April 2018

Linear Tube Audio ZOTL10 MK II Vacuum Tube Amplifier
A paradigm shift in a box.
Review By Paul Schumann

  Full disclosure here, I'm a tube guy. I was raised on Dynaco. I've had the same JoLida amp for 22 years. Sure I've listened to solid-state amps in my life, even had some in my home, but I've always gravitated back to those therimonic devices. Maybe I'm just brainwashed Dynascum. Maybe if I get a chance to hear one of Nelson Pass's well-received amps or a 47 Lab Gaincard I might change my mind, but right now that's where I plant my flag. And this brings me to my review of the Linear Tube Audio ZOTL10 MK II stereo vacuum tube amplifier. Yet first....

Of course valve-powered amps vary quite a bit. My listening experiences have ranged from the intimidating 200 Wpc Conrad Johnson Premier One to the always-lovable 3.5 Wpc Fi 2A3 'X' amplifier (reviewed here). But no matter what the design, all of them had output transformers so they could talk correctly to the speakers. No transformer is perfect. Even the very best ones have inherent distortions. It has been one of those tradeoffs in life. I love being able to haul all sorts of stuff in my minivan, but I'm never going to beat a Ferrari off the line when the light turns green.

All of Linear Tube Audio's amplifiers are designed by David Berning. Some people think he's a genius. Harvey "Gizmo" Rosenberg thought he was an alien who came down to enlighten us. Berning created and patented the ZOTL topology. He still personally builds his amplifiers under the Berning name, just like Dan Garber did with fi. I can't imagine how much soldering that man has done in his lifetime. But one man can only solder so much. That's were LTA has given poor Mr. Berning a break. Their equipment is made right here in the USA with his guidance. You may not own an amplifier personally made by its creator, but it's pretty darn close.

Now ZOTL stands for zero-hysteresis output transformer-less. Say that ten times fast and you win a lollypop. Now, I can look at a schematic of a simple single-ended or push-pull amp and I figure out what's going on. As soon as I look at a schematic of a ZOTL amp, I am lost. There is great explanation of ZOTL in an old issue of Glass Audio magazine I'd recommend if you are really interested. I read it several times and I think I know what's going on, but don't hold me to that.

As soon you take the ZOTL10 MK II out of the box, you notice how light it is. Duh, no output transformers! Even the separate power supply isn't heavy. Oh yeah, the separate power supply, that's where the Mk II designation comes from. You have to connect the amp to the power supply with a short umbilical. Easy-peasy. On the back there are two sets of inputs, one for using the volume control on the front, and one for bypassing it. Also in the back are sturdy binding posts, which were a little close together for my cumbersome Audio Magic speaker cables. Unlike many amplifiers, the ZOTL10 MK II is deeper that wide, so when I slid it into my modest stereo cabinet, it barely fit.

Now about the looks. If you like to show off to your friends that you have a tube amp, you might be a little disappointed. The front is a large square of brushed aluminum, a volume knob and a blue "power on" LED. The guts are covered by black perforated metal cover that more than reminds of me of the cover on my old Dynaco ST70. Turn it on, peer inside, and yup there they are, pretty glowing vacuum tubes. Luckily I'm a secure tube owner, so just knowing they are in there doing their job is fine. If anyone doubts me, they can take a peek for themselves.

Peering inside, the ZOTL10 looks more like UNIVAC than a traditional tube amplifier. Two 12AT7s and two 12AU7s are used in the driver/splitter stage and four EL 84s are used in push-pull configuration as the output tubes. Those of you who are guitar-amp geeks know that EL 84s were responsible for the British-blues sound in the 1960's. They also further evoke the Dynaco comparison. Back in the day, Dynaco had the ST-35, which was EL 84 based. The more famous ST-70 used the bigger EL34s. The big brother ZOTL40 uses EL34 vacuum tubes also. Coincidence? I think not.

Back to the insides. The 12AT7s and 12AU7s are NOS; the EL 84s are the new Genalex Gold Lions from New Sensor in Russia. Using these tubes as standard equipment demonstrates a dedication to quality. It's too bad that there aren't more NOS power tubes out there anymore. A set of EL 84 Bugle Boys would be nice. I'm sure Keith Richards has a stash somewhere. Oh well, one can dream. There are no biasing adjustments needed since the ZOTL10 MK II is self-biasing. This is about as much plug-and-play as it comes.

Whenever you listen to a truly great piece of equipment, it shows you its greatness right out of the box. This is definitely the case with the ZOTL10 MK II. As normal in my home, there were several family members wandering through the living room when I was hooking things up. I just happened to have Imogen Heap's Ellipse [Epic ‎– 88697 506052] laying around, so I put it on. The first track First Train Home started playing. My son, who happened to be walking by, stopped in his tracks.

"Is that the new amp?"

"Yup."

"It sounds so … smooth."

"You're right."

"How long do you get to keep it?"

"Until I'm done reviewing it."

Pause. "Well take your time, this sounds great."

It's good to know that your own flesh and blood appreciates some of the same things you do. But what elicited that reaction? I've had many different pieces of gear in my home and never before has anyone had such a visceral response. My wife and eldest daughter acted quite the same way. (My youngest always give a shrug) How can I even quantify this? One of my favorite Gizmo quotes is, "Once we get the hang of the words, we lose their meaning." I'll try my best.

I know it's sort of boring to talk about, but let's talk about the ZOTL10's performance across the frequency spectrum, starting with the bass. I have never heard a tube amp, check that - any amp, in my home with such deep, tuneful, freaking POWERFUL bass. Remember, this is 12 WPC! More about that later.

