by John Richardson

Linear Tube Audio and David Berning

Let’s start this review with a little pop quiz.

Question 1: Have you heard of Linear Tube Audio (the company)?

No? OK, I didn’t think so.

Question 2: Are you familiar with David Berning?

You are? No surprise there, as Mr. Berning is one of the best known amplifier designers of the last 40 years or so, especially when it comes to vacuum tube designs.

So why do I ask, you ask? As it turns out, there’s a very definite link between these two entities. Linear Tube Audio (LTA as I’ll call it for short) is a relatively new company, founded in 2015, specifically for the purpose of building, marketing, and purveying some of the newest, classic, and best amplifier designs by David Berning. Both Berning and LTA are in the Washington DC area, and the collaboration remains fluid and fruitful. As of today, LTA offers three different amplifier products, all based on Berning’s ZOTL (Zero-hysteresis Output Transformer-less) technology.

So what does ZOTL really mean? Most audiophiles are at least aware of Julius Futterman’s true output transformer-less amplifier designs, but ZOTL is actually something quite different. Unlike Futterman’s amplifers, Berning’s ZOTL amps do retain output transformers- they are just used in a very different way than in typical amplifier designs.

According to Mark Schneider, founder and head guy at LTA, normal amps using output transformers have tonally colored and distorted outputs due to the un-optimal turn ratio employed [as an aside, an AC voltage signal can be either stepped up or down depending on the ratio of turns, or coils, between the secondary (output) coils and the primary (input) coils of the transformer]. That’s why tube amps are known for those pleasant, but nonlinear, distortions. If the turns ratio between the secondary and primary could be jacked up by say an order of magnitude, then everything would just be honkey-dory. That way, the inherent linearity of vacuum tubes could be utilized to the fullest, and we’d be free of those pesky distortions.

So then, how does ZOTL technology accomplish this goal? Here’s the beef, explained better than I could do it in my own words, taken from the LTA MicroZOTL2.0 user manual: “The ZOTL amplifier uses radio frequency to change the voltage-current transformer characteristics of the output tube from its normal impedance plane to one suitable for driving a dynamic loudspeaker. The radio frequency re-mapping is implemented using special high-frequency power-conversion techniques. The high-voltage, low current tube impedance is remapped to the high current speaker impedance plane through special transformers operating at a constant RF carrier frequency of 250 kHz. Because the audio signal is riding on a carrier, it is not subject to parasitic elements of the transformer that would distort the audio signal. Unlike the conventional audio-output transformer, this impedance transformation operates on both the AC and DC components of the signal…”

The upshot of this technology is that now, rather than using a transformer with the less than optimal turn ratio limit of 25:1 or less, one with the much cleaner output afforded by the more optimal 168:1 ratio can be employed. In other words, bye-bye distortion. Oh, and a byproduct of this business is that the vacuum tubes employed can now be operated at lower voltages where their lifetimes are stretched out to 10-20 years, depending on frequency of use. For those of us a bit leery of tubes, that’s some good news indeed.

But wait… isn’t the device supposed to be transformer-less? Here we are talking about optimal turn ratios. So are there transformers in there or not? I posed the question to Mr. Schneider, who answered in kind: “the ZOTL actually does not have output transformers. Output transformers require iron (or some other metal) cores to create inductance. We use an air core transformer device called an impedance converter, used in RF and switching supply applications, and typically operates at much higher-than-audio frequencies.” So there you have it: yes, no, or maybe, depending on how you want to define “transformer.” I see it as something of a “transformer-like device”.

So back to product discussion. On offer from LTA are two somewhat traditional amplifier designs, the ZOTL 10 and ZOTL 40, which are meant to drive speakers with 10 watts per channel and 40 watts per channel, respectively. Then there’s the somewhat funky little fella called the MicroZOTL2.0, which is the unit up for review. All of these amps are based on the ZOTL technology described above.

