[AMRadio] Class AB and B audio XFMRS

John Coleman ARS WA5BXO wa5bxo2005 at pctechref.com
Sun Mar 12 23:16:54 EST 2006

Very interest Jim:
On magnetic bucking:	
	I guess just a few turns of some heavy wire would suffice and
the bucking current could be many amps to compensate for the fewer
turns.  But the DC CCSource would need to be fee of HUM or any
variations else it would modulate the secondary with a step up.  Perhaps
more turns in the bucking coil would be better. It is an interesting
concept though.  	

On LOW freq distortion in the output XFMR:
	I found it interesting, how the cross over distortion reappeared
(in my PP KT88s CLASS AB1) amp as the Frequency of the driving signal
when down (talking below 20 CPS).  It shows the need to limit the low
end of the pass band in the preamps to a value that will coincide with
the pass band ability of the output XFMR.  Accidentally hitting the
grids with too low of a frequency while trying to pass high frequencies
at the same time caused not only distortion in the lows but it modulated
the higher frequencies with the distortion.  And it really sounds bad!
Limiting the low frequency pass band in the preamps really cleaned up
the audio.  You might ask what low frequencies we are talking about.
It's the 4 - 10 CPS stuff that comes from turn table rumble.  On class A
XFRMerless solid state audio the speakers just moved with the rumble
(yes, a total waste of power and could damage the speakers).  But with
the tube equipment it created quite a stink with intermodulation right
in the output XFMR.  I am sure that some of the bad stuff we hear some
times on the air is due to incomplete modifications where the preamps
and drivers of the audio equipment have been modified to pass 20 to 30
CPS but the modulation XFMR circuitry can not pass below 100 with out
intermodulation occurring in the XFMR due to core saturation and/or
cross over distortion.

John, WA5BXO

More information about the AMRadio mailing list

This page last updated 15 Dec 2017.