[AMRadio] Modulator question.


Larry Szendrei ne1s at securespeed.us
Sat May 8 13:56:08 EDT 2010


RICHARD GEORGE wrote:
> I have over the years built several low power AM transmitters. I would like 
> to build something a little bigger say in the 400-500 watt plate modulated. 
> I have been kicking around the idea of using one of the solid state 
> amplifiers made for car audio driving a reverse connected audio transformer. 
> I have seen these audio output transformers available in the 200 watt range. 
> I understand that the RF output stage would have to be designed to fit the 
> impedence of the output transformer.  Does this sound like it could be done?
>   
Well, sort of...

The problem will be if you run the RF PA plate supply current through 
the modulation transformer (or modulation transformer wannabe) 
secondary. The transformer then needs to be designed for an unbalanced 
DC current flowing through one of the windings, because the core is 
magnetized by this current, leaving less effective core for the audio 
signal riding on top of it. Don't know of the output transformers you 
are talking about are single-ended (unlikely at 200W) or push-pull. If 
single-ended, you can run a DC current through the winding to the extent 
it was designed for. If PP, then no, you can't, you can have a 
modulation choke pass the DC current to the RF PA, and capacitively (AC) 
couple the transformer secondary to the choke (Heising system). But 
then, you need to find a suitable modulation choke, which could be as 
difficult as finding a suitable modulation transformer. Chokes and 
transformers designed for an unbalanced DC current have all the E and I 
leaves lined up, and have a precise air gap separating the two stacks. 
Transformers designed for no DC current, or DC currents that cancel as 
in a PP winding, have the E and I sections interleaved.

73,
-Larry/NE1S


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