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
