Speltz Zero Autoformer with Tenor OTL 75

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.

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.

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).