Let me now focus on the review sample, the MicroZOTL2.0. Unlike the other amplifier offerings from LTA, this one can actually serve three distinct purposes: integrated amplifier, active line stage preamplifier, and headphone amplifier. It’s actually marketed as a “headphone and office amplifier” which suggests that it is well suited to a desk-top system. Further, the design itself is nothing new. It was first developed by David Berning way back in 1996 and marketed primarily as a headphone amplifier, even though it also sported speaker outputs. It was popular enough that Berning built and sold it in its first iteration for over 10 years. Unfortunately, it was eventually discontinued, but that’s where LTA and Mark Schneider come in. Mark recognized the unique nature and promise of the design with its ZOTL technology and decided to revive it (with Mr. Berning’s blessing) as LTA’s first viable product. And he did something else I found to be exceptionally cool and useful: he added a set of preamplifier outputs to take advantage of the considerable gain offered by the device coupled with its exceptionally low output impedance. Long interconnects, anyone?

Internally, the tube complement consists of two pairs each of Russian made Tung Sol 12AT7 inputs and 6SN7 outputs in a push-pull configuration.   No tube rolling was attempted during the review.

Setup and User Notes

The MicroZOTL2.0 can be had in two configurations: with the stock switching power supply ($1100) or with a nicely made custom-built solid-state linear power supply, now sold as the Deluxe version for $1695. My sample came with the optional power supply upgrade, so that’s how I reviewed it.

The circuitry is housed in a nearly cubic, nondescript metal box with plenty of ventilation and a nifty see-through lucite top panel. I like being able to look down into the innards of the circuitry, though some people might find this unattractive. Mine came in the typical black finish, but midnight blue is also available. The thick brushed aluminum front panel has, from left to right, a power button that illuminates in its own “ring of fire” when turned on, a toggle source selector switch (for up to two analog sources), the volume knob, and finally, a headphone input. The rear panel is equally spartan and easy to navigate, with one pair of speaker outputs, two pairs of analog inputs (RCA) and one pair of preamplifier outputs (also single-ended RCA). Setup was as easy as expected, but users should be aware that all outputs are simultaneously active, so it’s best to only have one output device (e.g., headphones, amplifier, or speakers) connected at a time. However, I was informed by Mr. Schneider that the newest version of the MicroZOTL2.0 actually has an optional closed-circuit headphone jack that automatically disconnects the speaker and preamp outputs when headphones are plugged in; this is definitely a nice option for the prospective buyer to consider. Power output for both headphones and speakers is a single watt into four ohms, while the preamplifier gain is somewhere between 12 and 14 dB, with an output impedance of only two ohms. If I had only one quibble, it would be that the rear panel feels a bit flimsy when cabling is attached, most likely due to its ample venting.

With these items out-of-the-way, let’s see how the MicroZOTL2.0 sounds…

As a Power Amplifier

Evaluating the MicroZOTL2.0 as an integrated amplifier requires an especially efficient set of speakers, given the amplifier’s single watt of power into 4 ohms, and presumably less into higher impedances. The most logical mating given the speakers on hand would be Zu Audio’s Mk V Druid, which comes in at an efficiency of 101 dB. Seeing such a small amplifier driving these large, floor-standing monoliths almost made me laugh out loud. However, the sound coming out of the Zus’ drivers was nothing to smirk at; it was big and convincing, yet exceptionally lithe. While the Druids are crispy and quick in and of themselves, I definitely had the impression that the little MicroZOTL2.0 didn’t slow things down a bit. I sensed an almost electrostatic-like jump and acceleration in the sound that was more gazelle-like than anything else. Even the bass, though somewhat light in overall impact via the Zus, was nuanced and defined, without overhang or bloat. In other words, nothing at all like classic soft, ponderous tube-amp bass. I’d even go so far as saying that there’s very little to make one think of the MicroZOTL2.0 as a tube amplifier just by listening to it, at least when powering the somewhat lean and fast Druids. More appropriate descriptive terms might fall along the lines of linear, accurate, and uncolored. Given the technology behind the amplifier, I’m assuming that the tubes are being driven in an operating range that allows them to remain exceptionally linear in their output characteristics, thus further minimizing the typical pleasant distortions we are accustomed to hearing in more traditional amplifier circuits.

