Audioclinic (Q and A) (Jan. 1973)

by Joseph Giovanelli

Hum in TV Audio

Q. My problem is in attempting to hook the audio output of my new Motorola Quasar TV set into the rest of my audio system. The TV dealer tried to cooperate with me by installing a phone jack on the back of the TV set.

He took the audio signal from the volume control, the idea being to feed in a signal into my Dynaco PAT-4 preamplifier, which would be constant and not affected by the volume control.

When I attempted to use this arrangement, I got an unpleasant buzzing along with the TV audio. It sounded as though the set was not properly grounded. I discussed this problem with the dealer and he said that the ground was "incompatible." This does not explain much to me.

By experimenting I find that I can run the output of the TV into my Revox A77 tape recorder with no problem as long as the recorder is not connected in any way to the rest of the audio system. I can record from the TV set as long as the recorder is not connected to the system.

I hope you will be able to give me some suggestions as to what to do so that I can plug the TV into the Dynaco preamplifier.

-Leonard F. Blanchard, Clayton, N.Y.

A. I do not know exactly what TV set you are discussing. (There are a number of models which carry the Quasar designation.) Once I had this information I would then need a schematic which might shed some light on your problem. Therefore, what follows is based on past experience with TV sets in general.

Any television receiver has a number of circuits, which, when improperly adjusted, will enable video to enter the audio circuit; this will be heard as a buzz.

It could be that you have a set with a "hot" chassis, one having no power transformer. If this is the case, you stand a chance of damaging some components as well as introducing audible hum when attempting to connect this TV set to your sound system. You will require an isolation transformer capable of handling at least as much power as is required by your TV set, plus a reasonable safety margin. If all of these assumptions are correct, the addition of this transformer may solve your problem immediately.

If the hum still persists, try connecting the chassis of the TV set directly to the chassis of your preamplifier. Connect the shield of the interconnecting cable only at the preamplifier input. The other end of the cable should not be connected. Use No. 16 wire to interconnect the two chassis. (This wire will serve to carry the signal and not the shield.) If the hum is still present or if the audio of the TV set no longer operates as it should, the audio input circuit is not standard. I would then need to see a schematic of the set before proceeding further.

Dear Mr. Giovanelli:

Thank you for your letter regarding the problem I had in attempting to connect the audio output of my new TV receiver through my high fidelity music system. I have the problem solved.

I first tried a good-size isolation transformer as you suggested. I plugged it into the a.c. wall outlet and then plugged the TV set into the transformer. This entirely solved the problem.

A transformer of enough capacity to handle the TV set's requirements plus a safety factor was rather costly. I tried another approach. I got a small matching transformer and used it in the audio line only. This also solved the problem.

Now I am using the inexpensive matching transformer rather than the heavy isolation transformer. I plugged the output from the matching transformer into the high level input of a Shure 4-position mike mixer, which also enables me to add reverb. I am now getting very good sound from the TV set, and the added reverb further improves it ... at least I like the reverb.

The mixer, in turn, plugs into one of the inputs of my Dynaco PAT4 preamplifier. I will not carry the matter any further, as it is now commencing to sound like "the ankle bone is connected to the foot bone," etc.

EDITOR'S NOTE: The use of an isolation transformer is indeed an excellent solution to the problem. The television set must, however, never be grounded to a radiator, electrical outlet box, earth ground, etc. Grounding of this sort must be done only when an isolation transformer is used. (I am, of course, assuming that the television receiver in question does not employ a power transformer.) The impedance at the volume control of tube-type television receivers is of an extremely high value. It would be difficult, if not impossible, to find a matching transformer which could bridge this audio line without seriously loading it down. Therefore, I suggest that matching transformers be used only in the case of television receivers employing transistorized audio circuitry. The primary impedance of such a matching transformer should be approximately 10 to 20 K ohms; the secondary's impedance may be 10 K ohms. However, this will depend on the application. If the input impedance of the stage driven by the set is 600 ohms, the matching transformer should have a secondary impedance of this value. If the input impedance of the device is high, 100 K or so, the impedance of the transformer can be quite high. 50 K is not unreasonable here. Because the output from solid state audio circuits (TV) is low, the transformer will provide added voltage.

(Audio magazine, Jan. 1973)

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