Refrigerator Interference
Q. I recently bought a small refrigerator which produces so much interference that I cannot listen to my music system. The refrigerator starts every ten minutes and runs for two minutes. When it starts, I hear a noise. This noise isn't too bad, however. When the refrigerator stops, the noise is as loud as a circus whip.
I am living in an old house on the third floor without a ground wire. All outlets on the second and third floors are on the same circuit. Do you have a solution to this problem?
-John Hadzevich, Toronto, Canada.
A. Unless you can get at the contacts of the thermostat of your refrigerator there is probably not much you can do to solve this problem.
As you know, the operation of a refrigerator is controlled by contacts which are operated by a thermostat. Gas pressure acting on a diaphragm causes the switch to complete or disconnect a circuit, depending on the temperature of the box. Perhaps the gas pressure in your particular thermostat is not sufficient to cause the contacts to open quickly. If this is the case, your thermostat contacts will soon be permanently welded closed. I realize that your refrigerator is new, but you still might have a thermostat problem.
Assuming that the thermostat is in good condition, you may be able to shunt the contacts with a series combination of a resistor and capacitor. The capacitor will probably have to have a value of 0.05 µF. The resistor would have to have a value of no less than 100 ohms. The voltage rating of the capacitor should be 600 volts. The resistor can be a half-watt unit. You will have to experiment with the value of this capacitor until you arrive at a capacitance value, which suppresses the noise. Apparently when your contacts open, they arc. The purpose of this network is to absorb a portion of this arc voltage.
You might try grounding your refrigerator to your outlet box. This sometimes helps to a degree. Because so many circuits are fed from one line, it might be that the stopping of the refrigerator results in a dramatic rise in power line voltage. The suddenness with which this rise takes place could cause the transient clicks you are hearing.
Perhaps you should tell the dealer from whom you bought the refrigerator about the trouble that you are having, or you might write directly to the manufacturer of the unit.
I have not had a great deal of success in solving problems of this sort. It might be that another reader will have some observations which will be helpful. I will welcome the chance to hear from such a person and will print the material if it proves relevant.
Fig. 1--Click filter circuit. The resistor may have to be increased to 100
ohms.
Amplifier "Pops"
Q. When my amplifier is turned off: with or without an input source feeding it, my speakers produce a loud "pop." Is the “pop" considered normal with solid state equipment? If not, what is a solution?
- John Hanley, Woodside. N.Y.
A. From time to time I run into an amplifier which exhibits a "pop" when it is turned off. This has to do with the sudden interruption of the a.c. I think that this transient enters the amplifier as a pulse and becomes audible. I do not believe this is a matter of whether the amplifier is solid state or not. I have seen both tube and solid state gear exhibit this "pop." You might try bypassing the switch with a series combination of a 0.02 uF capacitor and a 200-ohm resistor. When the switch opens. this combination will act as a slight buffer to reduce the sharpness of the transient, and we hope, will eliminate the "pop" for the most part. (Sec Fig. 1.)
Letter on Thumping
From time to time I receive items which might prove interesting to my readers. The following is a case in point:
Quite a few issues ago, a reader asked you a question about a heavy thump when switching on powerful transistor amplifiers.
I have found a method of appreciably reducing it. I have been using this system now for a number of weeks without noticing any audible drop in power output. Here it is, for the benefit of other sufferers like myself:
Buy a so-called "tube saver" (300 watt type is adequate) and plug it between the power line and the line-cord of the power amplifier. The tube saver consists of a temperature-dependent resistor with a negative temperature coefficient (carbon, etc.). When it is used, there is an appreciable drop in line voltage at the amplifier. The voltage builds up gradually (few seconds) to a value about two volts lower than without the resistor. This gradual rise of the applied line voltage allows the electrolytic blocking and filter capacitors to charge slowly. The desired effect of a less violent "thump" is thus achieved at a cost of only about $2.50.
I hope that other solid state amplifier users will find this suggestion helpful.
-Alex Azelickis. Morton Grove, Illinois.
Treble Cleanness and Volume
Q. On the amplifiers to which I have listened there seems to be a "kick-in" point on the volume control, where the treble becomes clearer and not so subdued.
It does not matter what other components were used. It happens all the time. I usually have the volume past the "kick in" point to truly allow me to enjoy the music. Why do amplifiers do this?
-Sgt. Dennis Z. Muller. APO. San Francisco, California.
A. In most amplifiers the treble does not increase with respect to overall sound as the volume is raised. However, subjectively, the human ear perceives more treble and more bass, too, with respect to the middle frequencies, as the volume of sound is increased. Above a certain point the ear does not observe a further buildup of treble. This situation is a phenomenon of human hearing. It is not a problem associated with amplifiers. Therefore, if you wish to hear more treble, but not at so high a volume, advance the treble control to suit your needs. I notice that many of us tend to leave the tone controls on "flat." However, they were designed into the equipment to be used for just such situations as yours.
(Audio magazine, Mar. 1971, Joseph Giovanelli)
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