Why? Berning's patented ZOTL circuit upshifts the audio signal onto a 250kHz carrier wave where a tiny RF transformer with a ludicrous turns ratio handles impedance transformation. This allows impedance conversion wildly beyond traditional transformers which must operate in the audio band. Hence the OTL part of the name. It is short for output transformer less.
Unlike Atma-Sphere and related conventional OTLs however, there is no need for endless paralleling of very hard-driven tubes to match the output impedance to what loudspeakers require. With ZOTL, it's the minuscule RF transformer which weighs nothing but might contain an extreme non-lossy 1:165 turns ratio. Unusual switch-mode power supplies different from what's in typical class D amps complete the lightweight mechanical picture. Thermally it's lightweight too and far from the space heaters which conventional OTL are. Sonically meanwhile, tubes one might be very well familiar with for signature sonics—take the 300B in the Siegfried for example—sound very different zotl'd; as though steam-cleaned, purified and accelerated beyond all the ills of conventional transformer loading; ills which register fully only by contrast. Given my familiarity with Dan's Siegfried and how fast, lucid and wide-band normal 300B sound in it, I'm feeling alarmingly confident that the 40wpc class AB push/pull ZOTL 40 from Linear Tube Audio
should become a real stand-out destination in the contemporary valve audio arts; and a valve amp which transistor lovers like myself could embrace and fancy. If I can arrange for a review loaner, I might have still more to say about it. Here's another web link
on it; and one more
of Dick Olsher's Denver sighting.