After my amazing experience reviewing the MicroZOTL2.0 preamplifier back in May, I was informed by Mark Schneider, owner/engineer of Linear Tube Audio, that he was coming out with a tube amplifier based on the same David Berning ZOTL design. The ZOTL is a patented architecture that stands for "Zero Hysteresis Output Transformer-Less" and works qualitatively different than any other audio circuit on today's market. The ZOTL uses a super-imposed carrier signal for the music signal at 250 kHz, which then is extracted through an RF-converter transformer to obtain the high current and low impedance required by loudspeakers. This simply means that, unlike a normal tube amplifier that uses output transformers operating over the total audio bandwidth, the RF-converter transformer operates at a single frequency, which allows a very pure signal to be passed without bandwidth limitation/distortion into the speakers it is driving.
Mark asked me if I would be interested in reviewing his new model, the ZOTL40 MK.II amplifier, which retails for $5,800. Of course I was quite eager to audition/review this amplifier after hearing what the MicroZOTL2.0 preamplifier had done driving my reference system.
The ZOTL40 MK.II amplifier is a 40-watt Class AB push-pull tube amplifier that uses a pair of 12AX7s along with a pair of 12AU7s as input tubes and four EL-34 power tubes. The amplifier can use any brand of EL-34 power tube. Mark and his listening group discovered that amplifier sounded much better with current production Genelex Gold Lion KT-77 power tubes, instead of the EL-34s. Therefore, they ship the amplifier with these power tubes in place. The ZOTL40 MK.II is self-biasing, so there are no manual adjustments to do if you are retubing or trying out different power tubes in the amplifier. The ZOTL technology operates the power tubes in such a gentle manner that their lifespan can last for as long as 10,000 hours.
The ZOTL40 MK.II amplifier's chassis is black aluminum; its dimensions are 8.5 inches high by nine inches wide by 18 inches deep, and it weighs 9.7 pounds. On the front plate is a red LED that lets you know that the amplifier is on. In the middle is where a volume control is located if you run the ZOTL40 MK.II direct from a source. I found that the ZOTL40 always sounded better when it was being driven with an active preamplifier than with its passive volume control. Around back is where you'll find the IEC input/on/off switch, two pairs of high-quality speaker wire connections, two sets of inputs (single-ended and XLR), and one volume control input if you are going to drive the ZOTL40 MK.II direct from a source component. The ZOTL40 MK.II amplifier is well built using high-quality standard internal parts and has an "industrial design" appearance. This amplifier does not offer the "eye candy" of other brands. However, when you hear its sonic performance, its appearance really won't matter.
The ZOTL40 MK.II amplifier replaced my reference Pass Labs XA60.8 mono blocks and was driven by the MicroZOTL 2.0 preamplifier, just as the Pass Labs amps have been since the 2.0 preamplifier became my new reference line stage. I wanted to use the same musical selections that I had used in my review of the MicroZOTL 2.0 preamplifier to keep constant the comparison between the great Pass Labs solid-state mono-block amplifiers and the tube-based ZOTL40 MK.II amplifier.
The first selection was John Coltrane's album Ballads (Impulse). The ZOTL40 MK.II produced an even richer density of tonality/timbres with all the instruments than my great XA60.8s. Great tube amplifiers are known for their ability to reproduce gorgeous tone color. Examples of these would be SET 300B/845/211 amplifiers, which normally are famous for the tonality of the midrange band. The ZOTL40 MK.II not only created this magic in the midrange, but from the sound of the cymbals to the bass fiddle, this amplifier completely covered all the frequency ranges in a rich color/tonality in a seamless manner. I also was surprised by how quiet the noise floor was in the tube-based ZOTL40 MK.II amplifier compared with the Pass Labs amplifiers, which are some of the quietest amps in the world. This allowed for a pristine clarity so that all the micro details in the music could be effortlessly heard.
