Linear Tube Audio ZOTL40 Mk.II power amplifier

I have always been fascinated by audio power amplifiers. I even tried building about a hundred of them. My best friend in high school, Bill Brier, taught me the basics of soldering, wire management, and reading schematics. He loaned me his Dynaco Stereo 70, and gave me a hot-running, 20W, class-A transistor amp that he'd built on his mother's kitchen table. Bill took me to concerts, and taught me about classical and jazz music. He had perfect pitch, tuned pianos for money, played every instrument in the orchestra, and had memorized the complete keyboard works of J.S. Bach before he turned 16. And this stuff was all on the side—mainly, we built drag race cars together.

Bill taught me to judge audio components by how realistically they reproduced the impact and tone of voices and solo instruments, especially piano. He said that nothing could reproduce the sound of a whole orchestra. Bill was a supersmart science guy who quickly came to believe that the new Dynaco Stereo 120 transistor amplifier outperformed my old tubed Stereo 70. He said that the 120 had "lower distortion," and that I could keep the 70 because it was now "obsolete." Unlike Bill, I was much slower to embrace this new form of lower distortion—especially after his home-made transistor amp set fire to my living room.

Nowadays, I prefer either type of amplifier, solid- or hollow-state, as long as it's been created by a maverick designer who knows what a good amp should sound like when driving a range of quality loudspeakers.

David Berning ( is just such a maverick designer, and his highly original design for Linear Tube Audio's microZOTL2.0 preamplifier, headphone amplifier, and integrated amplifier ($1235) sounds so extraordinarily transparent, lively, and soulful that it gave me no choice but to add him to my short list of Ch'i Masters, which also includes such legends as John Curl, Mike Moffat, and Nelson Pass. And because the microZOTL2.0 remains an important cornerstone of my current reviewing practice, I had great expectations of Linear's new ZOTL40 Mk.II power amplifier ($5800).

The ZOTL40 Mk.II is a class-AB tubed power amplifier in an anodized aluminum case that's available in silver or black. Almost shoebox-shaped at 9" wide by 8.5" high by 14.5" deep, it's very light at 9.7 lbs, and runs cool for a tube amp that can put out 40Wpc into 8 ohms.

The ZOTL40 Mk.II is so light because it doesn't need heavy, impedance-matching output transformers. Instead, it uses an amplifier-load interface, designed and patented by David Berning, that superimposes the antiphase waveforms of the push-pull output tubes on a 250kHz carrier wave, and subsequently on pairs of diminutive, hand-wound RF transformers. The result: push-pull power pentodes without bulky, audio-frequency output transformers.

According to LTA's website, the ZOTL40 Mk.II automatically biases its four KT77 tubes (an EL34 substitute). No user adjustments are required, and LTA predicts a tube life of 10 to 20 years. The ZOTL40 further maximizes tube life with a high-voltage turn-on delay, and a form of automatic protection circuitry.


The ZOTL40's perforated tube cage triggered memories of my old Dynaco Stereo 70, but tubes aside, what was under it reminded me more of a 1970s receiver than a vintage tube amp: an electronic landscape dominated by hordes of small chokes and high-frequency inductor coils. There are two green circuit boards, one each for the power-supply and audio circuits. On the audio board are the eight ceramic tube sockets (this includes the ZOTL40's two 12AX7 and two 12AU7 small-signal tubes), as well as a plethora of generic surface-mount polypropylene capacitors, some Rubycon electrolytic caps, generic film resistors, and two remarkably small power-supply transformers.

On the front panel are a red On light and, far below it, a knob connected to an Alps volume control. On the rear panel are two pairs of RCA input jacks, one connected to the Alps control and the other for a direct input. (The user's guide stresses that only one input can be used at a time.) I used the direct inputs for all of my listening. Also on the rear are two pairs of good-quality speaker binding posts, and an IEC inlet with an On/Off rocker switch.


