So you’re on the hunt for an integrated amplifier in the mid- to high four figures. There are many options, and in some ways it would be hard to end up with one that wasn’t at least very good. But you’re a discerning shopper, with a refined taste who can’t be bothered with any of the run-of-the-mill, high-powered, class-AB options littering the audioscape. Class-D amps are out due to their lack of character. Most tubed amps require that the owner bias and occasionally replace the tubes -- and what are you, a mechanic? No, you’d rather be listening. Finally, you go weak in the knees when you see a product proudly emblazoned with “Made in the USA.”
I bring good news: Linear Tube Audio’s Z40 integrated amplifier ($7650 as reviewed, all prices USD) is the one you’ve been searching for.
Z-O-T what?
Linear Tube Audio crossed my radar screen last April, when, on sister site SoundStage! Hi-Fi, I read Oliver Amnuayphol’s review of LTA’s similar but less powerful Z10 integrated amplifier. Oliver began with this:
All Linear Tube Audio amplifiers are designed around David Berning’s patented Zero hysteresis, Output Transformer-Less (ZOTL) amplifier technology, and so march to the beat of a very different drum than do non-Berning designs. While the full details of Berning’s patent are beyond the scope of this review, in short, the ZOTL technology does the job typically performed by output transformers: converting the high-voltage, low-current signal from power vacuum tubes to a lower-voltage, higher-current signal capable of driving loudspeakers to sufficient volume levels.
No transformer. Cool, right? Those interested in the gritty technical details can check out Berning’s U.S. patent No. 5,612,646. In short, LTA says that the “warmth and presence” long described as “tube sound” is due to second-order harmonic distortion caused by transformers. In LTA’s view, if you eliminate the need for an output transformer and the second-order distortion it produces, you’re left with “the liquidity and linearity of tubes . . . [and] neutral accuracy without coloration.” As an audiophile who prefers solid-state amplification for just those reasons, I find Berning’s ZOTL architecture a fascinating alternative. LTA is the sole licensee of Berning’s patent -- in fact, Berning himself tests and approves every LTA design based on his patent. It’s helpful that he lives only 30 minutes from LTA’s home base in Takoma Park, Maryland.
LTA suggests that the transformerless approach reaps other benefits. Since the power tubes use a lot less current than they would in a traditional tubed design, the user can expect them to last somewhere between 10,000 and 20,000 hours. The soft, rolled-off bass that can plague traditional tube amps is also avoided, due to the ZOTL circuit’s inherently linear frequency response of 6Hz-60kHz, +0/-0.5dB, into 8 ohms. Finally, the ZOTL circuit’s low output impedance relative to a typical tube amp ensures more consistent performance over a wider range of loudspeakers.
The Linear Tube Audio Z40 has exactly the same amplifier circuit as is found in LTA’s ZOTL40 Reference stereo power amplifier ($6800). The Z40 is the most powerful integrated LTA makes, producing 46Wpc into 8 ohms or 51Wpc into 4 ohms, with 0.5% total harmonic distortion (THD) into either load. Its Berning-designed switching power supply is the same one that David Berning uses in his own, far more expensive electronics, and promises plenty of current. You may not be able to use it to yank tree stumps, but as long as your speakers are reasonably sensitive, the Z40 should be able to motivate them.
My review sample of the push-pull, class-AB Z40 came with its standard quartet of Gold Lion KT77 power tubes, though it can alternatively be ordered with NOS Mullard XF2 EL34 power tubes (add $600). Four 12AU7s and two 12AX7s comprise the rest of the tube complement. The preamplifier topology is pulled straight from one of David Berning’s own five-figure designs. Per LTA’s Nicholas Tolson, “The ZOTL [amplifier] circuitry cross-couples to the preamp gain stage, so it ends up being the same as having a preamp with its own ZOTL circuit. . . . It provides the ability for the integrated to accept a true balanced input.” Per Tolson, the ceramic-printed preamp board has the same sound quality of the same board if wired point to point. Connection-wise, there’s a lot to like: one balanced (XLR) and four unbalanced (RCA) inputs, and tape-monitor ins and outs (RCA). Optional, Berning-designed phono stages are available: moving-magnet only ($500), or moving-magnet/moving-coil ($1250). Include all options -- Mullard XF2s and MM/MC phono stage -- and the Z40’s price rises to $9500.
For that money you get high-quality components: Belleson regulators, internal wiring of Ohno Continuous Cast (OCC) copper, Vishay Dale resistors, Mundorf capacitors, and copper speaker binding posts from WBT. LTA boasts that the outward appearance of its products is handled by the design house of Fern & Roby, based in Richmond, Virginia, who design and manufacture all of LTA’s cases, as well as furniture and other goods.
