Speltz Zero Autoformer with Tenor OTL 75

I live in the twin cities and nobody uses Atmosphere amps on maggies.

I heard them once connected to maggies at the dealer, and they sounded poor, IMO. The store owner said they were a poor match.

I do not see the advantage of a 2 MHz bandwidth on the Zero.

Don, Implicit in any response to your sentiments is that (1) the Zeros are autoformers, not transformers. That is a BIG difference, because in addition to the low turns ratio, and like Ralph says, the bandwidth is huge and at the extremes autoformers don't delimit the sonics in the way even a good output transformer usually does do, and (2) you can't possibly know that "nobody" uses Atma-sphere on Maggies, unless you know "everybody", which is unlikely.  However, whether one would like the Zeros or not is a matter of taste.  I used them for years to drive my full-range Sound Labs and was very happy, until I modified the Sound Lab input circuit such that the Zeros were no longer needed, because my mod dramatically increased the input impedance of the SLs.  Yup; I like the speakers way better with intrinsically high impedance and no Zeros needed.  Which brings me to another point, ESLs are not intrinsically low Z devices except at very high frequencies (e.g., 10kHz and above) where the energy demands are so minimal that the low Z is not much of a problem.  What makes many of them problematic for any tube amplifier, not just OTL amplifiers, is the built-in circuitry that often precedes the ESL panels.  In the "solid state era", many designers were not shy about adding resistances in parallel, correction circuits, etc, that all tend to lower input Z artificially.  This is usually not felt by humongous SS amplifiers, so they usually get away with it.  To take the SL speakers for one example, they use(d) a 5 to 8-ohm resistor in parallel with the treble step-up transformer, along with a capacitor in series, in order to effect a high pass filter.  But that resistor sucks power and results in a net impedance at vital midrange frequencies that could go as low as 2 ohms.  I got rid of all those parts in my speakers, and the net impedance of the current configuration is >20 ohms from 50Hz to 5000Hz. My Atma OTLs love it. (At 20Hz, Z is ca 100 ohms; above 5kHz, Z drops gradually to 2 ohms at 20kHz.)  An ESL is a giant capacitor, which means intrinsically that Z goes down as Hz goes up.

I live in the twin cities and nobody uses Atmosphere amps on maggies.

I heard them once connected to maggies at the dealer, and they sounded poor, IMO. The store owner said they were a poor match.

I do not see the advantage of a 2 MHz bandwidth on the Zero.

I live in the Twin Cities too, and while nobody uses Atmosphere amps on maggies, Some people do use Atma-Sphere with Magnaplanars.

Mark did run M-60s on Magnaplanars at his store, but he ran them direct without the ZEROs.

You don't have to see what the advantage is for it to be there- things like that exist without our knowledge of them being necessary. In the case of the ZERO the bandwidth is not there for any other reason than it is a byproduct of the low turns ratio. It is important for bandwidth to 200KHz, as this allows for zero phase shift at 1/10th the frequency (20KHz).

Good one Ralph, I'm with ya :-) ! But it's Magneplanar, with an e. I see that all the time.