Why wire an output transformer as autoformer ? (parafeed)

[00940]

The designer of the amp shown in this post: http://www.diyaudio.com/forums/tube...-feed-choke-load-output-stage.html#post506410 uses the primary and the secondary of its output transformer in serie, to get an auto transformer rather than a classical transformer.

What's the point of it ? Any benefits doing so ? I felt curious about it and couldn't find any other exemples of this.

Thanks for any light on this.

 

[kevinkr]

That's a very good question and given the ratios and the fact that this is a conventional parafeed OPT connected as an auto-transformer leaves me questioning this design choice.

Using a purpose designed auto transformer with these ratios would probably be little better. It in theory it should eliminate secondary windings, (but here it really doesn't) however the common (output) windings would be to have wire sized to match the required load current and would reflect a very small % of the overall winding electrical length - so again this leaves me wondering if there is any real benefit. Essentially at best you save a little small gauge inexpensive wire and IMHO that could be it.

It would very slightly increase the overall effective inductance of the "primary" and the transformation ratio for what I think would be an unmeasurable improvement in LF performance.

Low ratio devices are a big win due to better winding coupling due to the lower number of overall windings, (and you can save significant copper) but here it doesn't seem to make much sense.

I'm also wondering about lumped LC constant induced HF phase shifts when connected this way - and whether or not this might adversely affect the HF FR of the transformer.

I'm afraid I've added more questions, and answered none. In short I hope someone who has experimented with this topology given the ratios involved can state whether or not on purely technical grounds this is more than a design fashion statement.

 

[Miles Prower]

What's the point of it ? Any benefits doing so ? I felt curious about it and couldn't find any other exemples of this. :confused

I've never seen anything like this done with hollow state, but you see it all the time in designs of wide-band, solid state, RF amps. That auto-transformer topology is frequently seen in "no tune" shortwave amps that can cover the 10M -- 180M ham bands without an LC tuner. Suspect its designer is a sand-head.

You can get away with that when using transistors since you can drive the impedances low enough so that stray and internal device capacitances have cutoff frequencies well below the 10M band.

Can't do that with hollow state, so you need to absorb those capacitances into an LC tuner or you lose all your gain.

Those %@#&^ no-tune SS amps are making good air variable capacitors virtually unobtanium these days.

 

[DF96]

Tighter coupling? At least part of the 'primary' is perfectly coupled to the 'secondary' as they are the same piece of wire. However, the effect will be negligible for any transformer with a highish turns ratio. Fashion statement, more likely.

 

[00940]

Thanks

 

[speakerfritz]

the componets in that part of the schematic fit real well into a 35hz 6db high pass filter. the key element in the filter is the impedeance of the primary winding.

with out knowing th eimpedeance of the secondary winding, we do not know how the shift in impedeance effects the high pass filter. evidently, the extra impedeance was needed, to align the values for the compoents used. with out the use of the seconday widings impedeance, we are at 35hz high pass.