Linear Tube Audio MicroZOTL MZ 2.0

Click pics to enlarge. The small box in front of the Akitika amp is the basic Switch Mode PSU, connected to the MicroZOTL2 by an umbilical cord.

Linear Tube Audio MicroZOTL MZ 2.0, Part I
Overview: OTL

4/10/20:Part II published (LTA drives Klipsch Heresy)

My recent acquisition of the LTA MicroZOTL 2.0 is like a dream come true, in more ways than one. The Delayed Gratification is all the Sweeter. Part I reviews the MicroZOTL mainly as an amp (something not adequately treated in reviews on line). Then I shall test it out as preamp (this would take a while) as well as take the unit to my friends Andy and Kevin for evaluation on their Altec horns (3 of them!). Watch for future Installments.

Overview: OTL
My experience with OTL goes back a long time. It started in the early 90's, when a friend (unwisely) used big Atmasphere MA-1's that employ the classic OTL tube 6AS7. They drove his early MBL 111 (an expensive but imho overrated loudspeaker with a top shaped like Wotan's Spear, with lousy woofers that were total disconnects). Naturally, I didn't hear much that was special; the sound was clean but there was no magic, not to mention quality bass. Later on he switched to a pair of Marantz 9's which I lent him and we were all much happier.

Fast forward to Hong Kong, where I heard more of everything. I remember hearing a Croft OTL (uses 6AS7 also) that was pleasant with vintage JBL loudspeakers. It was surprisingly not at all a clear sound but I thought there was potential. I toyed with buying it but chickened out. Sometime later, I saw a pair of Atmasphere M-60's (I love the look) in a second-hand shop. I asked to hear it, but the owner just refused (defective?). For quite a while, I toyed with building a Transcendent Sound OTL (I kinda liked their grounded grid preamp but had heard nothing else), but desisted because of cost. For this article, I re-visited their website; is it my imagination, as the prices now seem more reasonable (and I have tons of EL-509)? I'd love to hear from someone who has one of these.

What I heard most often, at several places, was the Graaf 20 (here) and also the Modena, and it was usually quite nice. My friend Paul is still using the 20 with his B&W Nautilus 804 to good effect. Graaf uses the high-heat 6C33 tube (Atmasphere does too, like in their Novacron), which I personally like. It should be said that the Graaf OTL aficionados are a very loyal group, and it is not easy to spot one on the second-hand market. Even second-hand, it is on the expensive side, so I never got one. I do like and have 6C33 amps, but they are all transformer coupled (Almarro, Audio Professor), not OTL.

Now, back in the US, I also heard Andy's iconic Futterman OTL (also using the EL509) play very well with his Quad ESL-57 (here). I talked to Andy and he said he always felt OTL is "problematic", meaning too lean in the bass. Perhaps, but in my experience the Graaf is quite even and not overly lean in the bass. So I still would like to hear and experience more. I'd love to hear the legendary Counterpoint SA-4 (using 6LF6)!

Most OTL amps, like other tube amps, require some maintenance, a re-biasing once in a while for some. So you see, I have paid attention and heard a lot but never owned any, till now...

Linear Tube Audio MicroZOTL 2.0 MZ2
Under Mark Schneider, since 2015, Linear Tube Audio (LTA) basically uses and at most tweaks a little David Berning's ZOTL design (the technical aspects are beyond me but you can read about it at David Berning). The only David Berning product that I am familiar with is the TF-12 preamp (here), which I heard and saw many times in the 90's in HK's second-hand stores. It is a very liquid sounding preamp, the sound of which I still remember but I refrained from buying one because I did not find dynamics to be one of its strong suits. That is digressing, the MicroZOTL uses the 2 triode halves of the 6SN7 in a push-pull configuration, though the implementation is by no means the PP we understand for the usual tube amp with transformers. There is zero negative feedback.

