Electronic Design

# Bob's Mailbox

Hi Bob! I have been reading and enjoying your stuff for many years! In this case (see "What's All This C-R Stuff, Anyhow?" Sept. 1, 2006, p. 18), I would say that capacitance has varied, not with frequency, but instead with applied potential. Various materials, especially ceramics, vary capacitance with applied voltage. (Yeah, but only a few percent... /rap) If we give cap time to charge up some, its capacitance can vary dynamically. (There may be some materials for which that is true, but most useful capacitors are fairly linear. I have taken some of these caps, with poor response and poor soakage, and charged them up to a lower voltage, and the shapes are just the same, scaled down. No kidding. Can I run them up to higher voltages? I'll try. I charged up a paper-and-oil capacitor to 10 V, shorted it out for a short time, and let go. The voltage went up to 2 V. There must be a lot of dipoles twisting around in that oil! If I charged it up to +100 V, could it charge to more than 20 V? It couldn't go much higher, or it would face some laws of conservation of energy. How about 1000 V? Can you name a material for which the errors scale differently? Most ceramics have minimal voltage coefficients. /rap) Above the R-C rolloff, the voltage across the cap does not vary, because the signal is being conducted across without much charging. (We agree. /rap) And way below rolloff, the signal does not get through at all, so we only see the change at rolloff. But we should be able to characterize it by measuring capacitance (perhaps with tiny sine waves) while varying a much larger dc bias. (Many capacitors will not change much with dc bias, but maybe some will. I'll have to check it. Thanks for raising the question. /rap)
Terry Ritter
Pease: I just tested every capacitor in my lab by putting a 20-V p-p square wave across them at 120 Hz. Even the ceramics were silent! Every one was silent. But there was an "Audiocap" that emitted some noise. You'd think that a \$20 capacitor wouldn't have a flaw like that.

John M. Cook

Dear Bob: I thought the guys with the golden ears couldn't stand to listen to any amplifier other than a single-ended vacuum triode with a non-thoriated direct-heated filament cathode, preferably with no capacitors or transformers, and no feedback, playing vinyl discs—and don't forget the \$1500 oxygen-free water-shielded litz speaker wires.
Ken Lundgren
Pease: Actually, my friends in the know say that electric heating of the cathodes is obsolete. Coal-fired pentodes take a little more time to warm up, but they sound warmer. And, do you know how many 6AS7GAs it takes, to just drive the grids of all the 6AS7GAs you need, to drive from their cathodes into the 4-Ω speaker? Lots and lots!

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