[AMRadio] Class AB and B audio XFMRS


Larry Will lhwill at verizon.net
Tue Mar 14 15:12:28 EST 2006


Bob and all,

Both of our discussions are correct but deal with a different part of 
the problem.  Core saturation is definitely one of the problems but 
hysteresis, the inability of the magnetic core to instantly change 
polarity is another.  The core saturation causes a spiking  due to 
field collapse as Bacon said but the hysteresis causes the "crossover 
distortion".  Both contribute to the overall distortion.  Higher 
quality iron and enough of it helps LF but the higher interwinding 
capacitance of the larger transformers can kill HF response.  Thats 
one reason many audio transformers can't go even to 10 or 20 kcs.




Nothing is simple.

Larry


At 03:47 PM 3/13/2006, you wrote:
>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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