr. 1975)

Connecting Several FM and TV Sets to One Antenna

Q. My problem is this. My signal source is a community TV/FM 75-ohm cable. I want to connect a black-and-white TV set, a color TV set, and an FM tuner to this cable. I now have them hooked as follows: The cable terminates in a 75-to-300 ohm adaptor and two-way splitter. The main output of the splitter goes to the black-and white TV set and the other output of the splitter is connected to the input terminals of another two-way splitter, with one set of output terminals connected to the FM tuner, and the other set to the color TV set. The black-and-white set works very well.

The color set, however, is adversely affected by the black-and-white TV set when changing from one channel to the other. This causes snow and even a loss of color on the lower channels when the black-and-white set is switched to higher ones.

Please tell me the proper way to straighten this out. I think I need a 75 to-300-ohm adaptor with three outputs, but 1 cannot locate one. I also wonder if a signal booster is called for in this situation.

-Keith J. Webster; Elko, Nevada

A. Your problem will be best solved by using a transformer to step up the 75-ohm impedance of the cable to the 300 ohms which is required by each of the various pieces of equipment you wish to connect to the cable. Chances are that this transformer will have but one 300 ohm set of terminals. You then must obtain a three-way coupler by which you can interconnect the sets. It would appear that your present signal splitters are not much good or you would not get the interaction between sets. The whole purpose of a signal splitter is to avoid just such problems as those you mentioned.

Obtain the best possible transformer and coupler because you do lose some signal in each of these devices. By the time signals reach the various sets, 4 to 6 dB of signal will be lost. This loss is not too serious if the strength of the signals is high, and where you have good isolation between sets, as is supposed to be accomplished by the splitter.

Playing Old Mono Tapes

Q. I have several 12-year-old reels of tape recorded from FM using a mono recorder. What type of track arrangement currently available should I use, 4-track stereo or two? What would one sacrifice with each kind, soundwise?

-Edward J. Sampson, Jr.; Andover, Mass.

A. You will get better results with a half-track (2-track) stereo head when playing mono tapes. The difference will be a 3 dB higher signal-to-noise ratio, because the longer half-track gap will pick up more signal than the quarter-track gap. On the other hand, using a half-track playback head, there is the possibility of poorer treble response owing to imperfect azimuth alignment. The longer the gap, the greater the treble loss for a given degree of azimuth misalignment between the head used in recording and the head used in playback.

Editor's Note: In his February Audioclinic column, Mr. Giovanelli stated that "the output impedance of tube ... preamplifiers is generally so much higher than the input impedance of transistor units that the signal output from the tube unit is inadequate to drive the transistor component." The Audio Research folks ask us to note that their tube preamp has an output impedance low enough to drive all current transistor amps, and ranges from 50 to 600 ohms depending on tone control setting.

(Source: Audio magazine, Apr. 1975, JOSEPH GIOVANELLI)

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