During my visit to Washington, D.C., in 1998, David Berning invited me to drop by his house in nearby Potomac, Maryland. At some point during that evening he pulled out a prototype, which he described as a low-power and low-cost ZOTL amplifier. David was eager to offer his impedance-conversion technology in a highly affordable product. When he finished describing the device, I suggested that it would make for a perfect headphone amplifier. Sure enough, when it was released in 2000 as a “personal amplifier,” it included a headphone input. When I reviewed the microZOTL some 15 years ago I concluded that its greatest potential was as a headphone driver for both low- and high-impedance designs. My opinion then was that it ran circles around headphone amps costing thousands of dollars. Sadly, the microZOTL was discontinued in 2007. It remained up to Mark Schneider of Linear Tube Audio and his distribution website, urbanhifi.com, to re-issue the microZOTL under license from Berning as the microZOTL2.0 reviewed here.
The microZOTL2.0’s internal circuitry is identical to that of the original, but there are some significant differences. The power supply is now outboard which reduces noise. Functionality has been enhanced in the form of preamp outputs, a second line-level input, and a selector switch that toggles between the two inputs. Premium tubes (Russian Tung-Sol reissue 6SN7 and 12AT7) are now standard instead of the Chinese tubes of the original. In general, parts-quality has been enhanced and the unit includes an Alps volume pot and silver-coated and Teflon-insulated copper wiring. And there’s no question that the microZOTL2.0 improves on the cosmetics of the original.
Go ahead and peek through the plexiglass top plate at the main circuit board and marvel at the circuit complexity. The microZOTL2.0 is configured as a Class A push-pull power amplifier. What is most unusual here is that the power tubes for each channel are the twin sections of a 6SN7. As far as the front end, one section of the 12AT7 input tube is used as a voltage amplifier while the second section is connected as a cathodyne phase-splitter. The output of each 6SN7 half is superimposed over a 250kHz RF carrier and transformed through an RF-converter transformer to obtain the high-current and low-impedance domain required by a loudspeaker. The advantage gained is that, unlike a conventional audio transformer that must operate over a wide bandwidth, the RF-converter transformer operates at a single frequency and can therefore achieve an extremely high turns-ratio without adversely impacting the power bandwidth. According to Berning, the microZOTL2.0’s effective turns-ratio is an enormous 168-to-1, making it possible to use a triode (normally only seen in amplifier input or driver stages) in the output stage. The resultant output impedance is 2 ohms with zero global feedback. Of course, the carrier frequency is filtered to the tune of 50dB before reaching the output terminals. Power delivery is a nominal 1W into a 4-ohm load.
My first instinct when surveying the microZOTL2.0’s enhanced functionality was to use it as a basic line preamp. It didn’t take me long to hook up a DAC to Input 1 and a phonostage to input 2. I asked David Berning about the expected voltage gain when driving a power amp, typically a 50k-ohm input impedance. He thought the microZOTL2.0 would offer a bit more than the specified 12.4dB gain into a 14-ohm load. That’s more than enough for any digital source and should suffice for a moving-magnet phonostage with a gain of 45dB or more. David did caution that the power amp should be turned on last and turned off first in order to avoid a low-frequency thump. Then there is the 2-ohm output impedance. That’s significantly lower source impedance than that of any preamp on the market, even those incorporating a cathode-follower output buffer. The payoff is the ability to drive long cable runs with no worries. It’s not just about output impedance but also about being able to sink current into the cable, and that the microZOTL2.0 does very well by virtue of being a miniature power amplifier.
At the 2015 RMAF I was surprised to find out that Mojo Audio’s Benjamin Zwickel instincts were aligned with mine. He was using the microZOTL2.0 to drive David Berning’s latest power amp, designed for Linear Tube Audio, the ZOTL40, which features a 40Wpc, EL34-based, push-pull output stage. In my setup, the microZOTL2.0 was coupled with the VTL Manley Reference 100/200 monoblocks and the Analysis Audio Omega planar loudspeakers. In this context I could only marvel at its explosive dynamics. The range from soft to loud was negotiated with a world-class startle factor that exceeded the performance of the Audible Illusions L3A line preamp, and if memory serves me right, actually equaled the performance of the much-more-expensive Pass Labs XP-30 preamp in this regard. That made the microZOTL2.0’s presentation visceral and involving, impossible to ignore. And as it turned out, it was far from being a one-trick pony. Bass response was spectacularly solid, extended, and pitch-perfect. And there was no denying that soundstage transparency bettered that of any tube preamp I’ve auditioned under $5k. The microZOTL2.0 was able to dig into a mix and resolve a complex passage with great precision. And that was a function of its excellent control over transient attack and decay. Its tube character was evident in its ability to do justice to spatial depth and width perspectives. That has been a weak point of solid-state designs and one of the reasons I’ve stuck with tube preamps for the past several decades. I suggest that you give the microZOTL2.0 a serious listen. It unzips the soundstage, allowing its boundaries to surge and expand with conviction. The upper midrange is smooth and free from tube brightness. However, the treble range sounds a bit electronic in character, lacking the textural purity of the midrange. My first thought was to categorize this as an artifact of the carrier frequency, especially since in more recent ZOTL designs Berning has deemed it important to raise it from 250 to 500kHz. But since I did not experience this problem during my headphone listening sessions, I’m inclined to think that the culprit is the VTL amp, which may be sensitive to the residual carrier frequency.