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Page 15 |
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2003 |
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Copyright © 2003 John Broskie All Rights Reserved |
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But the actual point of interest was the choice made to use Radio Shack carbon-film resistors; surely, that was something new and bold. Topology can exist without parts, just as geometry can exist without stone or wood or steel or plastic objects. For example, prior to the transistor actually being created, a schematic could have been drawn, and most certainly was drawn, that displayed valid topologies that used a part that hadn’t yet existed. If you wish for a more abstract analogy, think sentence structure: “Man bites dog” is structurally no different than “Man bites cat,” no matter what the dog and cat think. Grammar tells us what is possible; rhetoric, what is desirable and effective. Basic electronic theory tells us what can be built that will work; electronic design practice, what to expect in terms of sonics, reliability, efficiency. And while marketing can accurately tell us what will sell, (basically, what we want to believe) only a fool would try to learn basic electronic theory from ads in glossy magazines. |
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Cars and Planes Returning to the analogy of the airplane and a car, imagine that your neighbor returns from England with — as he was happy to tell you in a postcard he sent ahead — “an airplane.” Soon after he returns, he drags you over to his house and opens his garage to reveal…(a suspense building pause here)...a car. Yes, a Honda Accord, just like yours: same year, color, wheels and tires, sunroof and trim. You’re confused and ask where the plane is. He replies, “you’re looking at it; isn’t she a beauty?” He insists it’s a airplane and will not be persuaded otherwise. You ask him for the specs on his “airplane”: engine displacement, number of cylinders, miles per gallon, top speed, 0-to-60 time, braking distance and turn radius — and every answer he gives matches exactly your car next door. Finally you ask him why he thinks his car is an airplane. His answer is that it said so in the brochure and the salesman assured him it was...besides the personalized license plate reads “Not A Car.” |
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When asked if he has ever flown this airplane, he smugly replies, “Sure, lots of times.” But when asked what altitudes he has attained, he says that he doesn’t really know; and besides his airplane cannot be tested by normal means, as it is a very special design; so much so that the normal measurement techniques, such as hanging a measuring tape outside the window, just don’t work; the salesman was quite adamant about that. You cannot stomach any more and you drag your neighbor to your garage and show him your Honda Accord. “How is your airplane different from my car?” you plead. He then points out, in a tone usually reserved for small children, that your mere car has its steering wheel on the left side, whereas his airplane has its steering wheel on the right side. While you stagger from his answer, he says, “Besides, do you really think that I would be stupid enough to pay $70,000 for a Honda Accord?”
Recap We have seen that we can readily create at least six topological variations out five main components: two large power supply capacitors (each capacitor represents a voltage source, such a single floating power supply or the one half of a floating bipolar power supply), two tubes, and a signal reference. Other topological variations could be had simply by shifting the reference to the top or bottom of each of the circuits shown on the following page, but something has to be left others to invent and patent. Circuits with their reference at the center of the output, the Ultratron (C) and Circlotron (D), are the most confusing, but they are not physics-defying because of it (advertising copy and internet buzz to the contrary). Which is better? Better for what? The circuit with one tube atop the other, the Ultratron (it would teach me a lesson if this joke of a name stuck. So preemptively, I will christen it the “vertical center-referenced push-pull amplifier”) offers the advantage of using a single center-tapped power transformer (readily available) per channel. As the power supply the power supply must be left float, two channels cannot share the same power transformer, unless the transformer were decoupled via two center-tapped chokes. An added plus might be that this topology lends itself more readily to separate idle current and DC offset controls, as one potentiometer would adjust the current through both tubes by varying the bottom triode’s conduction and the other potentiometer would adjust the DC offset via the top triode’s cathode voltage. |