[AMRadio] Class AB and B audio XFMRS


Larry Will lhwill at verizon.net
Mon Mar 13 13:24:47 EST 2006


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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