[AMRadio] Class AB and B audio XFMRS


Bob Bruhns bbruhns at erols.com
Mon Mar 13 15:47:02 EST 2006


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


----- Original Message ----- 
From: "Larry Will" <lhwill at verizon.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>;
"'Discussion of AM Radio'" <amradio at mailman.qth.net>
Sent: Monday, March 13, 2006 1:24 PM
Subject: RE: [AMRadio] Class AB and B audio XFMRS


> Hi all,
>
> As I dimly remember from Motors and Machines 1 and 2,
THE TRANSFORMER
> IS A HIGHLY NON-LINEAR DEVICE.  WE USE THESE SIMPLE
FORMULAS for TR
> and ZR but in reality YOU NEED ADVANCED CALCULUS TO
adequately
> explain BOTH THE Hysteresis and eddy current losses
and
> distortions.  The open circuit and short circuit
tests can get that
> info out of a particular device, I haven't done these
in years but it
> is a valuable tool for analyzing any transformer.
You need power and
> the correct frequencies however.  Its the hysteresis
losses that
> result in the poor LF response.  The secondary
voltage lags the
> primary current (I think I remember that correctly)
more and more as
> the frequency is decreased.  The problem is
especially acute at the
> polarity change, ie the zero crossing where the
magnetic field must
> reverse instantly.  The non linear effects generate a
discontinuity
> in the waveform and the harmonic components and odd
phases are the
> result.  What is happening the primary power is
converted to a
> magnetic flux which is then converted back to power
in the
> secondary.  Its the medium - the IRON that causes the
problems.  This
> cannot be analysed except by non-linear mathematics
Messy at
> best.  The DC current, if present polarizes the
magnetic field making
> the effects worse.  MacIntosh got around this with
his patented
> transformer and circuit which greatly minimized the
magnetic
> non-linearity, circuits which are still used in
MacIntosh audio amps today.
>
>
> BTW audio analog tape recorders minimized this
problem with the head
> by using a high frequency bias, say 22 kcs, to keep
the flux
> constantly changing and allowing good LF response
while making the
> recording.  The HF signal is filtered out on playback
either by the
> playback head or immediately before the first preamp.
>
> Larry W3LW
>
> Some folks on here surely can amplify this and
correct my fuzzy
> memory if needed.
>
>
> The problem is the energy transfer medium - THE IRON.
>
>
>
> At 01:02 PM 3/13/2006, John E. Coleman (ARS WA5BXO)
wrote:
> >         Perhaps I should clarify one point that we
may all be forgetting
> >here.  A XFMR will only transfer energy during the
movement of the magnetic
> >field (EXPANDING OR CONTRATING).  If the magnetic
field becomes stationary
> >then no energy will be transferred to the secondary
regardless of the amount
> >of iron.  But if the magnetic movement is fast
enough then transfer
> >efficiency can be high.  As the frequency is lowered
the magnetic movement
> >is slowed down then the efficiency drops off.  I'm
not sure if this is the
> >proper term mathematically but it is as if the
coefficiency of coupling is
> >not as good when the frequency becomes too low.
> >
> >         I hope I'm not boring folks with this and
some may say I am making a
> >mountain of a mole hill.  I just find it
fascinating.  I guess it is just my
> >type of thing.
> >
> >John, WA5BXO
> >----Original Message-----
> >From: amradio-bounces at mailman.qth.net
> >[mailto:amradio-bounces at mailman.qth.net] On Behalf
Of John E. Coleman (ARS
> >WA5BXO)
> >Sent: Monday, March 13, 2006 11:36 AM
> >To: 'Discussion of AM Radio'
> >Subject: RE: [AMRadio] Class AB and B audio XFMRS
> >
> >The Band pass and energy Xfer of a transformer with
no load is one thing but
> >it all changes depending on the load and the DC
involved.  In class A
> >balanced PP circuits the XFMR will still saturate at
some frequency and load
> >even if it is perfect balance on the PP circuit.
XFMR saturation distortion
> >in class A single ended service has a trapezoidal
shape if the quiescent
> >current is too high but in PP class A the shape is
weird because the XFMR
> >remains balance as for as DC is concerned but yet
the XFMR will not produce
> >the sign wave on the output if the frequency is too
low.  It resembles cross
> >over distortion even though there is no cross over
in class A push-pull.
> >
> >
> >
> >
>
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