These days, I begin every review of an amplifier by listening to it through the Zu Audio Soul Supremes ($4500/pair)—not because the Zu Souls are the most accurate (I hate that word) or neutral (what is that?) speakers, but because they let me to listen through their full-range paper cones, directly into the electronics behind them.

In my room, the Soul Supremes have a slight bump in the midbass, a conspicuous rise in the presence region (2–4kHz), and something unnamable but audible in the top frequencies. Nonetheless, the Zu Soul Supreme remains on my short list of all-time favorite speakers, along with the Falcon Acoustics LS3/5a ($2195/pair) and the DeVore Fidelity Orangutan O/93 ($8400/pair). All three speakers are useful in showing me what an amplifier really sounds like when driving a benign load of high impedance. With each, Linear Tube Audio's ZOTL40 Mk.II sounded exceptionally lively and clear, and precisely detailed.

I played Varèse's Ionisation, with Robert Craft conducting 13 musicians playing woodwinds, brass, a piano, and 37 percussion instruments, including two sirens and several large timpani (from Music of Edgard Varèse, "six eyes" LP, Columbia Masterworks MS 6146). To my surprise, the ZOTL40 sounded a lot like my reference high-power amplifiers, Bel Canto Design's REF600M monoblocks ($4990/pair). Both amps delivered six octaves of high-energy, strategic-force impact and strong, articulate bass. With both, the surprise entrances of the various percussion instruments was startling and precise—just as Varèse intended.

The ZOTL40 rendered the dynamic contrasts in Ionisation, grand et petit, with a captivating vitality I won't soon forget. But strangely, and to my complete surprise, the Bel Cantos sounded more colorful and real in the top three octaves. Driving the Soul Supremes, the LTA ZOTL40 exhibited a sound that was direct, fast, and detailed, if somewhat deadpan in its stark evenness. How could a class-D amp sound more airy and colorful than one using push-pull KT77s?

I played Ionisation one last time, carefully comparing the ZOTL40 Mk.II ($5800) with Woo Audio's WA5 integrated amplifier-headphone amplifier ($5899). The Linear gave a distinctly better impression of percussion scale, bass character, and empty space. Timpani had realistic size and impact. Musical drama was well presented. The WA5's 300B triode tubes sounded a bit less dramatic, but achieved a greater amount of density and spatial reality. The Woo's midrange was jam-packed with what seemed an infinity of low-level information. The WA5 materialized the Varèse with a magical vibrating presence, while the ZOTL40 produced an emptier, more diagrammatic expression.

Recording after recording, the ZOTL40 Mk.II delivered joy and excitement through impact, momentum, detail, and control. With the same recordings, the more engaging wholeness of the Woo WA5 comprised extremely realistic timbres, a plush fabric of low-level dynamic contrasts, and something intangible but akin to blue skies reflected by still water. The ZOTL40 was less good at such poetic intangibles.

Linear Tube Audio
Washington, DC
(301) 448-1534

tonykaz's picture

I'm feeling that I now have to lower my expectations on tube gear reviews, for darn good reasons:

A piece of tube gear is like a Turntable/Arm with it's tubes being a (phono cartridge type) variable.

The Pro Audio guys I know will pull their hair out looking for "Good Sounding Tubes", say'n stuff like that tube "sounds like shit!!!"

My old supplier, Art Ferris of Audible Illusions, carefully selected the tubes in all the Modulus 2 preamps he built and sold. He said it was his secret sauce.

Today our very own Kevin Deal has become the KING of tubes by being a reliable supplier of quality sounding glass. His importing PrimaLuna and Mystere speaks to his mastery along with Schiit owners that I know buying glass from Upscale.

My own Conrad-Johnson MV-45a's performance was from superb tubes.

Willian Z.Johnson would send Audio Research gear to Harry Pierson at TAS, those pieces contained very special glass.