Lorde's Melodrama [Republic – 602557725070] is chocked full of great bass lines. At the beginning of the second verse of Green Light, some of that deep, tickle-you-in-the-loins bass kicks in. Whoa, where the heck did that come from? I didn't even know my old Thiels could get there. Who needs a sub when you have this testubular bass? Ok, let's move on to the midrange, sticking with Lorde's catharsis. The vocals on this album are intimate; up front with no reverb. There she is, front and center, growling out her pain. Hey, you don't have to scream when you are in pain. That growl, it comes up from her chest, reverberates in her mouth, and hisses over her exposed teeth. Despite this tortuous path, the ZOTL10 MK II never turns her vocals into Parseltounge. Lorde's natural hiss is enough. This is single-ended triode midrange territory guys.

This benevolent nature extends to the high frequencies also. Recordings that I had thought previously as "too screechy", like Snow Patrol's Fallen Empires [Fiction Records – 2786192], are suddenly tamed. There is absolutely no listening fatigue. The best I can describe it is like a really good 2A3 amp. That good. On top of that, there's a seamlessness. None of the frequency ranges get preferential treatment. Leopold Stokowski Encores [London Records – 433 876-2] is not what you would consider an audiophile disc. I've listened to it many a time and thought I knew its sonic flaws. Playing it through the ZOTL10, the string basses have a new fullness, but they are never bloated, the woodwinds have new tonal richness, and the violins are suddenly singing. This is a revelation.

I think another reason that the ZOTL10 MK II elicits such a gut reaction is the way it avoids smearing the subtle musical cues that you experience in a live performance. Steven is his review of the 47 Labs Gaincard perfectly described this property as "rhythmically musical freedom". I've never had a chance to hear an Audio Note UK Ongaku or the Gaincard, but I suspect this is what we have been hearing at our home since the ZOTL10 MK II arrived. Everything just flows like real music. James Horner's Pas de Deux [Mercury Classics ‎– 481 1487 DH] is a pleasurable listen because of the interplay between the violin and cello played by the Samuelsen siblings. What is it about family members playing together?

Listening to this performance through the ZOTL10 MK II, you can feel the back and forth between brother and sister; listening to each other in a way that only people who have played years together can. It's a thing of beauty. Another example of this is Bill Evan's Waltz for Debbie [Analogue Productions – 0009], where you can feel the jam between the members of this trio. Sitting there, I kept finding myself gently swaying or nodding, caught up in the rhythm. Evans, LaFaro, and Motian at times might seem like they were each doing their own thing, but then they all come together, letting you know that they had it under control all the time.

Now let's go into a paradigm shift here folks. The ZOTL10 MK II is a push-pull type amp. As typical, this amp employs negative feedback to lower the distortion. This tends to do two things, introduce a time-phase shift which blurs some of the musical cues, and flattens out the dynamics. As I said in the previous paragraph, the ZOTL10 MK II swings like crazy. Dynamically the ZOTL10 MK II is as free-wheeling as any SET I've heard. Looking through my old vinyl, I pulled out Brahms' Second Piano Concerto [Resonance – 2535 263]. This is lower-priced re-issue of a Karajan / BPO recording with GezaAnda at the piano. Not exactly the stuff of audiophile fetishes. Playing it through the ZOTL10, I'd never heard this recording so dynamic and alive. Just what the heck is going on here? I admit it, I've become a bit of a curmudgeon in the last few years of my life. I'd become convinced there was only one path to audio enlightenment, but now ZOTL has shown me an alternate path. That's the funny thing about life, no matter how old you get, you can still learn something.

Now before I wrap things up, let's touch on one more topic -- power. How much power do you need? I know some of you saw that 10 Watts and thought, "Well, that's not going to work for me." I listened to the ZOTL10 through my 4-Ohm Thiel CS1.5s, so the rated power output was 12 Wpc. As I have mentioned before, the Thiels are not the most sensitive of speakers at 86dB/W/m. Sounds like a poor match, doesn't it? Wrong. Never did I feel like the ZOTL10 was running out of steam. If you want to rock out to Muse or Foo Fighters, be my guest. You don't need to crank it up to "11" to get that killer bass you crave. This amp gives it to you at lower listening levels than you're used to. As long as you don't plan on pairing them with some hard-to-drive planar speakers, electrostatics, or giant arrays, you should be fine.

As you already guessed by now, I am smitten with the ZOTL10 MK II. Its brings out the best in any music you play through it. Rock, jazz, classical, pop – it doesn't matter. What made it so difficult to review this amp is the reviewer part of my brain would keep shutting off and I would just start listening to the music. I can't think of any better compliment than that. Now if I could just get my hands on some Bugle Boys, we could go to the next level of enlightenment.

Specifications
Type: Stereo vacuum tube amplifier
Amplifier Class: Push-Pull Class AB
Tube Complement: Four EL84, two 12AU7 and two 12AT7 tubes
Power Output: 12 Watts per channel into 4 ohms
Frequency Response: 6Hz to 60kHz（-0.5dB)
Hum and Noise: 94dB below full output
Input Sensitivity: 1.2V RMS for 10 Watts output
Dimensions: 9 x 8.5 x 18 (WxHxD)
Weight: 14 lbs.
Price $3200

Company Information
Linear Tube Audio
7316 Carroll Ave.
Takoma Park, MD 20912

Voice: (301) 448-1534
E-mail: hifi@lineartubeaudio.com 
Website: www.lineartubeaudio.com