[AMRadio] Class AB and B audio XFMRS

Bob Bruhns bbruhns at erols.com
Mon Mar 13 20:40:43 EST 2006

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)"
<wa5bxo2005 at pctechref.com>
To: "'Discussion of AM Radio'"
<amradio at 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: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net] 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
> They thought it should behave symmetrically.  That
> if DC passing through a coil produces a fixed
> field, they thought that a similar fixed magnetic
> 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,
> But no matter what he did, no DC came out of his
> Finally the great scientists had had enough.  I
> 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
> 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
> from the winding.  That means that the longer a
> wave needs to hold positive, the more flux there has
> 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
> 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
> of output power.  But as the frequency decreases, the
> magnetic flux needs to go higher and higher to
> 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
> 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
> and then saturates in the opposite direction.  This
> produces a pulse, followed by a drop to zero volts
> 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
> a positive pulse where we should have seen the signal
> waveform rising.  And unbalanced DC tends to make
> 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
> that or less gap than that is not as good.  This
> 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
> some point.
>   Bacon, WA3WDR
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