In February 2014, when I reviewed the David Berning Company’s ZH-230 power amplifier for SoundStage! Ultra, I instantly fell in love with it. Its sound consigned to the scrap heap -- or at least to the closet -- every other amplifier I’d tried in my system. This tubed design eschewed the output transformers used in virtually every other tube amplifier in the world, and which are responsible for lots of sonic problems. Everyone who heard it agreed that the sound was just splendid, and its 30Wpc easily drove my sensitive horn speakers.

But the ZH-230 had a few eccentricities, including its use of weird output tubes -- the 33JV6, long out of production, though still inexpensively available as new old stock (NOS). However, even if there are still ample supplies of 33JV6 tubes, the buyer might wonder if a set of 50-year-old tubes could still be in good condition. Sometimes, old tubes lose their seals and begin to leak air into their vacuums, and often the pins corrode. So wouldn’t it be nice if your favorite amp used only tubes that are still in production, and that come with a warranty?

Linear Tube Audio ZOTL40 Mk.II

Enter Linear Tube Audio’s ZOTL40 Mk.II stereo power amplifier ($5800 USD), also designed by David Berning. It includes the same fabulous ZOTL circuit as the ZH-230, but implemented and manufactured by Linear Tube Audio (LTA). ZOTL stands for zero hysteresis, output transformerless, and 40 indicates that the amplifier outputs 40Wpc. Apparently, that rating is conservative; I was told that my review sample actually outputs 46Wpc, and can be bridged to output about 70W as a monoblock.

The ZOTL40 Mk.II uses Berning’s basic circuit design, but uses 12AX7 and 12AU7 tubes for input and phase-splitting duties, respectively, and EL34 or KT77 output tubes -- the review sample had KT77s, which sound better and produce slightly higher output than EL34s. All tubes furnished with the ZOTL40 Mk.II were Genalex Gold Lions -- premium Russian tubes that, yes, are still being made. While the Berning ZH-230 is a pure power amplifier, the ZOTL40 Mk.II has a Japanese Alps volume control: you can plug in a single source, hook up your speakers, and enjoy music. The input sensitivity is 1.3V, the input impedance 50k ohms. Unfortunately, there’s no remote control, something we couch potatoes will miss. If you want to use the ZOTL40 Mk.II strictly as a power amplifier, it has inputs that bypass its volume control. It has XLR and RCA input jacks, though both are unbalanced connections -- the XLR jacks are there only for convenience, and are actually combination XLR/phone jacks.

Linear Tube Audio ZOTL40 Mk.II

The ZH-230 comes only in an enclosure of silvery brushed aluminum that to me looks elegant. The ZOTL40 Mk.II comes in two colors: black, like my review sample, or midnight blue. There’s a nice faceplate of brushed aluminum, and the case is also aluminum. A perforated cover protects the tubes from the careless, and their fingers from the open circuit board, on which lethal voltages appear. It also protects the rest of your house from the radio frequencies generated by this amp. DO NOT operate the amplifier without the cover in place.

The ZOTL40 Mk.II’s enclosure is in two sections. In the bottom is the power supply, which seems well isolated from the amplifier circuitry above it. While this layout makes sense, the resulting dimensions of 9”W by 8.5”H by 14.5”D could make it difficult to fit this amp into an equipment rack, especially if you leave a couple inches’ worth of space for ventilation above the tube cage -- few equipment racks provide this much height in their lower shelves, and many are less than 18” deep. So you can place the ZOTL40 Mk.II on a top shelf of your rack (here’s hoping it leaves enough room for your turntable) or on a dedicated amp stand.

The ZOTL40 Mk.II weighs only 9.7 pounds -- if it were any lighter, you’d have to tie it down to keep it from floating away. The light weight is another advantage of OTL design, and here’s another: If you run a conventional tube amp with no speakers hooked up to it, you can damage its output transformers; not the ZOTL40 Mk.II. However, running the amp without speakers connected is still not recommended -- it can trigger the ZOTL40’s very aggressive protection circuitry, which can take a while to reset itself.