Quite enjoyable was Coleman Hawkins’ album Sirius (LP, Pablo, digitally archived). The Hawk’s saxophone sounded airy and alive, with a nice woody tone. The plucked string bass was also quick on its feet, with a nice touch of attack and decay, though with not quite the same extension and power as I get with other amp/speaker combinations. I don’t think that lightweight bass is a characteristic of the MicroZOTL2.0 itself, but rather a limitation of its power when used as an amplifier for speakers.

Serving as an integrated amplifier driving my Fritzspeaks Carbon 7 SE speakers, at 89 dB efficiency, I found that I could actually get sound at reasonable volumes when the speakers were set up in a near-field configuration about my listening chair. Even so, when pushing the volume, I could detect some breakup and distortion in female vocal crescendos (e.g., Natalie Merchant’s Tigerlily) which indicates clipping. As suggested in the MicroZOTL2.0’s user guide, 89 dB speaker efficiency is probably about the lowest expected for reasonable performance at medium volume in a small room. However, within its limitations the amp offered up quite nice tone and detail retrieval with the Carbon 7 SE speakers. Bass was actually heftier and more refined than I would have expected, while the midrange was silky sweet with an excellent sense of presence. Through the mids, I could detect just a hint of “tubey” warmth; just enough to let me know that there are vacuum tubes hiding in the amplifier. Highs were also clean and extended, as long as clipping was avoided, at which point things could get ugly pretty quickly.

So yes, you can use the MicroZOTL2.0 as a power amplifier, but be very aware of its power limitations. You’ll need highly efficient speakers and a small space to get optimal performance. I’d imagine that the amp would work very nicely driving a set of small, passive, and efficient monitors in a near-field setup, perhaps on a desktop (more on this later…). Even though I thoroughly enjoyed listening to the unit as a stand-alone integrated amp, I was ultimately left wanting for a bit more dynamics, overall presence, and bass heft in the big-rig system, even with the ultra-efficient Zu Audio Druids.

As a Preamplifier

When I first read about Linear Tube Audio’s MicroZOTL2.0, I was most interested in auditioning it as a stereo preamplifier.

Over my long years as an audio enthusiast, I’ve had what amounts to a love/hate relationship with preamplifiers. We all recognize that back in the glory days of hi-end audio, these devices were a necessity: vinyl was king, and the listener needed a way of amplifying and controlling the outputs of those finicky phono cartridges. Oh, and we got balance controls, tone controls, and all sorts of other goodies along with the deal. What’s a kid not to like?

When I first started getting involved in high-end audio as a graduate student back in the day, I was able to cobble a nice but simple system together that consisted of a pair of speakers, a power amplifier, and a cd player with variable outputs and a built-in volume control. This system sounded great, but I really lusted after a preamp. All the magazines (no e-zines in those days…) touted the wonderful things a preamp could do for you. Heck, it might even clean up after me if I paid enough for it! Well, I finally saved up some dough and started auditioning various affordable line stage preamps in the system. Guess what I found out? Yep, they all changed the sound of the system, but often not for the better. Maybe dynamics improved a bit, but the overall sound somehow deteriorated in quality. How could this happen? I finally settled on a decent quality tube preamp that I thought changed the sound, but in a euphonically pleasing way. Plus, it had a phono stage, so I could expand into vinyl when I was ready. And so it went for a long time, and I was for the most part satisfied.

But I never forgot about that first simple system and how clean, pure, and good it sounded. With the advent of stand-alone DACs with passive volume controls, I sort of moved back to the preamp-less days. I’d digitally archived many of my most loved records, so I had little use for multiple inputs; besides, I had a separate phono stage for when I did want to hear my vinyl spun in the analog domain. And I had no problems with sufficient power: I was using decently powerful solid state amplifiers (such as my class D Merrill Audio Thor monoblocks) to drive speakers of moderate to high-efficiency. Besides, I rarely, if ever, listen at headbanger volumes anyway. Preamp? I don’t need no stinkin’ preamp…

That sentiment was pretty much how I approached my own definition of audio goodness until quite recently, when I became interested in exploring Nelson Pass’ low powered First Watt amplifiers. With these amps ranging in output from 10 to 25 watts per channel, gain suddenly became a factor. Under these circumstances, an active preamp suddenly didn’t seem like such a bad idea. Fortunately for me, such devices are a lot better than they used to be, so finding ones to audition at a reasonable price point didn’t seem to present too much of a problem.