Various Iterations MicroZOTL went through several. All use the same technology and tubes. The Original, in blue, was reviewed by Dick Olsher in enjoythemusic in 2000. In 2016, LTA issued the 2.0 version, which was again reviewed by Olsher, this time in TAS (though he did not test it as an amp). It was also reviewed by Herb Reichert in Stereophile (who did, good for him). There are many other reviews, which are focused on earphones and rather irrelevant to me. Current 2019 2.0 As you can tell from the pic, even within the designation 2.0 the current version is different in many small ways from the 2016 2.0 (see below). This one is reviewed on many internet sites, mostly with earphones, as on innerfidelity (which commendably included a comment on use as preamp). Second-hand buyers should be aware of the changes over time.

Spec's This is all laid out in the Official link. I shall comment only on a few aspects. Inputs The current 2019 version has, thank God, 3 inputs instead of 2 in the 2016 version, selected via a tiny toggle switch (which I am sure is sonically better than generic larger selectors). Power Output is 1 watt into 4 ohm, and 0.5 into 8 ohm. Output into my 15 ohm horns should be even lower. However, I noticed that in Olsher's 2000 review of the original cited above, the spec's says 0.5 ohms into 14 ohms. Numbers don't mean everything, as we shall see later. Output Impedance A very low 2 ohms, one reason for me buying one, as I run long interconnects from preamps, and I do intend to try it out as a preamp. Input Sensitivity Quite high at 0.6V, which imho is necessary for such a flea-powered amp. It proves friendly to sources as we shall see later.

Options My unit is the most basic one. Voltage I ordered the Switch Mode PSU (used in the original version), which comes at no extra charge and can naturally be used anywhere in the world (come to think of it, the amp is sort of "portable"). I of course am curious about the optional linear PSU, which is usually better. 6SN7 or 12SN7 This was misconstrued even by some reviewers. One can actually choose. I somehow did not read that there are internal dip switches to enable use of both; if I had known I'd have opted for the NOS 12SN7 (which I don't have) rather than the current production "Tung-Sol" 6SN7. Too bad, but I do have a lot of old 6SN7's. Remote No for me. Color The Black really looks great!

Built and Ergonomics Generally, the unit is very well built and component quality is high. Even the PSU feels sturdy. The umbilical cord that connects the PSU with the amp is well made and the locking collar is surely a good touch. But I do have Caveats: 1) this is a major one for me - if you look at the pic at the bottom, you will notice that the circuit board, which is not that thick, is completely unanchored around the SN7. This makes swapping tubes, particularly the SN7's, a worrisome and potentially perilous act, as the circuit board severely warps during the procedure. I was afraid that I'd break some of the tracings. I advise when pulling a tube to apply gentle pressure with a blunt probe to counter the pull; more difficult is when inserting the tube, where I'd advise rolling a paper tube the diameter of the clearance between the circuit board and chassis, bend it into L-shape, and stuff the short end under the circuit board to counter the pushing action. Or, if your fingers are very slender, you could provide some counter action down there. No matter, not optimal; 2) this one is minor, some of the screw holes for the acrylic top-plate are visibly rough around the perimeter, too bad for such an otherwise well-finished product, a beauty in my eyes (in black).

Listening

• As Amp, with stock tubes and Preamp I used the LTA with my 104 db YL horns. For ease of connection, I actually first connected the fully loaded Shindo Monbrisson preamp from my System II (see sidebar) to one of LTA's Inputs. On-Off On my highly sensitive horns, there is a faint thump turning on and off. With less sensitive loudspeakers, one may not notice. Instant Musicality Muddy Waters' Folk Singer (MCA/Chess, CD) was what was on the Sparkler S306 CDP. I turned the LTA's volume knob half way. As soon as I pressed play, I was blown away by the big and expressive sound. I sat down thrilled, and listened to the entire album, allowing myself to be swallowed whole. Bass The most amazing thing was the bass (the bass-strong Shindo also contributed some, I am sure), full and ripe, in one fell scoop dismissing the notion that OTL has no bass. Presence Although there is good air, I'd not say it is the airiest thing, but the treble was clean and very accurate (see music examples below). and the midrange sparkling. The guitar playing and interaction of Muddy Waters and young Buddy Guy were just mesmerizing, the picked notes flying at you like darts - as good as I have heard, and then some. I then turned up the volume of the LTA, and down the Shindo, and it was good too. I did not hear overloading. Background Noise None that I could hear. This is the quietest amp that I have. In contrast to many efforts, the quiet fortitude does not come at the expense of liveliness ( a topic I treated here).

