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by Joseph Giovanelli Definition of Terms Q. Please define the following: 1. Output Z in ohms; 2. damping factor. -James Leone Rochester, New York. A. 1. Z stands for the word "impedance". Thus, output Z in ohms means output impedance in ohms. 2. Loudspeakers are not perfect devices. When signal "hits" them, it takes their cones a certain time to get moving. When this signal is removed, it takes the cones time to stop moving. We would like to have our speakers start and stop immediately upon the removal or introduction of the signal. It just so happens that, if we could short the two speaker terminals together and still feed a signal into the speaker, the speaker cone would stop moving much more quickly than it would if the terminals were not shorted. This phenomenon is called "damping"; the ability of an amplifier to perform this task is a measure of the damping factor of the amplifier. The higher the number, the better will be the damping provided by the amplifier. Lightning Protection Revisited I have just finished reading your reply to Mr. James Boltz in the November, 1972 issue of AUDIO. I think your explanation of the operation of a lightning rod is in error. Lightning rods do not have points "because ... lightning is attracted to high, sharp objects" as you wrote. I think the operation of a lightning rod can best be explained as follows: The clouds become charged, either positively or negatively, by moisture activity until a region of charge is directly over an object on the earth, or the earth itself, and an opposite charge is induced in this object. Because unlike charges attract, the "desired state" of this system is to discharge. This occurs when the air dielectric breaks down and provides a conducting path for the charged particles. Thus, the lightning discharges. The function of the lightning rod is to keep the induced charge from building up in the ground object by allowing this induced charge to leak off to the air. This is accomplished by a pointed rod, well affixed to a ground source of charge. The point on the rod is necessary to provide the leakage. Because a charge always resides on the surface, the small surface of the point then provides a high charge density. Lightning rods do not attract lightning as you stated. Who wants to put up lightning rods to attract lightning; I would like to distract it. Most lightning discharges would disintegrate lightning rods, cable, and probably a house. -John Rich, Whitesburg, Kentucky High Resistance Grid Circuits Q. I have read that it is necessary to have a high resistance between the grid and ground of the initial preamplifier tube in a vacuum tube music system. Why is this necessary? -Name withheld. A. Any preamplifier will require some kind of return path for the grid circuit. The question here comes down to how high a resistance is needed for this return path. We see values ranging from 100 K to perhaps 10 megohms. I am assuming from your question that by "high resistance" you refer to those 10 megohm values. The reason that some input stages do use this extremely high value is because they operate on what is known as "contact bias." In such a circuit the cathode is grounded. The bias voltage is developed by cathode current flowing through this high grid resistance on its way to ground, thereby driving the grid negative with respect to cathode. The advantage of such a circuit arrangement lies in its economy. There is no need for a cathode biasing resistor and a cathode bypass capacitor. However, such contact biased circuits are subject to greater distortion than either cathode biased or fixed biased circuits. (Audio magazine, Aug. 1973) = = = = |
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