I wouldn’t call the Z40 pretty -- the 18-pound, 16″W x 5.2″H x 16″D amp has a mechanical, hand-built feel. Its front panel is made of solid aluminum -- nothing special there -- but the power and function buttons are of solid brass, and the latter are touch-sensitive: they don’t move in any way. The small aluminum volume dial isn’t remarkable, though the clicks of the relays as you spin it through its 100 steps of attenuation sure are nice, as is the dot-matrix LED volume readout. There’s a toggle switch for selecting between the speaker and headphone outputs, and two 6.3mm headphone jacks, one each for low-sensitivity (2.6W into 32 ohms) and high-sensitivity (220mW into 32 ohms) ’phones.
The rest of the case is of folded metal, with circular cutouts in the left side of the top panel from which peek the tops of the four power tubes. The Z40 has almost a steampunk-ish vibe that would look right at home in a chic loft apartment with exposed brick and metal beams. The only thing that spoils the theme for this picky reviewer is the single red LED that lights up the amp’s interior -- it draws attention away from the warm, soft glow of the tubes.
The Z40’s specifications include output impedance of 1.2 ohms, input impedance of 47k ohms, voltage gain of 23dB into 8 ohms, and hum and noise of 94dB below full output (20Hz-20kHz). A brief manual and power cord are included, as well as the silver-finished aluminum remote control that shipped with the original Apple TV. The Z40 is shipped in a single cardboard carton and is warranted for two years, with a 14-day, no-questions-asked return policy -- very cool.
Setup
Setup was dead simple. I wired up Mytek Digital’s Brooklyn DAC+ digital-to-analog converter with balanced and single-ended interconnects, to ensure that both types of inputs on the Z40 worked -- they did -- then, when I’d satisfied myself that I couldn’t hear a difference, settled on using the balanced inputs. The LTA and Mytek were both plugged into my Emotiva CMX-2 power conditioner, which helps eliminate the electrical hum in my century-old home. Loudspeakers were Dynaudio’s excellent new Contour 20i minimonitors, which, with a sensitivity of 86dB/2.83V/m, present a nominal impedance of 4 ohms, and might be a bit more difficult to drive than your average loudspeaker. For the latter half of my listening I swapped out the Mytek for Mola Mola’s far more expensive Tambaqui DAC, also into the Z40’s balanced inputs, and the two-way Dynaudios for my three-way KEF Reference 3 floorstanders. My source was a bruised and battered old Intel NUC computer running Roon, Qobuz, and Tidal, wired to my system via USB.
Listening
So much of the appeal of tubes, it seems, is that “tube sound”: that warm, ripe, vivacious midrange that can make solid-state gear sound a bit flat by comparison. I get it -- I don’t like it, but I get it. Associated with that sound is a certain nostalgia that just begs to be paired with classic rock or old jazz.
And the Linear Tube Audio Z40 didn’t sound like that at all.
That Berning-designed power supply was seriously quiet. It wasn’t until I put my ear right up to the soft-dome tweeter of one of the Dynaudios that I heard the slightest bit of noise. When I began streaming music, my first impression was that I was listening to a high-end solid-state amp. The sound was clean, clear, and thoroughly articulate. I would never have guessed that there was a tubed power amplifier -- let alone a tubed preamplifier -- in the signal chain. Fascinating.
“Open Your Eyes,” from Snow Patrol’s Eyes Open (16-bit/44.1kHz FLAC, Polydor/Qobuz), is one of those feel-good indie-rock anthems that has always lifted me up, regardless of the mood I’m in when I press Play. I adored how the Z40 reproduced Gary Lightbody’s soft singing at the beginning, and the accompanying guitar chords -- he sounded beautifully fleshed out and three-dimensional. That reach-out-and-touch-it palpability that solid-state, class-AB amplifiers -- even my reference integrated, Hegel Music Systems H590 -- can have such trouble reproducing, was very much on display. And yet I heard no warmth being imposed on the sound. LTA’s ad copy claims that the Z40’s ZOTL circuit marries the best of solid-state to the best of tubes, and from what I heard, they’re right. As I played this song’s first verse again and again, I kept noting the presence, the midrange finesse. Those characteristics will surely be valued by lovers of chamber music, jazz, and singer-songwriters -- music for which dynamics aren’t as crucial as sounding as lifelike as possible. There was a magic to the sound of this amp’s ZOTL topology -- as demonstrated by the goose bumps it raised on my skin as “Open Your Eyes” crescendos to its conclusion, backing guitarists Nathan Connolly, Paul Wilson, and Johnny McDaid supporting Jonny Quinn on the drums. Oof -- the Z40 was good.
