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There's always the iron concertina

Posted by Dave Cigna (A) on February 9, 2006 at 16:42:25

In Reply to: Split Load PI theory posted by BillyJack on February 8, 2006 at 19:32:23:

Jeremy Epstein deserves credit for developing this idea. You can read about it on his web page. As he writes there, the inspiration came from a circuit described in RDH4 (fig 7.51D) Simply put, the two matching load resistors in the usual split load PI are replaced with a pair of coupled chokes. The coupling is accomplished by winding both coils on a common core. In practice a 1:1 interstage transformer could be used.

The advantage identified by the author is a larger available signal swing. Another advantage (not identified by the authors) is that the impedances of both outputs remain equal under a wider range of conditions. My rendition of this circuit is below.

Jeremy's contribution was to notice that two more windings could be added to the same core. This makes the splitter fully transformer coupled eliminating the capacitors on the outputs:

Often with transformers they look great in the schematic, but designing and winding a good quality component turns out to be challenging. Sometimes though, luck points your way and it actually turns out to be relatively easy to make something that ought to work really well. This is one of those times. Winding the two top coils bifilar, likewise the two bottom coils, should make it easy to produce a transformer with very high performance and wide bandwidth. The interwinding capacitance that can make bifilar windings unsuitable in some situations actually help us here.

The only thing that could make things easier is if we could make the DC flux from the plate and cathode currents cancel, sorta like in a PP output transformer. That would allow us to use a smaller, minimally gapped core. That's not the case here; it acts more like an SE output transformer and needs to be larger with an air gap to compensate for the DC current, but no bigger than a typical SE interstage transformer. We can live with that.

I admit that I haven't tried this, so I don't know how it actually sounds. Jeremy used a Lundahl interstage (that is not bifilar) and reports very good results. Lundahl makes great iron, but I think an even better unit could be wound for this application. In any case, from a purely theoretical perspective, this might be as close to an ideal phase splitter as we can get. It should display VERY good balance and fairly low output impedance. Compared to a phase splitting input transformer it has very high input impedance making it easy to drive with any source. Anyone want to try one on their Amity? :)

-- Dave

• Jeremy Epstein's Iron Concertina page   (Open in New Window)

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