[AMRadio] Class AB and B audio XFMRS

John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Thu Mar 9 18:25:45 EST 2006

	I have been doing some experimenting here on the side and have found
a interesting thing that I've seen before but forgotten about.  I thought I
might share it here with you guys.

	When using a Class B or AB push pull output stage into an audio XFMR
such as Modulation XFMR or output XFMR for Speakers, there is of course a
cross over problem that must be contended with.  What many don't understand
is that the XFMR is the deciding factor for determining the lowest frequency
at which some distortion will NOT occur.  If the tubes are driven so that
peak current level is held constant for all frequencies, you will notice, as
the frequency of the driving signal is reduced you will find a frequency
point at which the output measured at the secondary begin to drop.  The
distortion that occurs in the shape of the signal is at this point is a
result of XFMR saturation and cross over distortion.  Reducing the bias and
drive will correct the distortion, at the expense of less output, but as the
frequency is reduced it will come back again.  Trying to pass frequencies
that are too low for your XFMR can cause intermodulation and harmonics, as
well as cause RF oscillations in the output of an audio amplifier or
modulator stage.  The distortion when viewed on a scope will appear as cross
over distortion.  It looks like a flat spot between the peaks.  It is there
at say 20 CPS but not at 100 CPS.  This is because the XFMR can hold and
release it magnetic flux energy over 1/100 of a second but it can not hold
it for 1/10 of a second.  It releases early and changes the shape of the
induced output wave on the secondary at the lower frequency.

More iron is better, have no or little gap in the E-I sections of the XFMR
and keep the current out of the secondary and balanced in the primary.

Hope this helps some one.


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