Linear Tube Audio ZOTL40 Mk.II

The ZOTL40 Mk.II’s price of $5800 is a lot lower than the Berning ZH-230’s $8360, but still pretty expensive for a 40Wpc stereo amp. The one-year warranty on parts and labor seems short for such an expensive product, but it includes the tubes; most makers of tube amps warrant their tubes for only 90 days.

Setup and use

The first thing I did after unpacking the ZOTL40 Mk.II was to read its five-page user’s guide. It’s clearly written, with a line drawing of the rear panel. To ensure enough space for ventilation, I placed the Linear amp on the top shelf of my rack, which had just been vacated by an amplifier that was even taller and weighed 70 pounds -- I really appreciated the featherweight ZOTL40 Mk.II.

The LTA’s speaker terminals are five-way, gold-plated binding posts. I used Crimson Audio’s Crimson RM Music Link speaker cables. And, because they’d worked so well with the Berning ZH-230, I hooked up to the ZOTL40 Mk.II my Affirm Audio Lumination horn speakers. An IEC connector on the amp’s lower section, on the right side of the rear panel as you face it, lets you connect the power cord, and the power supply is universal -- it automatically works with any AC voltage from 100V to 240V. That’s cool. But the on/off switch is part of the IEC connector, which means you have to reach around to the back of the ZOTL40 Mk.II to turn it on or off -- which is literally not cool. Fortunately, when you do, the hot tubes don’t fry your arm. I really prefer a component’s on/off switch to be on or near its front panel. When I turned the ZOTL40 Mk.II on, at first no sound came out -- there’s a 45-second delay as it warms up. I didn’t panic; after all, I’d [ahem] read the manual.

The ZOTL40 Mk.II’s output circuit is a class-AB push-pull design, which makes it sound like a Dynaco ST-70 -- except that, to save money, Dynaco used 7199 pentode tubes, not 12AX7s and 12AU7s. But the ZOTL40 Mk.II’s circuit is far from a standard design, using a radio-frequency signal carrier. Tubes are shipped installed. As with all David Berning designs, the tubes are rated for an enormously long life: 10-20 years! You may never have to replace them. But if you do, the output tubes are self-biasing, so you don’t have to bother with that unpleasant task. The user guide cautions you that rolling tubes -- i.e., replacing the manufacturer’s stock tubes with other types, to tailor the sound -- is done at your own risk. That’s fair.

Linear Tube Audio ZOTL40 Mk.II

Although a break-in period of 300 hours is recommended, the ZOTL40 Mk.II sounded quite good right out of the box. Of course, the review sample already had some 200 hours’ playing time on it -- I never heard it when it was brand new. I heard no bad habits from the LTA -- just a tiny thump when I turned it on or off -- and it was very quiet. The LTA’s turn-on delay was about as long as my preamp’s, so it imposed no additional waiting time.

The ZOTL40 Mk.II is described as a power amplifier, so for most of my listening I used it connected to an Audio Research SP20 preamplifier. I used Audience Au24 SX unbalanced interconnects plugged into the LTA’s Direct RCA inputs, which bypass the volume control. I also briefly tried connecting the source to the ZOTL40 Mk.II’s other input -- if you have only one line-level source, you might prefer that configuration, with volume control -- assuming you can live without remote control of volume.

As is appropriate for any test of a full-range amplifier, I turned off my subwoofer; otherwise, I’d have been hearing the sub’s own amp in the deepest bass.


“Folia: Rodrigo Martinez 1490,” from Jordi Savall’s La Folia 1490-1701 (16-bit/44.1kHz AIFF, Alia Vox), is one of my all-time favorites. It’s performed by a small band of renaissance instruments led by Savall, who also plays viola da gamba, a fretted viol that resembles a cello. The track begins with several whacks on cascabels, or sleigh bells, and the Linear Tube Audio ZOTL40 Mk.II clearly distinguished the minor differences in each stroke. Without the subwoofer, my system’s bass extension and power were reduced; however, the bass was still deep and powerful through my horn speakers run full-range. High frequencies had a slight additional sparkle I don’t usually hear, which enhanced detail, and the wood block’s transient leading edges were more prominent. This additional detail was welcome -- the extended highs didn’t sound peaky or etched. In softer passages, however, the percussion instruments receded somewhat into a background haze -- sometimes, with other gear, they sound distinct throughout the track. Instrumental harmonics were vivid and rich, while the continual changes in microdynamic levels were easy to follow. Sometimes, these were portrayed as series of discrete levels rather than as a continuous volume spectrum. There’s no measurement for this, but the ZOTL40 Mk.II revealed that Savall’s band was having a blast.

