[AMRadio] Class AB and B audio XFMRS

Bob Bruhns bbruhns at erols.com
Mon Mar 13 06:43:00 EST 2006

The constant current flux-balance idea would work,
but it would waste a lot of DC power.  You might
as well use a single-ended class-A modulator on
the opposite side of a center-tapped winding from
the output, so the RF PA DC balances the class-A
DC.  Something has to dissipate that DC-balance
power, and it might as well be a class-A
modulator.  But this means a lot of power
dissipation in the modulator tube(s).

  Bacon, WA3WDR

----- Original Message ----- 
From: "John Lawson" <jpl15 at panix.com>
To: "Discussion of AM Radio"
<amradio at mailman.qth.net>
Sent: Monday, March 13, 2006 12:09 AM
Subject: Re: [AMRadio] Class AB and B audio XFMRS

> On Sun, 12 Mar 2006, W5OMR/Geoff wrote:
> > John Coleman ARS WA5BXO wrote:
> >
> > How difficult would be it be 'sweep' the mod
> >
> > The reason I ask, is because so much of the
surplus stuff we get is still of
> > Mil-Spec design.  Therefore, if the mil-spec
transformer says it's freq
> > response is +/- 1db from 100Hz ~ 3000Hz, how
would one determine exactly what
> > this particular peice of iron will be capable
of handling, in the
> > 'non-comercial' application?
>   An instructive test for most audio
transformers is to apply nice, clean
> square-waves to them, and observe the result
both in the time
> (oscilloscope) and frequency (spectrum analyzer)
>    The 'better' a transformer is, the fewer
artifacts it will add,
> especially at the "ends" of it's specified
>    Of course you can always sweep a transformer
quite easily yourself - a
> stable audio generator and AC voltmeter (that
will read accurately over
> the range of frequencies you'll be using) is all
that is required.
>    First, choose some reference frequncy, often
500Htz or 1KHtz - adjust
> the generator output to some convenient reading
on the meter - doesn't
> matter what it is, as long as it's not down in
the microvolt range..!
>    If you have a dB meter - all the better, set
your output level so the
> meter reads '0' or '-10' dB - or .775 VRMS if
the transformer is near
> 600-ohms output...  but no matter what type of
meter you use, the object
> is to lower the frequency until a substantial
fall-off in voltage is noted
> (the generator must NOT change it's output at
all during this) - then
> raise the frequency until a similar fall-off is
observed.  You'll get a
> pretty fair idea of the working bandwidth of
that particular tranny.
> This will change under load, especially with
large DC currents flowing in
> the windings - but the wider, the flatter, the
better. If you want to get
> Fancy - get some (linear) graph paper and plot
the output reading vs.
> frequency from, say, 10 Htz to 50 KHtz.
>    Then do this again with square waves and
watch on a scope for ringing
> and rounding of corners - you can use a
distortion analyzer if you have
> one.
>    If you really want to get obsessive, you can
plot out the magnetic
> curves, too - but this is outside the scope of
this post.
>    If you're completely insane, you can also
calculate the acoustic
> environment of the transformer... speed of sound
in steel, acoustic energy
> coupling between Mass 'A' of copper and Mass 'B'
of laminations...
>    At any rate, soon you'll get a 'feel' for the
various transformers at
> your disposal.
>   Cheers
> John  KB6SCO
> AMRadio mailing list
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