Tubegroover, according to David's discussions, the ZOTL circuit is a transformer emulator circuit. It provides the impedance matching without the turns-ratio limitations and saturation limitations of traditional output transfomers. The circuit emulates the "perfect transformer" and doesn't suffer from the sonic limitations of these transformers. Therefore, the audio signal is not subjected to the performance limiting factors associated with output transformers. But the impedance matching is done in a much more efficient way than is possible with a transformer, without degrading signal integrity. This allows virtually vertical rise times in square wave response well beyond the audio range, and this is exhibited by the incredible speed of this amplifier. No audio output transformer can deliver this type of performance.
As far as comparing the ZOTL method to traditional OTL methods of impedance matching, the traditional OTL circuits use multiple sets of tubes in parallel to reduce the output impedance of the amp to a level which could be practical for driving a speaker. Due to the fact that they don't get the output impedance very low, they do better into higher impedance speaker loads. The ZOTL circuit gets the output impedance down to 1.8 ohms, and provides better damping factor into normal speaker impedances. And it does so with fewer tubes in the signal path.
To take this one step further, the Single-Ended ZOTL amps have only 2 tubes per channel(1 driver and 1 output triode). No traditional OTL can do that. The coherency provided by using one output triode in OTL, instead of a multiplicity of output tubes, is something that has never been heard, outside of the Berning circuit amps. So this goes beyond just impedance matching. There has never been a true single-ended triode OTL before this. That, in itself, is a major breakthrough, and is worthy of accolades for allowing closer insight into the music.
