[AMRadio] Class AB and B audio XFMRS

Jim candela jcandela at prodigy.net
Sat Mar 11 19:34:52 EST 2006

Geoff, W5OMR said: More iron is better.

Reply by Jim, WD5JKO:

  Well not exactly. We are only part done when we take our mod transformer
that has a gap in it, and pass the DC through a choke instead of the
modulation transformer secondary. To get the full benefit from this we need
to eliminate the gap. If the mod transformer had E & I laminations with all
E's together, and I's together, then if possible, remove these E & I's and
then interleave them. This will increase the inductance of the windings a
bunch, and allow the same amount of iron to work down to a lower frequency
for the same amount of power. The P-P load impedance at the low end will be
better too, so distortion will go down as a result. In the example given
with the single ended audio output xfmr, the primary inductance (with gapped
transformer) only increased about 30% by removing the DC current from the
CCS. Combine this with eliminating the gap, and the inductance would go up a
whole lot more.

   Now instead of using a zillion Henry choke to feed the DC current, and
capacitive couple the audio as Geoff is doing, why not go back to that
gapped mod transformer, and add an additional winding, and use a power Fet
CCS (constant current source) to buck the DC field? If done well, maybe the
gap can be lessened, or eliminated? Sure the FET CCS would need to be nearly
ideal from DC to at least 5Khz, or it's mere presence would introduce

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of W5OMR/Geoff
Sent: Saturday, March 11, 2006 5:28 PM
To: Discussion of AM Radio
Subject: Re: [AMRadio] Class AB and B audio XFMRS

Jim candela wrote:

>John E. Coleman (ARS WA5BXO) wrote:
>>	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
>>might share it here with you guys.
>Geof W5OMR Wrote:
>I can't believe that no one else has replied to this, on the reflector.
>From: "Patrick Turner" <info at turneraudio.com.au>
>Subject: Re: SE Output transformer design idea
>Date: Monday, August 25, 2003 8:06 PM
>Jim WD5JKO pasted in the following:
>This from an old thread about single ended audio transformers, and how some
>folks deal with the high DC flux from the class A audio output tube. In
>respects this is the same problem we AM'ers face with push pull modulators
>where the mod transformer has to bear the RF final plate current without
>core saturation. Pay attention to the stuff about using a DC constant
>current source into one winding to buck the flux from the Class A output
>tube plate current. It may not be practical, but it sure is interesting...

What I surmise about their discussion is that they've basically came to
the same conclusion that John has...

More iron is better.  For us, that means we need more iron at the Class
B Modulator level.  All of the Henries of reactance wont matter, if the
modulation transformer iron isn't suffiecient enough.  Not necessarily
for passing low frequencies.. rather, having enough iron in the
transformer, to not have to worry about cross-over modulation, ergo,

Good post, Jim!  I enjoyed it, and am looking for more.

73 = Best Regards,

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