[AMRadio] Class AB and B audio XFMRS


Bob Bruhns bbruhns at erols.com
Thu Mar 16 11:12:10 EST 2006


Magnetic hysteresis is a complicating factor.
When you apply a magnetic field to a magnetic
material, it is partially magnetized.  When you
saturate it, it is strongly magnetized.  When it
is magnetized, it opposes the application of a
reverse magnetic field, and it takes a certain
reverse applied field to overcome the residual
field in the material, and it takes a little more
reverse applied field to reverse the residual
field.  This causes a timing delay in the magnetic
transitions.  The amount of delay depends on the
applied signal level.

Even below saturation level, hysteresis causes
increasing distortion as level increased, because
the residual magnetization is greater at higher
levels of applied magnetic field.  At the same
time, the magnetic transfer of the core falls off
at high levels, introducing more distortion.  This
is why bigger iron and better iron are favored.
But the windings are important too.

Issues with transformer windings can really affect
high frequency response.  Magnetic coupling of
outer windings is not as good as the coupling of
the inner windings, and different "leakage"
inductance results.  This, and different amounts
of stray coupling between windings and to ground,
cause high frequency rolloff, and it causes
different windings to have slightly different
frequency responses and throughput time delays.

In class-B operation, these effects cause
distortion of high frequency signals.  Half of the
waveform is delayed more than the other half, and
you get asymmetrical crossover distortion. This
distortion sounds terrible, especially on
sibilants.  The two halves of the waveform may
have different amplitudes as well.

I have improved this distortion by adding some
capacitance to the faster side at the driver
level.  This tends to balance the delay and
eliminate this effect.  When a driver transformer
is used, reversal of the grid connections may help
by making the unequal delays of the driver cancel
those of the output stage.  The plate connections
to the driver transformer can also be reversed to
maintain the original audio polarization, if
necessary.

Eddy currents in the core absorb high frequency
energy.  This is why we 'laminate' the core,
slicing it into thin layers, and coating them with
insulating paint.  The thinner the layers, the
less high frequency eddy current loss there will
be.  This is why audio transformer laminations are
usually thinner than power transformer
laminations.

In analog magnetic recording, hysteresis causes a
huge crossover gap that must be compensated, or
unbelievable crossover distortion results.  Modern
analog recorders use about 100 KHz AC bias,
adjusted slightly higher than required for maximum
recorded output, to achieve minimum crossover
distortion. Typically this overbias reduces
sensitivity just a little bit at 1 KHz , about
0.25dB.  However, it reduces higher frequencies
more strongly, about 2dB at 10 KHz.  This is
equalized out during recording.

  Bacon, WA3WDR


----- Original Message ----- 
From: "Larry Will" <lhwill at verizon.net>
To: "Discussion of AM Radio"
<amradio at mailman.qth.net>
Sent: Tuesday, March 14, 2006 3:12 PM
Subject: Re: [AMRadio] Class AB and B audio XFMRS


> 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.
> > > >
> > > >
> > > >
> > > >
> > >
> >
>_________________________________________________
_____
> >________
> > > >AMRadio mailing list
> > > >Home:
> >http://mailman.qth.net/mailman/listinfo/amradio
> > > >Help: http://mailman.qth.net/mmfaq.html
> > > >Post: mailto:AMRadio at mailman.qth.net
> > > >AMfone Website: http://www.amfone.net
> > > >AM List Admin: Brian Sherrod/w5ami, Paul
> >Courson/wa3vjb
> > >
> > >
>
>_________________________________________________
______
> >_______
> > > AMRadio mailing list
> > > Home:
http://mailman.qth.net/mailman/listinfo/amradio
> > > Help: http://mailman.qth.net/mmfaq.html
> > > Post: mailto:AMRadio at mailman.qth.net
> > > AMfone Website: http://www.amfone.net
> > > AM List Admin: Brian Sherrod/w5ami, Paul
> >Courson/wa3vjb
> > >
> >
>
>_________________________________________________
_____________
> >AMRadio mailing list
> >Home:
http://mailman.qth.net/mailman/listinfo/amradio
> >Help: http://mailman.qth.net/mmfaq.html
> >Post: mailto:AMRadio at mailman.qth.net
> >AMfone Website: http://www.amfone.net
> >AM List Admin: Brian Sherrod/w5ami, Paul
Courson/wa3vjb
>
>
__________________________________________________
____________
> AMRadio mailing list
> Home:
http://mailman.qth.net/mailman/listinfo/amradio
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:AMRadio at mailman.qth.net
> AMfone Website: http://www.amfone.net
> AM List Admin: Brian Sherrod/w5ami, Paul
Courson/wa3vjb
>




More information about the AMRadio mailing list

This page last updated 15 Dec 2017.