Hello Markus,


Thanks for the detailed reply, let me see if i have this straight
though please, sorry if I appear a bit dense, it's probably because I
am.... :)

Right now I run one voltage mode Class D push pull amp into a T type
low pass filter. All works great, nice gate and drain wave forms, and
my Scopematch box shows perfect sinusoidal wave forms for voltage and
current after the T type filter. Modest output transformer warming,
each pair of FET's near as damn it cold.

I add another duplicate amp and the Wilkinson combiner. The combiner
feeds the T type filter. With either one or both amps running the gate
and drain wave forms go to pot, the FET's run mad hot, as does the
output transformer (and the power input choke gets hot, too, which it
doesn't with a single amp straight into the filter).

Are you suggesting I build a duplicate T type filter bank and put a
filter between each amp and the combiner, with no filter on the output
of the combiner, just run the antenna co-ax to the matching
transformer and loading coil which are outside? I can do this, it
means finding and buying some more big toroids, but it's perfectly
possible....

Is the W1VD combiner what people call a "broadband" combiner? And if I
built this instead of the Wilkinson one would I be able to run it in
front of my single T type filter? Or do I still need a pair of filters
between it and the two amp's outputs? To remind, the W1VD one is here:

http://www.w1vd.com/137-500-500WCombiner.pdf

Thanks  for  your patience, and thanks Graham for your input too, just
don't  want to waste time and money building things that are incorrect
due to my ignorance!



> Hi Chris,

> the PI-type combiner per se is not the problem. All I was trying to
> say is that you have to prevent it from short-circuiting the
> harmonics directly at the output of a (voltage-mode) class-D PA. In
> that case, I would recommend a T-type low-pass filter between each
> PA and combiner input, starting with a series inductor rather than a shunt 
> capacitor.

> Note that the desired harmonic load impedance depends on the type
> of Class-D PA: Voltage-mode class D with rectangular voltage and
> sinusoidal current should see a high impedance termination for
> harmonics, i.e. an inductor or series resonant circuit. On the other
> hand, current-mode class D designs would have sinusoidal voltage and
> rectangular current, and require a low-impedance shunt for harmonics.

> Best 73,
> Markus





-- 
Best regards,
 Chris                            mailto:[email protected]