Linear Tube Audio ZOTL40 Mk.II

Another old fave is a disc of a cappella music of Allegri and Palestrina, performed by the Tallis Scholars (24/96 FLAC, Gimell). Allegri’s Miserere is performed by a small main chorus at the front of the stage, a smaller group of soloists some distance behind them, and a solo tenor. The ZOTL40 Mk.II spread these singers widely across the soundstage. The solo tenor sometimes sounds a little brittle, but not through the ZOTL40 Mk.II -- there was lots of air around his location. The distant solo group sounded farther back than usual, with a slight loss of detail. Some components have made the solo group sound closer, with less hall reverberance, which sounds more accurate to me.

Beethoven’s 32 piano sonatas are regarded as being among the pinnacles of the solo-piano repertoire. Recently I acquired a complete cycle of them, performed by Mari Kodama (DSD64/DSF, Pentatone/Primephonic). To my ears, the DSD recording format is especially well suited to recordings of solo piano, reproducing with unusual accuracy the actions of hammers hitting strings. In Sonata No.32, the ZOTL40 Mk.II made Kodama’s Steinway sometimes sound slightly clangy in the upper registers. If you’ve ever heard a Steinway, you know it doesn’t sound at all clangy, unless it’s broken. It projected enormous power or great delicacy, as required -- and, again, a terrific dynamic range.

I’m not sure why I enjoy Amber Rubarth’s Scribbled Folk Symphonies (24/192 FLAC, Chesky). Perhaps it’s because she actually interprets her songs, instead of just singing words. It’s certainly not because it’s one of the very few albums I have that sports an Explicit Lyrics sticker. Maybe it’s because of songs like the relentlessly cheerful “Any Time,” a welcome respite from the abundance of songs with depressing lyrics. Or maybe it’s because of Chesky’s typically crystal-clear Binaural+ recording. The ZOTL40 Mk.II sharply revealed the nuances of Rubarth’s vocal production, along with those of the accompanying small orchestra. I felt I could visualize the way she formed each word and how she phrased the lyrics. The ZOTL40 Mk.II captured the instruments realistically, with full, rich harmonics. Although the song sounded very detailed, its sound was also quite relaxed -- two qualities that don’t always go together.

Linear Tube Audio ZOTL40 Mk.II

To hear how the ZOTL40 Mk.II sounded through its own volume control, I connected the output of my DAC to the LTA’s volume-control input. The sound became more open, with more detail -- there’s no preamp like no preamp. Soundstages expanded, musicians’ positions were more precisely delineated, and the echo chorus in Rubarth’s “Any Time” sounded three-dimensional. If you have only one source component, direct input is obviously the better choice, in terms of sound quality.


I compared the Linear Tube Audio ZOTL40 Mk.II with -- what else? -- the David Berning Company’s ZH-230, the latter using Berning-provided 12AU7 tubes in all small-signal positions, and the aforementioned 33JV6 output tubes. I used the same Audio Research SP20 preamp and the Affirm Audio speakers, and the same interconnects and speaker cables. Measuring 16.5”W x 6.75”H x 12"D, the Berning looks more like a standard amp, and will fit in more equipment racks than the LTA. It weighs 15 pounds -- more than the ZOTL40 Mk.II, but still a featherweight. The ZH-230’s only inputs are unbalanced RCAs, and it has no volume control. Rather than typical rubber feet, it’s supported by Stillpoints, which reduce its susceptibility to vibrations transmitted through whatever it sits on. To my eye, the Berning looks considerably more elegant than the LTA -- but considering that a power amp will probably be stuck out of sight on a bottom shelf, that may not matter.

