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One more time: autoformers?

Not sure if this OP belongs in cables, speakers, or amps. Since I listed cables first -- it's cables. There are already some old threads which touch on the use of autoformers. Seems like most of the threads come up in the context of OTL and SET amps.

My Qs are: What are benefits and downsides to using autoformers? How does one know if an autoformer would be a good match for his/her rig?

As with most things in life, I suspect that autoformers must have downsides. Otherwise, everyone would use them. Open and intellectually honest responses are welcome.

Thanks?
bifwynne
zd542
5,339 posts
 
Are you looking for a comparison on output transformers vs autoformers, or autoformers vs not using either?
clio09
3,355 posts
 
If you haven't already check out the Paul Speltz Zero page where he discusses the benefits of using autoformers between the amp and speakers. I used a set we built between my OTL and planars to increase the impedance of the speaker which allowed my amp to drive them a bit better. My amp puts out more power as the speaker impedance increases so this was a benefit, but not all amps do this. I didn't notice any downside using the autoformer. I suspect with other types of amps what the autoformer does is affect damping factor and provide better amp/speaker matching.
bifwynne OP
2,206 posts
 
Thanks Zd... and Clio.... I own a power amp with output transformers. I also read the Paul Speltz white paper as well as the Atmasphere white paper on Voltage Paradigm and Power Paradigm amps. I get his basic point about impedance matching and the benefits of increasing the level of impedance the amp "sees."

My concern is that I do not have the full picture. If autoformers are so great, why don't amp manufacturers just slap that on the back of their amps for all the perceived benefits.

I would appreciate hearing from our "EE" type members about the benefits and downsides of using autoformers. One thought is that if a speaker's impedance curve is a rollcoaster with some pretty tough dips and hills . . ., sure, the autoformer presumably can lift the entire plot up 2X, 3X and so forth. What happens, for example, if the impedance hills touch 20ohms or 30 ohms?? Can any amp (SS or tube) push power into a 40 ohm (2 x 20 ohms) or 60 ohm (2 x 30 ohms) impedance??

Thanks again .
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