Atma-Sphere S 30 & Zero Autotransformer Boxes

Does anyone out there have experience with using the Atma-Sphere S 30 integrated tube amplifier with speakers that have low sensitivity -- around 84? It looks like the best option for solving the impedance problem is Paul Speltz's Zero Autotransformer boxes.
Low efficiency? Yes, I think the S-30 is going to run out of gas. There's no way to solve this other than be using a different amp or different speakers.

Agree, to solve the low impedance problem, the autoformers will do the trick. To solve the low efficiency problem, you need more watts.
I had a very early production S-30 and it was definitely load sensitive. It sounded great with my Merlin VSMs, but absolutely wilted when driving my Verity P/Es. I understand (from a post in this forum by Ralph, IIRC) that Atmasphere subsequently modified the design and that later production was more robust. So, I'd be careful re the S-30 if you're looking at the used market.

I have used mine with a pair of Spendor 3/1 and Maggie 1.6QR speakers. My business partner had a set of autoformers lying around that he made up a while back that are a 1:2 ratio. So essentially it doubled the impedance of the speaker. The amp sounded fine with the Spendor's even without the autoformer, but the autoformer did help IMO. Especially when I pushed the sound level up over 90 SPL which is kind of rare for me. As for the Maggie's, it would have helped to have a 1:4 ratio on the autoformer. Better yet a pair of M-60's and the autoformer would have been better.

A lot will depend on the impedance curve of your speaker and how loud you plan to listen. The Atma-Sphere S-30 will put out more power into higher impedances, but after 32 ohms power will drop. So you really need to understand the entire impedance curve of your speaker.
Wilsynet and Clio09,
Thank you for your informative posts. The speakers are 8 ohm with an impedance curve no lower than 6.5 ohms. Do you think the Zero Autotransformer will solve the problem? I do not listen to unreasonably loud music since my hearing is very sensitive. I am uncomfortable with the high volume levels that others are often able to tolerate.
Wilsynet and Clio09,
John Atkinson's measurements of my Joseph Audio Pulsars stated: "However, as indicated by its plot of impedance magnitude and electrical phase against frequency (fig.1), the Pulsar is an easy load for the partnering amplifier to drive. Not only is the phase angle relatively low, the impedance remains above 8 ohms for most of the audioband, dropping below that value only in the lower midrange, to reach a minimum magnitude of 6.5 ohms at 210Hz."

Here is a link to those measurements:
Looking at the graph from the Stereophile review the impedance curve has a couple spikes in the bass region under 100 Hz. Other than that it is quite smooth. Using a 1:2 ratio on the autoformer would mean that the S-30 power would be around 45 watts for the most part. Those two spikes notwithstanding which appear to be just over 20 ohms. So at those points you might get slightly less than 45 watts, but at moderate listening levels should be just fine. Probably no different than my experience with my Spendor's.

My advice would be to try it standalone first and see how the amp fares. I think you might be fine. If you feel more power is needed then you can try the autoformers. If you live in the US and would like to try my set then contact me offline and maybe we can arrange for you to borrow them.
Thanks for your informative post -- greatly appreciated. This confirms Ralph's feeling and my own. That's what I was planning to do -- try the S-30 on its own and then add the Zero boxes if needed. I am looking for a pair on the used market. Regarding those spikes, it might help that I have REL sub.

Thank you very much for your kind offer. I would like to be able to take you up on it but I live in Asia which is a big problem when trying to put together a high end system. If I want to audition good equipment it means flying to Hong Kong, Bangkok or Singapore. I get to audition equipment infrequently and end up flying blind most of the time. I have had phenomenal success but have ended up selling off a lot of equipment over the years.