Take a look at the ASOG forum where this was recently discussed:
As someone who tried the Zero Autoformers with my AtmaSphere M60s, they are not a very good option, in my experience.
Yes, they allow the OTL amps to see a higher impedance. Yes, the allow the amps to drive a speaker they would not normally mate well with. No, they do not make good sound. The MAGIC of the Atmas are that there is life and soul to the music, combining the best of what solid state(speed, clarity, frequency extension) and tubes(reality, presence, beauty) and remove the tube part of the equation.
We found the sound in my system and my friends to be bleached out and very white sounding, in addition to having a tendency to become overdamped. At that point, why wouldn't you just use a solid state amp?
I think the ZEROs are very much case dependant. I had fabulous results with mine on a previous set of speakers. The speakers used were N.E.A.R. 50Me II's with a nominal rating of 8 ohms. They are rated at 87dB and the M-60's drove them exquisitely in a large room.
My current speakers have a higher impedance and are better served by using XLO Signature Shotgun cables.
You are oh so right, Tom! If it measures better, it must be better.
Wellfed, I agree that the Zeroes allow one to use the Atmas, just feel that they seriously hurt the sound in the various cases me and a friend have tried them in. A buddy here loaned both of us a pair to play around with for a month. My friend's comment was that he wanted to try them as he felt very strongly a transformer was needed somehere in the audio chain. After playing around with two different pairs of speakers, I felt like I must have been a misfit, because these things destroyed what the Atmas were best at. Sheepishly telling him my experience after a few weeks, he immediately agreed, saying he came to the same conclusion. When we both let our lender friend know this, even he did not disagree.
My suggestions for someone interested in AtmaSphere: There are enough good speakers that mate well with the M60s to choose from, and if you insist on having one that doesn't, move up to a bigger AtmaSphere amplifier.
Audio experience seems to have a way of sneering at absolutes. My experience with the N.E.A.R.'s and ZEROs yielded one of my top 5 audio epiphany moments.
On paper the N.E.A.R. 50Me II speakers appeared to be a fine match, 8 ohm nominal impedance, minimum 6.5 ohms. In fact I purchase these speakers knowing they would be driven by the Atma-Sphere amps and checked into the specifics beforehand.
I would be the first to agree with your last paragraph. Perhaps the reason for the disparity in our experiences has to do with the fact that I never went more than 2X on any of my ZERO configurations, I don't know.
I do know that the ZEROs provided PROFOUND improvement on my N.E.A.R.s which was not the case on my Köchel horns which are rated at a nominal 10 ohm impedance.
What speakers were you and your friends using when evaluating the ZEROs?
Wellfed, you are correct in saying that each invidual will have their own experiences and perspectives. I shouldn't be speaking in absolutes, but I guess I get a nerve pinched, as I often see a lot of people going for Zeroes and felt they were absolutely the wrong choice.
My speakers were Coincidents (8 ohm impedance) and Frieds (8 ohm impedance). His speakers were Infinity IRS V(yes, the biguns'), Audes Blues, and a few other speakers he had around (probably Kappa 8 or 9).
I can see where my own experience would dictate a wholehearted recommendation of the ZERO autoformers and where your experience would dictate the opposite.
Ralph Karsten has told me that different speaker cables might have given me some/or all of the benefit I obtained from the ZEROs as well.
Also, if you still have access to a pair of ZEROs I would be interested in hearing if the "alternative" 2X configuration would improve your results.
If ZEROs are available and if you care to try the "alternative" configuration let me know and I will give you details. I personally found the "alternative" 2X much more pleasing than the standard 2X on the mid/upper frequencies.