In the second version of the circuit board, there is a “park” spot to put the unused jumper
when configured for 6V tubes.
Technology
In 1996 David Berning developed a new and technically advanced vacuum tube architecture
designated as ZOTL. ZOTL stands for Zero-Hysteresis Output-Transformer-Less amplifiers.
The ZOTL architecture eliminated the problematic sound quality issues that audio-output
transformers caused.
Since ZOTL technology was introduced in 1996, the MZ2 amplifier has the lowest power output
of all ZOTL amps so far.
The ZOTL amplifier uses radio frequency to change the voltage-current transfer characteristics
of the output tube from its normal impedance plane to one suitable for driving a dynamic
loudspeaker. The radio-frequency remapping is implemented using special high-frequency
power-conversion techniques. The high-voltage, low-current tube impedance plane is remapped
to the high-current speaker impedance plane through special transformers operating at a constant
RF carrier frequency of 250kHz. Because the audio signal is riding on a carrier, it is not subject
to parasitic elements of the transformer that would distort the audio signal. Unlike the
conventional audio-output transformer, this impedance transformation operates on both the ac
and dc components of the signal.
In the transformer-coupled amplifier, the turns ratio of the output transformer determines the
impedance matching between the output tube(s) and the speaker. There are practical limits to
how large this ratio can be made because of the parasitic elements of the windings, and it is
difficult to make an output transformer with more than a 25:1 ratio.
With the ZOTL technology, the impedance matching is determined by the effective turns
ratio of the RF converter transformers. Without the parasitics to affect the audio, these
RF transformers can have much higher effective ratios, opening the door to using various
tubes under unusual operating conditions that cannot be implemented with output transformers.
In the MZ2, the effective (plate to plate) turns ratio is 168 to 1, making it possible to