Enter, then, the Linear Tube Audio MicroZOTL2.0. When I first read that it could be used as a simple line stage preamp, my interest was piqued. Gain was at hand, and thank my lucky stars, vacuum tube gain to boot! The deeper I got into my investigation of the MicroZOTL2.0, the more intrigued I became…

As we all should know, a preamplifier’s most important job is to produce gain when needed; and it must do so while imparting the smallest fingerprint possible on the source signal. A straight wire with gain, as it were. As I ruminated on how the MicroZOTL2.0 handled signal amplification and how it offered a crazy low output impedance (usually a good thing for a preamp…), I figured I might have indeed stumbled upon just the thing I needed to make my system sonically complete.

I started out by putting some serious time into listening with the MicroZOTL2.0 driving the First Watt F7/Zu Audio Druid combination. The first thing I noticed was that some of the over-energetic treble I’d gotten earlier on with the F7 alone driving the Druids seemed to disappear, only to be replaced by some seriously smooth and nicely balanced highs. Another improvement was in soundstage and image quality. Here, the MicroZOTL2.0’s tubes made themselves known, providing a wider, deeper, and overall more three-dimensional soundstage than I had noted with only the F7 in the game. The large Zu Druids actually came much closer to “disappearing” than I had ever thought possible. So maybe the MicroZOTL2.0 in this case isn’t exactly a “straight wire with gain,” but I can say I was really liking what I was hearing.

With things looking so promising, I went ahead and brought in my reference ATC SCM19 Version 2 monitors. Here’s where the gain would be truly needed, as the SCM19s are quite inefficient at only 84 dB. Indeed, with the little First Watt amp putting out only 20 watts per side, whatever the MicroZOTL2.0 could add to the equation would be most welcome. I’d found previously that the sound I got with the F7 amplifier driving the ATC speakers was top-notch, if a bit lacking in overall punch and headroom. I can say that adding in the Linear Tube Audio unit as an active preamplifier has without a doubt kicked things up a notch! That added bit of zest and energy are most welcome, and the music jumps from the speakers like a spring lamb. I can attest that the MicroZOTL2.0 is no slouch in the pace, rhythm, and timing arena; if anything, it seems to “keep watch” over the rest of the system, never letting the musical flow become stifled or constipated. If foot-tapping and air conducting/drumming are your thing during listening sessions, then I don’t think it will disappoint!

Playing Oregon’s album Crossing (ECM LP, archived digitally) was sublime. This record was a find while visiting Tacoma, Washington, where my daughter lives; it was the result of a joint effort during a dad/kid crate digging session, so it has some special meaning for me. The music itself, which falls somewhere between new age and jazz fusion, is especially engaging and somewhat wistful. It’s musically solid throughout, but I really love the title cut, which comes at the end of the album. This song was written by guitarist Ralph Towner (one of my favorites…) and is exceptionally lovely in a very touching and poignant way, especially in light of the death shortly thereafter of the group’s percussionist, Colin Walcott, in November 1984, just prior to the album’s release.

After taking a few listens to “Crossing” in its entirety, I got to thinking about how to best sum up what I was hearing.   Would it be best to dive into the normal audiophile lingo, discussing exactly how many inches that instrument is from my right speaker, or how extended, yet non-fatiguing the treble response is? No, not here. What I’m experiencing is more of a feeling or connection with the music streaming forth before me. It’s the kind of feeling you get when you know the system is firing on all cylinders, smoothly, effortlessly, and in top form. You forget about the stereo gear and focus on the music, and the time, place, and overall mental state in which it places you. It’s that sense you get when after months (or maybe even years…) of fiddling around with your system, you one day sit down and realize you’ve finally gotten it right, well, at least for now.