The last headphone Meet I went to probably had 50 headphone amps active, the group of headphone tryers ( including me ) gravitated to a Schiit Valhalla 2 Amp who's owner loaded the darn thing with Russian Glass, it was easily the best sounding Amp at the Meet. Jude Mansilla's Schiit Rag/Yggy was there but sounded flat by comparison ( breaking my personal Yggy bubble, oh well ).

During my Retail days, I sold Solid State electronics for it's consistent performance. I also had the complete lines of Conrad-Johnson & Audible Illusions Pre-amps which didn't quite sell well compared to my Electrocompaniet & PS Audio Sales. Now, I realize that I wasn't focusing on careful Tube Selection. I was focusing on Phono Cartridges and Turntables, our specialties !

Tube guys are West Coast, I think.

UpScale in LA
Bottle head up in Washington
All those Schiit Valhalla & Lyr tube rollers seem to be West Coasters.

But the Garage 1217 folks, the Feliks folks are internet and thus International. These are the most serious Tube Rollers on the Audiophile Planet.

I'm coming to think that a piece of Tube Gear with it's chassis, power supply, circuit board & wiring make up a Textured Canvas with the Tubes themselves being the Amplifier reviewed.

Tony in Michigan

John Atkinson's picture
tonykaz wrote:
A piece of tube gear is like a Turntable/Arm with it's tubes being a (phono cartridge type) variable.

I understand that in The Absolute Sound's review of this amplifier, the reviewer "rolled the tubes" until he ended up with something that he felt sounded better than those supplied. Our strict policy is to review tube gear with the tubes supplied and recommended by the manufacturer, because that is what our readers will be auditioning.

John Atkinson
Editor, Stereophile

tonykaz's picture

Of course, I understand your position.

My position ( and the position of any owner/purchaser ) is the variable nature of the specific glass tube doing the amplification work.

The mechanical stuff making up a Valve piece is the support architecture that enables the tubes to provide their magic.

A Tube Gear Review, therefore, seems to be a good piece of writing to accurately describe how this circuit & chassis design is likely to be useful for Valve Amplification in an audiophile's ( stereophile's ) home music system.

This type of writing is exactly the type that our Mr.Reichert is the Master of and why he is at the very top of the Audio Journalism pyramid... and probably why Stereophile feels lucky to have discovered him.

Glass Tubes are a variable... OK, we all can accept that fact, can't we?

Many of us seem to realize that when a Tube piece of gear is "on Song" it's darn near impossible for any Solid State piece of gear to sound anywhere near as good...

but, a superb design of Solid State gear, such as: a D'agostino , a Levison, an Electrocompaniet, a Pass and probably a good few more including my little Schiit Asgard 2 are consistantly satisfying and therefore well worth owning.

I contend that today's Stereophile readership are aware of Vacuum Tubes possibilities and therefore expect the reviewers to provide insight into how to make a reviewed piece of gear sing like Pavarotti.

In a great many ways, Stereophile has raised the bar.

It's time to take Tube Gear Reviews to the next level.

Betcha Kevin Deal is ready to help.

Tony in Michigan

ps. tubes ( themselves ) are cheap $, probably the cheapest upgrade anyone could make.

Birdstone's picture

you say your reviewrs only use the tubes supplied by the manufacturer but AD used upgraded tubes on the prima luna progue review. this is what he said .. "And then, when I replaced the Premium's stock, PrimaLuna-branded EL34 tubes with the Russian-made Tung-Sol KT120s sent to me, for trial, by PrimaLuna distributor Kevin Deal, I was even more impressed with those—an $80 option at time of purchase. I also preferred to use the KT120s in Triode mode." Kevin Deal was allowed to upgrade the tube to make his amp up to snuf. that Kevin gets away with murder.

John Atkinson's picture
Birdstone wrote:
you say your reviewers only use the tubes supplied by the manufacturer but AD used upgraded tubes on the prima luna prologue review.

Correct. And as you correctly quote Art as saying, the Russian-made Tung-Sol KT120s were supplied by the PrimaLuna distributor and are an $80 option at time of purchase.