It was a similar story with “Passenger Seat,” from Death Cab for Cutie’s Transatlanticism (24/88.2 FLAC, Barsuk/Qobuz). Dynaudio’s Contour 20i’s proved capable partners for the Z40: this model is slightly polite in the treble, with a smooth yet detailed midband -- perfect for reproducing the voice of lead singer Ben Gibbard in his prime. Gibbard’s piano in the opening melody sounded rich and resonant, and his first sung lines exhibited a lush, golden quality that, more than any other amp I’ve heard in the last few years, harked back to solid-state class-A sound. And for cerebral, cinematic music such as “Passenger Seat,” that’s a terrific quality to have. So far, so good.
I moved upscale. Out went the Dynaudio minimonitors and Mytek DAC, in came my KEF Reference 3 towers ($13,999.99/pair) and Mola Mola’s Tambaqui DAC ($13,400) -- some $7000 of gear making way for almost $28,000 worth. This was a different take on “Passenger Seat” -- Gibbard’s piano suddenly had a lighter tonality, with less weight and color for each note, counterbalanced by more detail, and a more focused stereo image. I chalk up most of this sea change in the sound to the KEFs rather than to the Mola Mola. Gibbard’s voice also had better spatial definition, with a seemingly tighter and faster delivery; sibilants were cleaner, hard consonants were more intelligible, and his falsetto sounded more delicate. The Z40 was not overawed by having to drive more resolving loudspeakers.
Changing gears, I cranked the Z40’s volume knob and cued up “Just What I Needed,” from a 2016 remastering of the Cars’ eponymous debut album (24/96 FLAC, Rhino/Elektra/Qobuz). A few things stood out. First, LTA’s flagship integrated could definitely do dynamics. The signature opening, featuring the late Ric Ocasek’s guitar chords and David Robinson’s drum thwacks, were delightfully taut and articulated, nary an ounce of fat or indolence to be heard. Likewise with Ocasek’s voice, set several feet back on the beautifully rendered soundstage. I don’t doubt that some solid-state amps out there measure better than the LTA Z40, might unearth just a bit more microdetail, and sound a touch more spacious while doing so. But would they score as highly at the Z40 in terms of outright engagement and musical satisfaction? I’m not so sure.
I played the Cars as loudly as I could bear but could hear no compression from my 87.5dB-sensitive KEFs. More impressive, this self-professed solid-state guy who’s a sucker for immaculate-sounding, high-power amps found the Z40 as generally enjoyable as my reference Hegel H590 integrated ($11,000). The only performance parameter in which the Z40 came up short against the solid-state Hegel was in bottom-end power and control. While David Robinson’s drumming had none of the softness or bloom that it might through a traditional tube amp, I think that most of the solid-state integrateds I’ve reviewed in recent years -- most of them far more expensive than the Z40 -- edge out the Z40 in terms of outright slam.
But a recording from 1978 -- even in a high-resolution remastering such as this one of the Cars’ “Just What I Needed” -- isn’t the sternest test of a tube amp’s linearity and transparency. So I opted for a torture test: “Brothers in Arms,” from the stupendous score for Mad Max: Fury Road, composed and performed by Dutch DJ Thomas Holkenborg, aka Junkie XL (16/44.1 FLAC, WaterTower Music/Qobuz). This track is an entropic melding of wailing strings, thunderous drums, and distorted electric guitars, all coalescing into rock-operatic madness that makes a lot more sense if you’ve seen George Miller’s visionary film. The Z40 more than held its own up to reasonably high volumes with this demanding cut, allowing me to make out all the cacophony laid out before me. Every aspect of the performance was highly resolved -- and, even better, I could hear no bowing of the LTA’s frequency response at either end of the audioband.
But the more complex “Brothers in Arms” became, and the higher I clicked the Z40’s volume dial, the harder I found it to focus on individual elements of the mix. By the time I hit my volume limit for the track, it was clear that the LTA was having serious difficulty retaining control: the soundstage depth shrank, and the sound turned flat and shouty. The Z40 never clipped hard -- for which my KEFs and I are eternally grateful -- but the compression was nonetheless noticeable. In a surprise to no one, this 46Wpc integrated amplifier couldn’t keep up with sweeping orchestral works at high volumes, in larger rooms, when paired with moderately sensitive loudspeakers. The Z40 won’t be ideal in every scenario.