The ZH-230 revealed the same minor differences in the sounds of the cascabels that begin “Folia: Rodrigo Martinez 1490” that the ZOTL40 Mk.II did. Bass, too, was extended and powerful through the Berning. If I say that the highs sounded just right, that’s probably because I’m used to hearing them through the ZH-230. Percussion instruments were slightly more distinct in softer passages, clattering more persistently in the background. There wasn’t much difference in harmonic accuracy and macrodynamics, both of which have always been strengths of Berning designs. The differences in sound with this track were tiny.

In Allegri’s Miserere, the tenor sounded a bit more human than with the ZOTL40 Mk.II. Oh, great, more reviewerspeak -- is Forrester suggesting that this recording uses space-alien singers? No, I mean that the track sounded more like a human voice, not a recording. The distant solo group sounded closer to the front of the soundstage, and their voices were easier to understand. As with the ZOTL40 Mk.II, the width and depth of the soundstage were convincingly carved out.

Mari Kodama’s recording of Beethoven’s Piano Sonata No.32 exploded out of the speakers, with huge dynamic reserves. The Berning amp’s ability to track small changes in tempo made Kodama’s phrasing just a smidgen more evident. Here there was no clang in her Steinway’s sound, just magnificent power. Exciting!

Finally, in “Any Time,” I thought I could distinguish Amber Rubarth’s words a bit better. The recording sounded a bit more like someone singing instead of a recording of someone singing, and the echo chorus was spread more widely across the stage.

At times, it was hard to hear much difference between these amplifiers, so I used a variety of different types of recordings -- solo voice, chorus, small instrumental group, solo piano -- to try to make any differences in sound more obvious. With a few of these, there were areas where I slightly preferred the Berning amplifier’s sound. Earlier, I questioned whether the ZOTL40 Mk.II is a good value for $5800. My answer is a resounding “Yes!” Were the differences worth the $2560 premium for the Berning? For me, probably, also yes -- but I suspect that most people would be glad to pocket the savings.

Bottom line

Linear Tube Audio has built an amplifier with downright splendid sound. I loved its predecessor, the David Berning Company’s ZH-230, enough to buy it, and it’s still my preferred amplifier. The ZOTL40 Mk.II updates that design, using currently available tubes. Its sound is nearly indistinguishable from the ZH-230’s, while producing 50% more power and costing $2560 less. If that sounds like a bargain to you, you’ll hear no argument from me.

Of course, that assumes that the ZOTL40 Mk.II’s power output will drive your speakers. If you need more power, that’s another story -- but a story that’s true of almost all amplifiers. The ZOTL40 Mk.II’s volume control makes it more flexible than the Berning, and if you can endure being deprived of remote volume control, it even sounds better than my standalone preamp. The LTA’s only drawback is that its dimensions could make finding room for it in your rack a challenge. But if it doesn’t fit, you can place it on a separate amp stand, where it will probably sound better anyway.

Strongly recommended.

. . . Vade Forrester

Associated Equipment

  • Speakers -- Affirm Audio Lumination, JL Audio Fathom f110 subwoofer
  • Amplifier -- DavidBerning Company ZH-230
  • Preamplifier -- Audio Research SP20
  • Sources -- Sony XDR-F1HD tuner (modified by Radio X); SOtM sMS-1000SQ streaming music player with sPS-1000 power supply; QNAP T-251 NAS for music file storage; PS Audio DirectStream DAC
  • Interconnects -- Audience Au24 SX (balanced and unbalanced) and Au24 SE (USB), CablePro Freedom (unbalanced), Crystal Cable Piccolo (unbalanced)
  • Speaker cables -- Crimson Audio Crimson RM Music Link loudspeaker cables
  • Power cords -- Audience powerChord e, Clarity Cables Vortex, Purist Audio Design Venustas

Linear Tube Audio ZOTL40 Mk.II Power Amplifier
Price: $5800 USD.
Warranty: One year parts and labor, including tubes.

Linear Tube Audio
Takoma Park, MD
Phone: (301) 448-1534