[AMRadio] Class AB and B audio XFMRS

John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Mon Mar 13 19:44:14 EST 2006

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 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

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