[AMRadio] Class AB and B audio XFMRS


Bob Bruhns bbruhns at erols.com
Mon Mar 13 06:54:11 EST 2006


Actually you would want an off-center tap to B+
with the single-ended Class-A technique.  The
modulator tube should be closer to the tap and the
output should be further away.  That way you can
get 100% or more modulation.  You would finagle
the turns ratio and modulator bias for DC balance
and the desired modulation.  Assuming you have
asymmetrical audio, positive peaks would peak at
more than 2X unmodulated quiescent, but the
negative peaks would only go down to zero.  It's
probably not too difficult to do.

But actually, a series modulator would be better
way to go.  One of those QIX sorta-class-B
thingies if you want analog modulation.  No big
audio transformer, caps and chokes, etc, in the
way.  Or some switched modulator if you want to go
digital.

  Bacon, WA3WDR


----- Original Message ----- 
From: "Jim candela" <jcandela at prodigy.net>
To: "Discussion of AM Radio"
<amradio at mailman.qth.net>
Sent: Saturday, March 11, 2006 7:34 PM
Subject: RE: [AMRadio] Class AB and B audio XFMRS


>
>
> 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
> distortion.
>
> Jim
> -----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
> I
> >>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
> many
> >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,
> distortion.
>
> Good post, Jim!  I enjoyed it, and am looking
for more.
>
> --
> 73 = Best Regards,
> -Geoff/W5OMR
>
>
>
>
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