Ralph-
The transformers, of which there is a pair for push-pull in the ZH270, are power-conversion transformers that operate at fixed frequency. They have low-loss ferrite cores (not air core) optimized for 250 kHz. They are indeed critical components for the impedance matching in the ZH270. Audio output transformers in conventional tube amplifiers perform this impedance-matching function, and are major limiting factors in obtaining the wide bandwidth needed to prevent the amplifier from having the filtering properties that both you and I are trying to avoid by not using them.
Now to try to get to the bottom of this controversy as to you thinking that the ZOTL has output transformers and my claiming that it doesn't, we need to come to an understanding of what an audio output transformer is. To me, an audio output transformer is a transformer used to couple the audio-frequency signal from the output tube(s) to the speaker, and for high-fidelity applications it is designed to cover three to four decades of frequencies starting from at least 20 Hz. To do this they must be physically large. In the design of these devices, compromise must be made between high frequency and low-frequency performance because of parasitic capacitance of windings and number of turns needed to achieve sufficiently low magnetizing current to reduce core saturation and distortion at low frequencies.
All I have been saying all along is that the ZOTL does not have any such device. I see the term OTL as an acronym for output transformer less; I believe that that was the original intent when the early pioneers such as Futterman had the dream of finding a way around the main limiting factor in audio power amplifiers of the time. Any amplifier that by its design eliminates this problematic device could be called OTL for that matter, including most transistor amps. Amplifiers that are free of these devices avoid the filtering (frequency-dependent) type coloration associated with coupling output devices to the speaker, assuming no other audio-frequency dependent coupling network is used.
The ZOTL falls into a generic class of amplifier described by the acronym OTL both technically and in spirit. The power conversion transformers called out by the patent cannot and do not operate at audio frequencies. The semiconductor devices called out by the patent drive these transformers at a fixed 250 kHz and do not operate at audio frequencies or as amplifiers. The patented ZOTL circuit is a two-way coupling circuit that performs impedance matching like an audio output transformer, but does not have any of the limitations and distortions imposed by audio output transformers. This circuit is in no way a buffer, such as a transistor output stage in a hybrid tube-transistor amplifier. There is no power gain, and more importantly, the speaker “sees” the tube transfer characteristics and the tubes “see” the speaker’s dynamic characteristics. This two-way power flow separates the ZOTL from the hybrid amplifier in that sonically it is pure tube. Further technical details and graphical demonstration can be found on my web site davidberning.com in the form of white papers.
The circuit topology of the ZH270 is completely different as to how the transformers are connected. I simply fail to see how anyone familiar with audio-amplifier circuits can confuse these transformers with audio output transformers. At least one winding of an audio output transformer is connected to the tube(s) in the non-OTL amplifier, and a winding is connected to the speaker. Neither of these connections is made with the conversion transformers in the ZOTL.
While I realized that the earlier OTL amplifier designs, that is pre ZOTL, have wonderful sonic characteristics when used with compatible speakers, I did not want to make OTL amplifiers before I invented the ZOTL because of the high heat and reduced reliability that comes from having many output tubes running in parallel. I also realized that speaker selection was more critical because these tube OTLs are starving the speaker for current. Since the ZH270 was introduced in 1996, it has slowly gained respect; so much so that I can no longer keep up with demand and there is a waiting period that now extends into next year. Clearly more people are finding that the ZH270 can provide sonically what previously only older-type OTL designs could provide, and can do so for a wider selection of speakers and without the excessive heat and fuss of replacing tubes and chasing drifting bias settings.