• As Integrated Amp, with stock tubes Gratified that the amp is more powerful than I thought, unusual for me, I decided to forego the preamp. Now, the Sparkler CDP goes directly into the LTA. For vinyl, I employed my Thorens TD-124/Thomas Schick 12" Arm/"Midas" Denon-DL103 (see last article; just roll down) and my trusted Parasound JC3 as phonoamp. No problem whatsoever with gain. With CD, it is loud enough when the volume knob is at 1 o'clock or so; with vinyl and the very high-gain Parasound JC3, often just 11 o'clock would do. The sound has just about the same attributes as with a preamp, though the bass, still prodigious, was a little less without the Shindo.

• As Integrated Amp, with Old Stock Tubes Impatient, I randomly rolled in what's on hand, well used old-stock Tung-Sol grey plate 12AT7's and Tung-Sol smoke-glass 6SN7's. The sonic attributes remain, but now the bass is a still little less full, more lithe and defined. While I'd prefer these tubes, I was actually surprised at how the current Russian "Tung-Sol" tubes have improved upon their predecessors. From this point on I listened to album after album, always with great pleasure and surprise, to the point that I expected some surprise in each! An example, in Everyone, on Van Morrison's Moondance, an album I am very familiar with, I was startled by how the flute punctuated past the confines of my loudspeakers and, no kidding, reached and dangled about 2 ft in front! In the great Into the Mystic, the undulating guitars lines also similarly advanced and retreated like waves. That is physicality, something rarely savored in hifi! It was so good that I pulled out Astral Weeks and gobbled that up whole too. In classical's, recordings are not that dramatic, but if you listen carefully, examples of superb articulation and expression in the reproduction can be found. I just played LAPO/Mehta's Virtuoso Overtures (London, LP); the sheer presence of the crisp double bass attacks in Die Fledermaus and Der Freischutz brought smiles to my face (and I was just in Carnegie Hall last night)! And Rienzi, that stirring potpourri (in the right hands), had tender strings, noble brass and big swells, the latter full and urgent where lesser gears would pinch up. What is really important is that, while details are abundant (but not artificially super-abundant, like in some so-called hi-ends), nothing is etched or at the expense of others. Strings are really impressive - whether solo, small group or tutti, they sound real. Sometimes, even with very good equipment, tutti strings sound either somewhat one-dimensional or homogenized; not here, it sounds like many violins playing, whether in VPO/Bernstein Sibelius Symphony No. 2 (DG, LP) or the thrice-familiar LSO/Kertesz Dvorak 9th (London, LP). In both of these, the brass chorales were absolutely lovely - one could separate every section and hear vertically with ease. In the latter, the filler, Othello Overture, was uncommonly gripping, thanks to the microdynamic details on offer.

• Redux: As Amp, with old stock tubes and Preamp After all that, I reconnected the MZ2 with the Shindo Monbrisson Preamp and System II, like when I started. I again played with the volume knobs. It seemed to hardly matter the two knobs' positions. Even with the MZ2 volume maxed, there was no detriment. The first record I played was Mozart's Piano Concerto No. 24 (Anda, DG, LP), one that I had just played before the re-connection. The Thorens TD-309/Koetsu Black/Aurorasound Vida sounded quite a bit sweeter and more emotive than the Thorens/Schick/Parasound (which I am not about to move for A/B comparison), but I think that may well be due to superiority of the Koetsu Black over the more clinical "Midas" Denon DL-103 (property of the aluminum shell). Seeking a more valid comparison, I re-connected the Sparkler S306 CDP to the Shindo (but the cable is still different; both are still Gotham but the former setup used a very long GAC-2, whereas the setup now was a much shorter DGS-1 into preamp and then a long one into the MZ2). I then played Moondance again. Compared to before, there were subtle but audible differences: addition of the Shindo preamp shifts the tonality downwards. The flutes did not quite startle as much on Everyone, but there was more gravitas, more force, better bass profile, and more fulsome interaction between the very talented musicians. My Van Morrison CD's are rather late pressings, and compared to my older copies in HK they usually sound somewhat lean (believe me, CDs, like LPs, differ in sound quality from pressing to pressing). The combo eliminated that problem. Readers should interpret this properly, as the difference is more subtle than blatant, and in part due to the difference in interconnects (even if both are Gotham). While some would view this as a case against preamp, for others like me the greater control would sound better with most material. The latter was definitely the case in Shostakovich's Symphony No. 11 (BSO/Nelsons, DG, CD). With the preamp, in the middle of the first movement, the passage leading to the depiction of massacre just had more menace, central to the meaning of the score. No, it did not sound as good as in the humongous WE system I heard just before I left HK (here), but adjusted for room size and power, perhaps it was close! One of my favorite movie soundtracks 桂花巷, (oop, now commanding a hefty second-hand price) which I have heard more than a hundred times (usually late night) since the mid eighties, sounded absolutely lovely, the oriental rhythms tensile, the reflections subtle, as well as I have heard it. I can go on and on, and I am going to go through a lot of music. Perhaps Part II would be just about music played! The important thing is, although the preamp does to a certain degree impose its own sonic signature, it does not detract from the formidable sonic merits of the MZ2, and in the best case, add solidity (just make sure your preamp is of similar merit, no easy task).