John Atkinson
Editor, Stereophile

woodford's picture

it would be interesting to know *which* KT77s were in use. i recently replaced the GL KT88s in my amp with GL KT77s, and it sounds like a completely different amp.

Linear Tube Audio's picture

Current production Genalex Gold Lion KT77s were used. We've been considering a return to EL34 as the stock power tube, especially for efficient speakers. When we made the change to KT77s for the Mk.II version, the primary listening systems had speakers less efficient than any of those used in this review.

If anyone is headed to RMAF, you can role-play as Herb and Mr. O in front of the ZOTL40 and the Orangutans, as we'll have that pairing along with a Fern & Roby turntable and phono stage as a source.

mrkaic's picture

I am sure I speak for many when I ask -- what happened to Bill Brier?

Herb Reichert's picture

joined the navy became a radar tech rolled a Chevy V8-powered MGA got out of navy quick became an electronic engineer at HP and a major local drag racer -- lost track of him years ago

mrkaic's picture

What a great guy!

Anton's picture

Heck, I can't believe we didn't get 2500 words on power cord rolling!

I like reviews where, to steal an auto racing phrase, ya run what they brung.

Fine review, Herb!

tonykaz's picture

Now, I ask you, would you consider spending $6,000 for this little amp that doesn't seem to perform up to it's price class?

Geez, we're going up against one of Blackie's Stereo 70's, a $2,000 PrimaLuna, a $600 Fisher 500C from eBay, those new Schiit Amps for $600 each or even a Mint Krell KSA 100 used for $1,500.

For $6,000 Grand I'd be expecting Kevin Deal's better tubes and the ability to play Quad ESL 57s, wouldn't you?

Run what ya brung:

I'd presume that you'd first get your auto "tuned-up" nicely, enough air in the tires, ball joints adjusted, brakes not pulling or rubbing, low restriction air cleaner, slippery synthetic Oil, low rolling resistance tires, the entire chassis stripped out of weight and the best darn Sunoco 260 plus the Nitro bottle hidden under the front seat. ( if you could )

The Manufacturer's comments: the Print Edition mentions their pursuit of Clarity, Neutrality with Uncolored Detail. Mr.Schneider also suggests that this Amp seems to be a Polarizing product.

My contention:

If we're gonna give this little Amp ( a possible prickly performer ) made by one of our favorite Electronics Builders, a fair chance of a successful debut, we should at least get our Glass selected properly. The manufacturer isn't building confidence so it's pretty much up to the reviewer who accepted this challenge, isn't it?

I have to accept that all Tube Gear is Tube selection dependent, I'd like the reviewers to go the extra mile to present the possible range of performance a reviewed piece presents. Isn't that what reviewing tube gear is all about? Solid State is fixed, tube gear is variable. Review Loudspeakers with a few differing amps, try a range of phono cartridges with a Turntable/Arm review, have a closer look at all the variables.

Reviewing Tube Gear is hard work that can be well done, I hope that our better reviewers step up to it's challenges and excel.

Tony in Michigan

Herb Reichert's picture

spent a lot of time in the pits 'runnin' what we brung' at Union Grove dragstrip in Wisconsin

Ludcam's picture

Hi JA and HR. I was looking at this amp for my system and found this review helpful.

I'm glad that Herb feels comfortable identifying some tube gear as "sweet and feminine" sounding and that Stereophile doesn't edit this informative content. Moreover, I agree that it is hard to understand let alone measure how a piece of gear is "neutral" (right HR, what does that mean?) or "accurate" (I hate that word too). Those concepts lack specificity and are not related to sound reproducing equipment.

I've been anxious that my current system was sounding too "good-with-money and Jewish" so adding a little bit of that "sweet and feminine" sound would really help me get to the soul of the music. Sometimes, though, when I go to my friends house I like his more Black sounding system. So much PRAT! Get's me tapping my toes like Al Jolson.

Oh well, time to go beat my chest.