OTL (Output Transformer-Less) amplifiers manufactured by other companies are a completely different technology than ZOTL amplifiers.
ZOTL technology involves a linear amplification process using a carrier frequency as well as not having a traditional audio output transformer, instead using air-core Impedance Converters.
Noted pioneer of amplifier technology, Julius Futterman, has inspired many different amplifier designs over the years. Based on Futterman’s principles, other OTL amplifier designs incorporate two separate banks of parallel-connected tubes. The two banks are joined in series by connecting the effective cathode of one tube bank to the effective plate of the other tube bank, and driving the loudspeaker in a push-pull fashion directly from this junction point without a transformer. There are alternatives to Futterman’s principles that use low-impedance triodes, but the basic principles are the same: given a sufficient number of parallel-connected tubes in each bank, enough drive current can be obtained to drive the speaker.
The output impedance of other OTL amplifiers is nowhere near the actual speaker impedance. In an OTL amplifier, a large amount of negative feedback is required to force the push and the pull to work together properly in order to provide sufficient damping for the speaker. Even with the lowest impedance triodes available, there is still a basic impedance mismatch between the tubes and the speaker in the OTL circuit.
Traditional OTL technology requires a large number of power tubes which need to be driven hard to obtain the required output. This results in drastically reduced reliability and tube life and generating lots of heat. Fans or additional room air-conditioning may be required. Power consumption is typically very high for OTL amplifiers, often exceeding one kilowatt for a stereo pair.