• Extra: As Headphone Amp It was late and I didn't want to do it, but I couldn't help it. I got out my AKG K701 and Beyerdynamic DT880 600 ohm. I listened to the aforementioned movie soundtrack whole on both. Just on memory, I am confident the MZ2 significantly bettered my small headphone amps (Schiit Magni and Vali, Micromega MyZic), though without comparison I cannot be sure about the formidable outputs from my Nagra PL-P and Manley 300B Preamps. Of those 2 cans, the darker Beyerdynamic, although seemingly more distant, brings out more of the sinuous quality of the MZ2. The AKG seems immediately more attractive, more airy, but ultimately less subtle. Those 2 cans are probably not even run-in, but the performances are compelling. Based on my listening, unlike some other reviews, I'd not have any any issue with high-impedance cans; in fact, they bring out the best of the MZ2 (with the material I listen to). 

Conclusions The LTA amp could do no wrong, with any material. Even if I have only tested it in one of its three roles, it is already the best thing I have bought in quite a long time. It shall definitely feature prominently in the impending Year in Review.

• Power This is the number one concern for folks with high efficiency loudspeakers who want to try it out (which Dick Olsher did not really do). LTA illustrates perfectly what I (and some others) have always said, that numbers don't mean everything. For my 15 ohm YL horns, it sounds much more powerful than expected. Before I bought it, via a friend I actually got to briefly speak to Mark Schneider on the phone (he was at the Capitol Audio Fest). I told him I wanted to use it as an amp and, unprompted, he immediately asked me the sensitivity of my loudspeakers. I said 104 db and without any hesitancy he said it would be fine. Now, even I had my doubts, as definitely not all 0.5 watt amp are equal, but now I know why he sounded so utterly confident! Being someone who enjoys flea-powered amps and regards low-powered tubes as better sounding, I could not be happier. I count myself, and likely my inner circle of high-efficiency horn users, lucky to be able to exploit a product like this. Detour, on this issue, I am highly skeptical of the current crop of "high-efficiency" loudspeakers, like Devore and Volti, which through my readings in many Stereophile articles in amp matching simply seem lower in actual efficiency, certainly much lower than those I have used (my current YL; and my Klipsch La Scala, Tannoy Canterbury and TAD 3401 of yore). That is a postulation, but I'd love to be proven wrong (I doubt it). Similarly, I'd think Olsher's loudspeakers are not nearly as efficient as their spec's, not to mention ours (hence his reluctance to use the LTA as an amp). With my loudspeakers, there was at most a little congestion at really loud peaks, no ugly distortion or clipping at all, which in a way surprises me. That said, I do think, unless you have proven high-efficiency loudspeakers (preferably vintage horns; like I said, I am not so sure about modern ones), I'd be cautious about getting the MZ2 to use as an amp. BUT, I am demanding, and I listen mostly to large orchestral works - if your music is smaller scaled, and you have a smaller room, by all means investigate.

