Atma-Sphere S 30 & 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.

Marty

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.