My next selection was Rosemary Clooney's album "Blue Rose/Duke Ellington and His Orchestra (Columbia/Legacy) to hear what the ZOTL40 MK.II would do in the spatial categories of soundstaging and three-dimensional imaging. The ZOTL40 MK.II's handling of Rosemary Clooney's voice was the best I have ever heard in my reference system. The amount of palpability and the holographic nature of the sound created an eerie illusion that she was in my room at that moment. The air and space between her and the individual players in Ellington's band completely filled my listening space with great depth, height, and width in a realistic and totally natural manner.
My final selection was Steve Winwood's album Nine Lives (Columbia), which I use to test an amplifier's overall macro dynamics, low bass extension, and sense of aliveness/presence. Winwood's album was an extremely well-recorded set of electrical blues-influenced rock and roll with powerful dynamics and deep visceral 3B organ bass notes. The ZOTL40 was the equal of my solid-state mono blocks when it came to addressing the deep bottom-end bass extension. Where the ZOTL40 MK.II pulled ahead was the overall aliveness and a sense of quickness that was unlike any other tube amplifier I have ever heard.
• The Linear Tube Audio ZOTL40 MK.II amplifier is hand built in the U. S. and contains high-quality internal parts and an excellent overall standard of craftsmanship.
• This amplifier produces beautiful overall tonality/timbres/color that ranks with any amplifier, regardless of cost.
• It is very inexpensive to re-tube with current production power and input tubes. It is self-biasing and runs its power tubes so gently that it might be years before you have to replace them.
• This is one of the quietest amplifiers, either solid-state or tube-based, which allows the smallest details to be heard clearly and effortlessly.
• One of the most outstanding virtues of the ZOTL40 MK.II is how it creates three-dimensional images of the musicians with a sense of air/space around them.
• The ZOTL40 MK.II's overall macro dynamics create a sense of aliveness and punch that gives a system the excitement of real music.
• Like all tube-based power amplifiers, the ZOTL40 MK.II runs hot and cannot be placed in an enclosed rack.
• Like all tube-based gear, you will have to replace its tubes in the future.
Comparison and Competition
Two tube-based amplifiers that fall in the price range of the ZOTL40 MK.II are the McIntosh Labs MC275 V1 and the VTL ST-150, both of which retail for $6,000. In the case of the McIntosh MC275 V1, it was totally out-classed by the ZOTL40 MK.II's performance when it came to clarity/transparency, and it sounded fuzzy/clouded in comparison. Both amps sounded washed out when it came to tonality/timbres and were almost "dry" compared with the signature of the ZOTL40 MK.II's beautiful natural colors. Neither amplifier had the high dynamic level of the ZOTL40 MK.II.
After the amazing experience I had with the Linear Tube Audio MicroZOTL preamplifier, I can't say that I'm shocked by the performance of the ZOTL40 MK.II amplifier. Both are based on David Berning's ZOTL designs that are universally acclaimed and hand-built by Mark Schneider to a high level using quality parts. In all the important areas that can be used to judge an amplifier's performance--overall clarity/transparency, seamless integration of all frequencies, beauty of tonality/color/timbres, spatial dimensionality, overall speed and dynamics, and the palpability of individual players--the ZOTL40 MK.II ranks with the very best that I have ever heard. When I paired the ZOTL40 with other preamplifiers, instead of its companion MicroZOTL, these virtues still could clearly be heard. However, the synergy of the MicroZOTL preamplifier with the ZOTL40 MK.II brought a beauty to my reference system that I have never heard with any other preamp/amp combo.
The Linear Tube Audio combo did not just produce a quantitative shift in the performance of my reference system, but a qualitative transformation, creating the illusion of real music being played in my listening room. This combo does not just sound like the best of what tubes have to offer (color/tonality/spatiality) married to the best of solid-state designs (low noise floor/transparency/macro dynamics/bass extension). It brings a more realistic, natural, and organic way of creating the illusion of live music. I purchased the ZOTL40 MK.II amplifier to join my stable of other great amplifiers I use in my different systems.
• Visit the Linear Tube Audio website for more product information.
• Check out our Stereo Amplifiers category page to read similar product reviews.