As I listen through the album, I’m transported back to my late teen years when I first became interested in the genre known as jazz fusion. I couldn’t get enough of it, and I feel that way again. It’s like I just shaved 30 years off my life and I’m that invincible (and somewhat cocky) kid again. It’s an emotional thing that only music can bring forth, and that’s what’s happening right now. The LTA preamp coupled with the First Watt F7 amplifier driving my ATC SCM19 monitors engulfs me with music and emotion; the sounds swirl about me as my thoughts take me to times long past.

I’m an analytical person, and that’s how I listen and review. So I offer my humble apologies to my more analytical brethren, but this experience is really more about the emotion: something that just can’t be quantitatively measured or described. I think most of that emotion comes from an overall opening up or effortlessness of the music reproduction that removes the fetters that remind us that what we are hearing is electrically reproduced. There’s a sense of freedom of constriction of frequency range, dynamic, and soundstage. I know that all of the above items are in reality truncated, but my mind lets me work easily around these limitations; the sound is just so right!

To see how the new MicroZOTL2.0 might have been affecting the sound, I next removed it altogether and ran the output from my DAC straight into the F7. Volume was controlled directly from the attenuator on the DAC. Differences were subtle but apparent. Firstly, I was somewhat amazed that the overall signature really didn’t change; the little bit of warmth I love about the F7 was of course there, and the timbral characteristics were pretty much the same with the MicroZOTL2.0 both in and out of the system. I’d therefore have to deduce that the little Linear Tube Audio unit functioning as a preamplifier is pretty darn transparent. I did, however, notice a few more small, but obvious differences with the preamp out of the system. One difference was bass extension and definition. With the MicroZOTL2.0 absent, I noted that the bass got a little woolier and less punchy, just looser and fatter overall. Furthermore, I noticed a minor collapse of the soundstage accompanied by a notable loss of imaging precision. Placing the preamp back in the system caused an immediate improvement, with the soundstage “popping” back into its previous glory. I immediately noted each instrument in its own space, surrounded by an airy layer of space. Soundstage layering and depth became immediately more apparent.

By itself the First Watt F7 is a superb amplifier, though I now suspect that it may struggle a bit to fully drive the inefficient ATC monitors. Adding the MicroZOTL2.0 as an active line stage preamplifier serves to bring the F7 to a different level altogether in this otherwise challenging configuration. Think of it as the jolt of energy needed to bring a world-class athlete to the brink of perfection. Color me impressed.

As A Headphone/Desktop Amplifier

Linear Tube Audio’s MicroZOTL2.0 user guide labels the device specifically as a “Headphone and Office Amplifier,” thus suggesting that its intended purpose is to live on a desk, so on to my desk it did go. In this configuration, I used it as an amplifier to power my relatively inexpensive Audio Technica ATH AD-900 headphones as well as a pair of older, but fairly efficient single-driver desktop speakers from Eric Alexander of Tekton Audio. I really didn’t expect to spend a long time assessing the MicroZOTL2.0 as a desktop amp- I’d already determined it to be a killer preamp in the big rig — but I found it to be an especially compelling performer in this application as well. Setting up the speakers on either side of my computer monitor, I let the tunes play. I wasn’t initially too impressed, but as I continued to listen, my attention was increasingly drawn in to the music.

The little Tekton speakers are of the single full-range driver variety, and as such, have always produced a highly integrated and cohesive sound. Powered by the MicroZOTL 2.0, what I got was a nearly mesmerizing organic experience. Kicking my chair back so that I was somewhere between three and four feet from the baffles, the aural image just hung between the speakers as if by magic. Even better, the musical presentation was lively and punchy, yet highly lifelike. I listened intently through the entirety of Ralph Towner’s solo album Anthem (CD, ECM) while marveling at the tonal accuracy of the guitar. I heard a resonant woodiness that often seems lacking in bigger, more expensive systems; it’s just this sort of thing that allows that momentary suspension of reason and allows the listener to believe he or she is hearing a live performer in a real space. Of course, deep bass was totally AWOL, but who cares?