• Sonic Attributes If the OTL is supposedly more direct and gives you more of the music, the LTA most certainly is. It plays everything well, so much so that I have yet to find an album that I do not like. Usually, equipment that does this is on what we called the "forgiving side", something that can be faint praise or even damning to the exacting audiophile who wants as much detail as possible. Nothing to worry here, the LTA is that very rare thing, gear that gives great detail and yet is forgiving (WE and good SET amps can do that). This is the kind of equipment that makes me angry at the current high end, which seems to pride itself at being able to play only "audiophile" recordings. As importantly, it draws my attention to pieces that I have neglected before (like the Othello mentioned before; or the Vienna Octet's London LP filler to Dvorak String Quintet, the Spohr Piano Quintet). The LTA also has something that is missing in most OTL's (indeed most amps) that I have listened to - an unperturbed quality when the going gets rough. It also belies the OTL's albatross of lean bass. The distance between the performer(s) and the listener is significantly shortened, the venue is naturally incorporated and listener engagement is significantly enhanced, rarities in the hi-end. In another word, superbly involving. The LTA rubs shoulder with the likes of Western Electric and Sparkler, the highest accolades in my book.

• Run-In What run-in? Unlike output transformer based amps, this amp was great from the word go, and is uber consistent. How lovely not having to run-in things!

• Tube Rolling With any tube equipment, tubes always matter, and the old-stock tubes are always better. The great thing about the LTA is that, the difference is minimized. Unlike in many conventional tube amps, whether using the current production Tung-Sol tubes or my old-stock tubes, the sonic signature is uncannily similar. Grant you, the older tubes have a little more finesse, but I really did not run-in the new tubes. I do know from experience that the current-production tubes require quite a bit of run-in to smooth out. Initially, they usually have just too plump a bass, and it is the case here. With run-in, I am sure the bass will align even better. That said, I'd not be surprised that some people will prefer the new tubes (just as in older ARC products, where old tubes could be lacking in oomph). AND, as I have not encountered new tubes in a while, I am surprised by the difference in construction of the new tubes from their earlier Russian cousins. The 12AT7 has broader plates (whereas many are half-width) that smack of NOS; so does the 6SN7, which also has supporting rods reminiscent of NOS 5692. Just by eye, they are significantly better than their predecessors. Sonically too, they did not shame themselves. The important thing is, for those who want plug-and-play, these are irreproachable. Also, I shall try to get hold of some 12SN7's to compare with 6SN7's.

• Preamp? Most people would not be entertaining this question. For me, using the MZ2 as an amp, running the source directly in or via a preamp are both immensely satisfying. I can easily use one of the 3 inputs for the preamp and the other 2 as direct-in. If pressed, especially since I use many sources at once, I'd opt for the greater control and flexibility of using a preamp, but the preamp better be of superb quality (as mine are).

• Upgrade? Many of the reviews mention upgrades. The most obvious one for the MZ2 is the Linear PSU, which I'd be curious about. I have also read about their more expensive and more powerful amps that use power tubes in PP (though they differ in having NFB). The MZ3 is perhaps an upgrade (and it looks even better, sleek!) but the ones that interest me the most are the ZOTL10 MkII amp and its cousin, the integrated Z10. Not because of the increased power (10 watts), but because they use another of my favorite tubes, the sweet EL84 (which imho has great bass quality). It is too bad LTA does not make a 6V6 or even 6L6 version (imagine the possibility of rolling in WE 349A and 350B). Nonetheless, to my thinking, if you have horns as efficient as mine, despite limitations the 6SN7 just very well may hold up sonically to more powerful tubes. Many reviews use the ZOTL40, but that one uses EL34, not one of my favorite tubes (though many like it). I'd love to hear any of these though.

Formidable!

 Note the intricate components, some specialized transformers that we don't see in others tube amps. Note the myriad fine wires and the banks of diodes; no ordinary DIY, this. As mentioned in the text, the circuit board has too few anchoring screws. Note the 2 screws flanking the 12AT7's (also 2 on the opposite side). In each lower corner, a full 2" of circuit board, where the 6SN7 sits, is unsupported. Note the 2 dip switches just above the 2 left tubes can be connected in parallel or in series for 12SN7 or 6SN7.