Speltz Zero Autoformer with Tenor OTL 75

I have a Tenor OTL 75 amp. It sounds great but there are times where you really miss having more power/damping to control the sound better. Has anyone tried the Zero Autoformer by Paul Speltz ? My primary concern is, is it just a band-aid like a make-shift solution or is it really effective ? Is there any loss of resolution or speed ?
It works quite well if you are trying to drive a lower impedance with your amplifiers. There is no loss of speed- the ZEROs have bandwidth to about 2 MHz, which is more bandwidth than most amps made.

People ask me if the ZEROs obliterate the OTL-ness of our amps and IMO they do not- this is because the ZERO is a problem solver- that problem being that the amp is being asked to drive a lower impedance with which its not particularly comfortable. The result will be more impact, lower distortion and more power. If the ZERO is being used with a speaker that the amp already likes then there is no point IMO/IME. You use it when you want to drive a lower impedance.

FWIW, Paul Speltz has comments from manufacturers of solid state amps essentially telling him that their amps sound better through the ZEROs driving a 4 ohm load rather than driving direct- even though their amps have no trouble doubling power into 4 ohms. This is likely because all amps, tube, solid state or class D, have lower distortion when driving higher impedances.

We have a good number of customers running our M-60s with Magnaplanars. The ZEROs make that happen with good success.
OTLs with external transformers, are not really better than tube amps with transformers, into lower impedance speakers. IMO!

What is the OTL advantage, over most tube amps, when using most non electrostatic speakers???? NONE!

I’m sure Ralph will comment (though he already offered information on the difference between the Zero Autoformer and the output transformers in tube amps just above---did you miss it?), but may I say that when you say "non electrostatic speakers" you are assuming ESL’s are a high impedance load. Yes, the original Quad (57) and other 50’s designs were nominally 16 ohm (though very, very low at some frequencies), but modern ones sure aren’t. The current Quads are at most an 8 ohm load, other brands often 4 ohm. So they too, though being electrostatic speakers, benefit from the Zero.

As to whether an OTL does or does not have any advantage with non-ESL speakers, there is more to that question that simply the matter of the presence or absence of an output transformer in any given amplifier. The Atma-Sphere amps have many other elements unique to their design than that.

The advantage an OTL driving a set of ZEROs has is that the ZEROs don't limit bandwidth the way conventional transformers do. The ZEROs have impressive bandwidth - 2Hz to 2MHz, which is beyond most amps made. This is because they have a very low turns ratio, which of course requires that the amp portion of the equation be something fairly low. By low I mean a few ohms rather than hundreds or thousands of ohms.

That turns ratio makes a difference!

OTLs can be quite successful driving speakers other than ESLs. Perhaps because Magnaplanar is in town, we have a lot of locals that drive Maggies with our amps- and most of them use the M-60s! I've yet to run into a specific speaker technology that OTLS can't drive really well. What matters more is the impedance.

However the impedance thing has an interesting property- turns out that it really doesn't matter what kind of amp you have, if its being asked to drive a lower impedance its distortion will be higher. This is true of all amps (I added that because its worth repeating) and you can see it in their specs. The fact of the matter is if you are investing in any amplifier, your amplifier investment dollar will be best served by a higher impedance speaker is sound quality is your goal. If sound pressure is your goal than there is a weak (3 db) advantage to running a 4 ohm speaker as opposed to 8 ohms.

This is why the ZERO can help solid state amps in driving lower impedance loads even though they are already comfortable making the power to do so.
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.