Linear Tube Audio MicroZOTL MZ3 Headphone Amplifier Review

1974 was a year that saw Happy Days debut on ABC, Richard Nixon resign, the Ali-Foreman Rumble in the Jungle fight in Zaire, and the establishment of the David Berning Company, which produced vacuum-tube amplifiers based around a unique impedance converter known as ZOTL for ‘Zero-Hysteresis Output Transformerless.’

This design was created to replace traditional amplifier output transformers and is the core tech that Linear Tube Audio licenses and builds their own amplifiers around, like the $3,700 USD MZ3 under review here.

The idea behind ZOTL is to operate the power output of the amp at a “fixed high frequency without traditional audio-output transformers, the ZOTL Impedance Converter eliminates the frequency-dependent performance limitations inherent in all transformer-coupled tube amps,” according to Berning’s white papers and website.

As someone who has a great love for tube amps and massive hand-wound output transformers I remember being skeptical about the design before hearing one in Chicago a number of years ago. I wouldn’t say I was a convert, but I definitely became a fan. So, when LTA approached me about reviewing their new MZ3 headamp/preamp/integrated amp I was happy to take the unit on as I’d not only heard that original Berning amp back in the day, but I’d also spent time listening to their new ZOTL10 MK II power amp at a friend’s place in Virginia a couple summers ago – along with hearing it during show coverage – and been duly impressed.

There’s no lack of information online about ZOTL, and OTL for the circuit freaks out there, but I’ll leave the bulk of that to the white papers for reading at your leisure HERE, and with this more in-depth description of the technology that is attached to the patent info: This patent teaches the principles behind the Berning ZOTL technology that can properly match the impedance of vacuum tubes to the impedance of speakers without using audio output transformers. The impedance conversion is done with dc-dc switching converters operating at fixed frequencies well above audio frequencies. The actual voltage and current impedance transformations are done via special high-frequency transformers. This is an RF carrier system that eliminates the frequency-dependent distortions present in audio transformers, and can enable the amplifier to have a wider frequency response and be dc coupled.

The LTA MZ3 with remote.

What I will touch on for this review, however, are the claims made about the technology that it is an improvement over traditional tube-amp output designs. The key takeaways are that an ZOTL/OTL amp will; produce less distortion, allow more detail retrieval because of less signal distortion, make for easy speaker/headphone matching, produce less heat, allow longer tube life because of less heat, and produce better bass (because of a claimed linear frequency response down to 8Hz, as opposed to standard output transformers which are usually rated down to 20Hz).

Build and construction

Chassis design by Fern & Roby.

Like all their products, Linear Tube Audio amplifiers are built by real people in Takoma Park, Maryland. The company says that their technicians are just as likely to be musicians, as they are audiophiles, or music lovers. The MZ3 is a classic-looking product of well thought-out industrial credentials and in handling the low-resonance aluminum chassis designed by Fern & Roby, it feels solid, heavy and purposeful. It has a small LED screen on the front fascia along with a soft-touch power button, toggle switch for inputs, a stepped attenutuator and a switch for either headphone or speaker/preamp output. Vishay Dale one per cent precision resistors are used throughout, as are Ohno OCC wiring, Mundorf caps and hermetically-sealed relays with silver contacts and built-in electrical isolation from the analog signal path for volume/input/feature controls. It is also equipped with a fixed-input level ‘Home Theatre’ mode. It comes with a supplied Apple TV remote. Weighing-in at a not inconsiderable seven pounds for the amp chassis (10-inches wide, 4.75-inches tall, 8.5-inches deep), and another five pounds for the separate umbilical-connected power supply (5.25-inches wide, 4.25-inches tall, 12.25-inches deep), the unit takes up a fair amount of space on a desktop, rack, or amp stand when everything is hooked up. This isn’t a small portable-themed amp, so if you’re thinking that, think again. This is for a serious home or office set-up in its guise as a headamp paired with a DAC or phono stage and turntable, or as a preamp or a one-watt integrated driving suitably appropriate high-efficiency loudspeakers (think 95dB or better). Whatever way you slice it, the MZ3 slots-in nicely as a Swiss Army Knife of high-fidelity sorts.

LTA MZ3 external power supply.

The MZ3 is a push-pull, Class A, zero-feedback circuit design with two-Ohm output impedance for both speaker and headphone outputs, and a 50k-Ohm input impedance constructed around a pair of 12AT7 input tubes and 12SN7 power tubes along with the aforementioned ZOTL circuit architecture. Channel separation is listed as 46 dB, 100Hz~10kHz with a four-Ohm load and 54 dB, 100Hz~10kHz with an 14-Ohm load. Sensitivity is claimed at 0.6 RMS at full output and, as mentioned, the unit uses an oversized toroidal linear power supply featuring “dual banks of organic poly ultra-low ESR capacitors, for over 100KuF of output choke isolated storage. Oversize transformer secured with aluminum and brass mountings (no steel). Regulation is performed using low noise Belleson super regulators. Current transformer controls output protection circuit.” What does this mean? In practical terms, this is a power supply which maintains high-electrical capacity to easily handle large dynamic swings and fast transient response, it is designed to maintain an ultra-low noise electrical environment which helps contribute to isolating the audio signal over a super-black background.

High quality circuit detail throughout the PS.

Inputs consist of three single-ended stereo pairs, outputs are a 1/4-inch headphone jack, two single-ended stereo pairs for preamplifier out and a pair of stereo speaker binding posts.

Linear Tube Audio
7316 Carroll Ave. Takoma Park, MD, 20912

Simply Nobody's picture

Stereophile has reviewed another model LTA microZOTL with measurements :-) ........

Simply Nobody's picture

It would be interesting see a comparison review with the Manley Absolute tube headphone amp, $4,500 :-) .........