None of which is to say that it couldn’t be utterly engrossing with the right instrumental music. I sat enraptured while listening to Sanctus: London, from Ola Gjeilo’s self-titled 2016 album (24/96 FLAC, Decca/Qobuz). The choir Tenebrae, under the direction of Nigel Short, sounded magnificent, and the word presence again sprang to mind. The ambience of All Hallows’ Church, in Gospel Oak, London, was on full display through the KEF Reference 3s, the linear top end extending the soundstage more than a couple of feet beyond my listening room’s front wall. If the true mark of great hi-fi gear is how quickly and effectively it transports you from, say, a small inner-city home and into a wood-beamed church on the south bank of the Thames, well, the Z40 is something very special.
The Z40’s headphone amplifier proved no mere afterthought. I used it for a couple hours with my AKG K371-BT and NAD Viso HP50 headphones, with good results. Neither set of cans is particularly difficult to drive, each with an impedance of 32 ohms. But the headphone amp’s noise floor was commendably low, and much of what I heard from the LTA-Mola Mola pairing through the KEFs was what I heard through my headphones. This headphone amp is a useful feature that could make the Z40 the centerpiece of a killer office system.
Comparison
After anchoring my system with the Linear Tube Audio Z40 for so many weeks, it was nice to get reacquainted with the Hegel H590. Generating 301Wpc into 8 ohms, the Hegel integrated is an $11,000 howitzer -- but in terms of appearance and feel, the Z40’s exposed power tubes, clicky volume control, and touch-sensitive brass buttons give it a lot more personality than the minimalist, monochromatic H590. And while the Z40 has a built-in headphone amp and LTA offers an optional phono stage, the H590 counters with a high-quality built-in DAC as standard. I can’t imagine that these two integrateds would ever find themselves head-to-head with one another, vying for the same audiophile’s hard-earned bucks or euros.
Nonetheless, it was fascinating to hear their similarities and differences. With the Mola Mola Tambaqui DAC plugged into the H590’s balanced inputs, the LTA was actually the quieter of the two integrateds -- and the more involving. With Death Cab for Cutie’s “Passenger Seat,” the LTA fleshed out Ben Gibbard’s voice a bit more than the Hegel could; for all its power and current, the H590 lacked that holographic quality with voices. But in other ways, the Hegel was more accomplished: The Cars’ “Just What I Needed” sounded more spacious, with a deeper soundstage, and Ric Ocasek’s voice was more finely delineated -- the combination let me more easily survey this classic track’s component parts. And with Junkie XL’s “Brothers in Arms,” the Hegel highlighted what six times the output power and a monster toroidal transformer can buy: extraordinarily high output and obscene bass control. I found myself preferring the featherweight LTA with more intimate material, such as the Death Cab and Ola Gjeilo tracks, and the Hegel for Mad Max and the Cars. You can never have it all.
Final thoughts
Linear Tube Audio’s Z40 tubed integrated is one of the coolest integrated amplifiers I’ve reviewed. Its ZOTL architecture paid real sonic dividends -- the sound was tonally neutral, linear at the extreme high and low ends of the audioband, and boasted a holographic midrange for which some audiophiles search decades without ever finding. Within its modest power envelope it did everything well, and was downright sensational with human voices and intimate instrumental music. The Z40 will no doubt work in a variety of systems and settings. But give it the right partnering loudspeakers and this amp won’t merely play music -- it will sing. That is most uncommon.
. . . Hans Wetzel
hansw@soundstagenetwork.com
Associated Equipment
- Speakers -- Dynaudio Contour 20i, KEF LS50 and Reference 3
- Integrated amplifiers -- Hegel Music Systems H590, Vinnie Rossi L2i-SE
- Digital-to-analog converters -- Mola Mola Tambaqui, Mytek Digital Brooklyn DAC+
- Sources -- Intel NUC computer running Roon, Qobuz Studio Premier, Tidal HiFi
- Speaker cables -- AudioQuest Rocket 33, DH Labs Q-10 Signature, Dynamique Audio Caparo
- Analog interconnects -- Dynamique Audio Shadow unbalanced (RCA), Nordost Blue Heaven LS balanced (XLR)
- Digital link -- DH Labs Silver Sonic (USB)
- Power conditioner -- Emotiva CMX-2
Linear Tube Audio Z40 Integrated Amplifier
Price: $7650 USD. NOS Mullard XF2 tubes, add $600; MM phono stage, add $500; MM/MC phono stage, add $1250.
Warranty: Two years parts and labor; 14-day money-back guarantee.
Linear Tube Audio
7316 Carroll Avenue
Takoma Park, MD 20912
Phone: (301) 448-1534
E-mail: hifi@lineartubeaudio.com
Website: www.lineartubeaudio.com