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.

Are you looking for a comparison on output transformers vs autoformers, or autoformers vs not using either?
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.
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 .
Well, as far as I know, OTLs are the only amp design that increases power into higher impedance. Though many amps may struggle when impedance dips too low (not sure if autoformers help this or not).

Amps certainly can push power into 40 or 60 ohms. These are not unusual impedance values for headphones (and some headphones have much higher impedance values). But a SS amp and a tube amp with transformer and speaker taps for 4,8,16 ohms would be putting out considerably less power into these impedences.

That's about as much as I can help here (if this could be called help).

+1 on reading Speltz's webpage.
Thanks Roscoe. There's a number of "EE" type members who I hope chime in too.

No +1 point for reading the Atmasphere white paper too??
The Atma-Sphere amp I own will put out more power into higher impedances up to a certain point, which I believe is 32 ohms. After that point the power will drop. So you really need to know the impedance peaks of the speaker and where they occur as opposed to the nominal impedance.

Music Reference amps are designed for light loading. For example, the RM-10 I own will exhibit lower distortion and provide other benefits if you use the 4 ohm tap on an 8 ohm nominal speaker. The trade off is about 10 watts of power. I am not certain that this applies to all amplifier designs.

BTW - Some McIntosh solid state amps used autoformers and among the McIntosh crowd these amps are quite desirable.
Thanks again Clio. I recall reading somewhere that certain Atmasphere amps had a 32 ohm ceiling. I tried the 4 ohm tap. The music su*cks. Went back to the 8 ohm tap.

Btw, been using my sub-woofer a bit more aggressively than before, i.e., raised the gain a little and raised the cut-off frequency to about 80 Hz. Going by ear, I think the sound stage opened up quite a bit. I don't know why. Maybe my finished basement is a giant bass (or is it base? - LOL) trap.

I'm speaking from experience and don't really care about theory or numbers. My Atmasphere MA-1's were not doing a good job of controlling the lower frequencies of my Aerial 10t's. They sound flabby. With autoformers the bass dramatically thighten up and allowed for a more linear presentation without a loss of sound stage. I already owned the 10t's when I bought the Atmasphere amp and preamp so I had to do something to solve the mismatch. In my case it worked very well but I must add the cavet that I'm in the process of setting but my brand new Merlins which should be a much better match to the OTL's. Autoformers are only suitable for certain very specific situations. If all works well with the Merlin's my autoformers will be for sale as the 10t's are headed for the Man Cave and will be driven by a Bryston amp. No autoformers necessary.
Hi Drambuie. Thanks for your post. I'm waiting for ARC to come back to me with its thought about how well my ARC amp matches with my speakers. If ARC says all is well, I'll stand pat. But if not, I may be in the market for Zeros. If you decide to unload your Zeros, please send me a message and I'll give you a status update. I'm hoping to hear back from ARC very soon. If ARC thinks Zeros might improve my sitution, maybe we can help each other out. Thanks again.