Heh heh, I hate typos!!! Forget all this technical stuff, let's make "I Hate Typos" T-Shirts.
Yes, by all means John, I'll fix the spelling and send you a copy. Now, where's my dictionary... Must be with my glasses... Bacon, WA3WDR ----- Original Message ----- From: "John E. Coleman (ARS WA5BXO)" <[EMAIL PROTECTED]> To: "'Discussion of AM Radio'" <amradio@mailman.qth.net> Sent: Monday, March 13, 2006 7:44 PM Subject: RE: [AMRadio] Class AB and B audio XFMRS > That is a perfectly written answer Bob. May I use it on my web site? Do you > want to have Geoff, W5OMR, check it for spelling first? HIHI > > John, WA5BXO > > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of Bob Bruhns > Sent: Monday, March 13, 2006 2:47 PM > To: Discussion of AM Radio > Subject: Re: [AMRadio] Class AB and B audio XFMRS > > Electromagnetism really confused the early scientists. > They thought it should behave symmetrically. That is, > if DC passing through a coil produces a fixed magnetic > field, they thought that a similar fixed magnetic field > should produce DC from a coil. This would have been > every cool, because they had permanent magnets from > which free power could have been derived. > > The problem was, it didn't work that way. There is a > story about how this problem was solved. Michael > Faraday was trying everything; he held a magnet in > every possible place around a coil, he tried holding > the magnet at every possible angle and direction, etc. > But no matter what he did, no DC came out of his coil. > > Finally the great scientists had had enough. I imagine > him standing up, cursing, and throwing the magnet > violently at the coil, in anger. > > But something happened when he did that. The > galvanometer twitched when the magnet passed through > the coil! Faraday had discovered that the magnetic > field needed to be changing in order to produce a > voltage from the coil, and the output voltage would > alternate. (And this sort of comedy has been entirely > typical of the process of scientific discovery from > earliest antiquity.) > > OK, now about an audio transformer. The flux must be > changing in one direction to produce a steady dc output > from the winding. That means that the longer a square > wave needs to hold positive, the more flux there has to > be in the core. Even without unbalanced DC in the > windings, the core will saturate at some point. This > places a limit on the lowest frequency square wave that > can be produced at any given power level. > > The situation with sine waves is similar. At high > frequencies, the alternating flux does not have to > build up to very high levels to produce a given amount > of output power. But as the frequency decreases, the > magnetic flux needs to go higher and higher to maintain > the necesssary rate of change over the slower cycles, > in order to produce the necessary voltage and power > output. And at some point, the core runs out of > magnetic capability. > > When that happens, the flux can not continue to rise. > It can only hold steady until the applied current > falls. The coil can not produce DC in this situation, > and the output voltage falls to zero and sits there > until the current falls, which happens at the next > crossover. At the crossover, the magnetic flux changes > and then saturates in the opposite direction. This > produces a pulse, followed by a drop to zero volts and > another flatline. So we get a flat line where the > signal should have had a positive peak, we get a > negative peak where we should have seen the signal > waveform falling, and we get another flat line where > the signal should have had a negative peak, and we get > a positive pulse where we should have seen the signal > waveform rising. And unbalanced DC tends to make this > happen with an offset. > > So. A given transformer can handle more power at > medium and higher frequencies than it can at low > frequencies, and the situation gets worse when > unbalanced DC is applied. Unbalanced DC is bad news, > because it builds the core up to significant magnetic > flux levels. > > It turns out that for a given amount of DC magnetic > flux, there is an optimum "gap" that produced the > maximum efect a given core can produce. More gap than > that or less gap than that is not as good. This gives > less inductance than no gap, but the inductance > survives unbalanced DC better, so it's a winning > compromise. But if there will be no unbalanced DC in > the winding, then we want to eliminate the gap. That > gives us more inductance from a given winding, which > gives better low frequency response. But remember, > those lows will saturate the core all by themselves at > some point. > > Bacon, WA3WDR > > > > > _______________________________________________________ _______ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:AMRadio@mailman.qth.net > AMfone Website: http://www.amfone.net > AM List Admin: Brian Sherrod/w5ami, Paul Courson/wa3vjb >
