Linear Tube Audio microZOTL2.0 line stage/headphone amplifier

This is a true story about a surprising 1W integrated amplifier—a push-pull, class-A, output-transformerless tube amp—that drove my DeVore Fidelity Orangutan O/93 speakers to 90dB average levels with grace, spiderweb detail, liquidity, and—unbelievably—a small degree of bass slam.

What the hell is it?
Have you ever observed an object for which you had no name? I mean, no whatchamacallit name at all? In the anthropological story of humans, the act of naming has powerful significance. In our everyday world, humans, objects, and forces don't fully exist for us until they're baptized, anointed, given a proper name. ("It's a girl!") Naming is transformative—it categorizes. ("It's a miracle!") Names generate pictures and memories. Names provoke feelings. (Think women named Bridget, Beyoncé, Barbie.)

Like those B-names above, names preform and deform our direct experiences. How? By their very nature, names assign virtue and value to our experiences before we even experience them. Worse, they convert pure sensory data into something that is mostly a cultural and emotional projection. Do I need to explain how much this naming game has corrupted our sacred hobby of listening to reproduced music in our homes?

Every adjective I use in this review has been predefined and precolored by my reviewing ancestors. Thank you, Gordon and Harry! So let me start again . . .

This is a true story about a surprising 1W integrated amplifier—a push-pull, class-A, output-transformerless tube amp—that drove my DeVore Fidelity Orangutan O/93 speakers to 90dB average levels with grace, spiderweb detail, liquidity, and—unbelievably—a small degree of bass slam.

This is a true story about a new tubed headphone amplifier that plays every set of 'phones I tried with extraordinary levels of liquid transparency, detail, and tuneful ease.

This is a true story about a world-class line-level preamplifier designed by a venerable audio designer named David Berning and manufactured by a company named Linear Tube Audio, founded in 2015 by inspired engineer and product designer Mark Schneider.

Linear Tube Audio has christened this futuristic trident of amplification the microZOTL2.0 ($1100).

David Berning
I have admired David Berning since the 1970s, when I enjoyed his EA230 tube amplifier. The EA230 was a sonically well-balanced variation on the classic Williamson circuit (a push-pull amplifier, made with carefully designed output transformers and using global negative feedback), which sounded especially good with electrostatic speakers.

In 1995, after more than 20 years of designing tube amps, Berning rejected what he believed were the limitations of output transformers. Hoping to take thermionic valves to a new, less "tubey"-sounding level of performance, Berning introduced his patented zero-hysteresis output-transformerless (ZOTL) circuit. He says that his ZOTL circuit is an "impedance converter" that replaces a traditional tube output transformer by using "DC-DC switching converters operating at fixed frequencies well above the 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."

I've owned a few output-transformerless (OTL) amplifiers, including Julius Futterman's H3aa and a pair of New York Audio Labs OTL1s. But for me, OTLs never really solved the frequency-extreme problems of output transformers. Even with my beloved Quad ESL 57s, I thought, The cure is worse than the cold. My sensitive audio sensitivities didn't appreciate the anxious feelings, oscillations, and circuit failures caused by all the feedback required to make row upon row of hot tubes achieve a low enough output impedance to drive even a pair of 15-ohm BBC LS3/5a's.

David Berning introduced the original low-power, modestly priced microZOTL ($680) in 1998. He called it a "personal amplifier," and it included a headphone output jack. It was discontinued in 2007. Then, in 2015, smart guy Mark Schneider licensed the design for the inaugural product from his new company, Linear Tube Audio. Schneider added a few wise touches—a nicer case, a second line-level input—but kept Berning's original circuit. This uses half of one 12AT7 dual-triode tube as a voltage amplifier, and the other half as a split-load phase inverter. Surprisingly, this common configuration drives not a pair of EL34 or KT120 power pentodes, but a 6SN7 medium-mu twin triode—as a single-tube push-pull class-A output stage!

Antiphase waveforms from the two halves of the 6SN7 are superimposed on 250kHz carrier waves, and the voltage difference that results is impressed across a radio-frequency (RF) converter transformer with a turns ratio of 168:1. This unusual strategy produces 1Wpc into 4 ohms with no global negative feedback, and an output impedance of only 2 ohms.

The original microZOTL came in bright blue casework with a store-window glass front so that the listener could admire the glowing tubes. The power switch, volume control, and headphone jack perched awkwardly at the top rear. Schneider's microZOTL2.0 sits on your desk or equipment rack with considerably more high-tech presence and subtle masculinity. Its case is made of black anodized aluminum, with a top plate of clear Lexan, a polycarbonate. The Power button on the faceplate is elegantly encircled by red light when the amplifier is switched on. To the right of this is a toggle switch whose two positions are labeled 1 and 2, for the two line-level inputs; then comes an Alps volume pot with silky-smooth action, and a locking ¼" headphone jack. The rear panel, too, is simple, with high-quality speaker-binding posts, those two line-level inputs (RCA), a preamp output (RCA), and a jack for the 12VDC power supply.

Listening: DeVore Fidelity Orangutan O/93s
Allegedly, a journalist asked Beethoven, "What does your D Minor piano sonata mean?" The master replied, "Read Shakespeare's The Tempest." Surely, Beethoven understood: art does not have meanings; it has uses, and Beethoven's provocative Piano Sonata 17 in d, Op.31 No.2, "Tempest," is proof. Among its range of uses: the Sonata in d can serve as a primer in the German literary movement (ca 1770) Sturm und Drang (tempest and drive). The repeating theme of first one, then two notes sounds almost like the composer speaking the words storm and urge. Artur Schnabel plays Sonata 17 with a unique range and balance of contrasts—calm vs excited, loud vs quiet—masterfully used as tools to express not only Beethoven's musical invention but his vision of the century that lay ahead (CD, EMI CDHH 63765). I don't know how, but driving the 93dB/W/m DeVores, the 1Wpc microZOTL2.0 showed me more pianistic power and subtlety from this recording than ever before. The sonata's—and Schnabel's—dynamic contrasts felt radically enhanced and more obvious than through any other amp I'd used with these speakers. When I say power, I don't mean loudness; I mean tensile strength, clarity, and the effectiveness of dynamic contrasts.

I rarely play my music loud—just 88dB on average—but it took only a few recordings before I forgot completely that the microZOTL2.0 was putting out less than 1W into the 10-ohm load of each DeVore O/93. If you have a small room and listen at modest levels, this is a truly magical combination.

Listening: Zu Audio Soul Supremes
The microZOTL2.0 played Zu's Soul Supreme speakers with a bell-clear, pure-water transparency that gripped my mind and would not let go. I believe the Soul Supreme ($4500/pair, review in progress) to be of even higher impedance than the DeVore O/93—slam and wham were reduced even further, but, if anything, transparency and grainlessness were enhanced. Musical flow seemed more slippery and alive, like a trout in a clear mountain stream.

Linear Tube Audio
Washington, DC
(202) 450-6480