I got a similar experience with male voice, as demonstrated when I threw on Chris Jones’ audiophile favorite “Roadhouses and Automobiles.” Not only did the plucked and strummed strings sound deliciously real, but Jones’ voice was imbued with a chesty resonance that I haven’t picked up to such a great degree in previous listening and on far more expensive systems.

I’m sort of sorry that Tekton Audio no longer makes simple speakers such as these, as they are a fantastic mate to the MicroZOTL 2.0 amp for desktop near-field listening.

I also spent considerable time with the LTA amp driving my Audio Technica ‘phones. As I’ve mentioned before, I’m not really a headphone kind of guy. I use ‘phones fairly regularly, but mainly for listening to the audio portion of movies late at night, or for monitoring when I digitally archive my vinyl records. Just a personal preference, but I’d much rather listen to speakers when enjoying music, even if it means a close-in near-field experience such as the desktop system I described above.

That said, I’ve used my ‘phones with a number of headphone amps, mostly integrated into DAC or amplifier units. These include devices from the likes of Antelope Audio, Metric Halo, Lavry, Benchmark, Sound Devices, and a few others I can’t think of off the top of my head. What I can say, without waxing too poetic, is that the MicroZOTL 2.0 probably gave me one of the most enjoyable headphone experiences that I can recall. With the exception of possibly the excellent SPL Phonitor (a dedicated headphone amp), the Linear Tube Audio unit rewarded me with a most realistically satisfying “out of head” listening experience. It doesn’t exactly throw the image out in front like the Phonitor did, but the soundstage doesn’t seem to be restricted to the space between my ears. Rather, it extends outside of my ears, and even somewhat beyond the ‘phones themselves, and I could even discern some nice front-to-back layering as well. A nice trick indeed. As far as tonality is concerned, I’d have to say that the amp provides pretty much the same tonal balance through headphones as it does via a neutral and resolving pair of speakers such as the Tektons.

So yes, in my limited experience, the MicroZOTL 2.0 holds up its end of the bargain as a highly satisfying and effective headphone amplifier. I’ll leave it to the headphone gurus out there to split the finer hairs on the issue.

Conclusions

All right, so what we have here is a true jack of many trades and a master of all. While I mainly reveled in the MicroZOTL2.0’s excellence as a simple line stage active preamplifier, it held its own as an integrated amp (under certain understandably limited conditions) and as an engaging headphone amplifier.

Thanks to the MicroZOTL2.0 serving as a preamp, I’ve been able to continue my love affair with the First Watt F7 amplifier, a real honey of an amp on its own, but now with just the right amount of added zest to let it truly shine powering my challenging ATC speakers. Before the MicroZOTL2.0 hit the house, I truly considered myself a sans preamp kind of guy. Now that’s all changed. I’m opening up the checkbook and keeping this handy little fella around.

Finally, I’d like to offer kudos to the folks at Linear Tube Audio for reviving such an important, though little known, design in the audio pantheon. Well done!

LTA MicroZOTL2.0 Deluxe amplifier/preamplifier, $1695 as reviewed

About the Author

John Richardson has been interested in music and audio since his early teen years, or stated another way, as long as he can remember.  He has been involved in the audio community in one way or another for around 20 years and for the last five has been a regular contributor to the on-line magazine Stereomojo.  There, he has been the resident computer audio guy and “value conscious audiophile” (aka “cheap bastard”).

John is a professor of analytical chemistry and a forensic chemistry consultant in his spare time, when he isn’t listening to music or evaluating gear. He tries to fit in plenty of time to hang out with his two teenage kids, his lovely wife, and the family cat, though only the cat also seems to harbor audiophile tendencies. John also enjoys running, cycling, golfing, hiking, or just about any other activity that sucks up time and money.

John is also a contributor to Stereo Mojo.