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Linear Tube Audio microZOTL headphone amp, preamp and integrated amp

04.01.2016..
Linear Tube Audio microZOTL
..


ReMusic Spark Award

Introduction

The title of this review is correct. We are not going crazy. The object is firstly a headphone amplifier. Moreover, it can be used as a preamplifier and integrated amplifier, although with not so many watts.

 

The Linear Tube Audio microZOTL has been suggested by our Personal Scientist and Official Pusher of Extreme High-End, Angelo Recchia-Luciani. The best is that ALL his titles are true, included those of the official degrees…

 

Angelo says: “As we are testing headphones and headphones amplifiers of top quality, we must review the microZOTL. In the States, people say that it is more than a simple headphone amp…”. Easy done. I find the website of the manufacturer. I verify with some cross-readings the potential quality of the product. I feel in love with the story that could be written and with the music that, on the paper, could be heard. Got in touch, got the product and…wonder that still lasts.

 

Step backwards for the suspense.

 

I do not want to spend many words on technical terms – see here how we test on ReMusic – but an extreme synthesis is due.

The vacuum tubes, in themselves, would be some perfect audio signal amplifiers. They are the only electronic devices at our disposal, if you consider that they work by emitting beams of electrons in the vacuum. They remain the most linear audio amplifiers. Purity counts. The problem is, that working in tension, they need output transformers in order to supply the current to the drivers. The output transformers, although theoretically perfect, limit, color, modify and mortify the qualities. Of course, there are circuitries with OTL tubes, but they are difficult to implement, they are instable, untrustworthy and often dangerous. A prove is the fact that they have never reached a significant commercial success: few, very few, almost inexistent are the famous brands.

Instead, transistors and MOSFETs are devices capable of suppling current directly. That means lots of power, but with effects at the listening more or less acceptable for an exigent and trained ear. We can talk of a sense of “discontinuity” of the signal, of granularity, of flatness of the soundstage, of few fluidity and musicality in a wide sense. They are caused by the applied counter-reaction, by the operational class, by the intermodulation distortion, by the odd-order distortion or by the great number of components in use.

It is known that those circuital problems are equally shared by the tubes and by the solid state devices. This could make thinking that EVERY amplifier represents the best compromise and, obviously, that all the audiophiles and designers have agreed to compromises. Beyond a certain level, we can talk of “designing choices”, to pursue with awareness and coherence. The quality of certain amp-speakers matches is not an opinion but the Holy Grail of our passion…In any case, if we could use the tubes WITHOUT the output transformers, this would be a step forward to a hypothetic Audio Paradise.

 

Design

It seems that David Berning, one of the “gurus” of the American Hi-Fi made that step. The word guru is not ironic. From a long time, his realizations have been strongly awarded by the specialized magazines and appreciated by the visitors in the audio shows. From 1976, Berning has been registering a series of patents in the audio field, which have contributed to define in 1996 the ZOTL circuitry, i.e. Zero hysteresis Output Transformer-Less, a tube amplification without transformers and with zero hysteresis. The circuitry has considered by many the most significant improvement in the tube amps technology since the ‘60s.

 

In brief, the principle is similar to how a radio station and receiver work. I quote. “The audio signals “ride” a carrier frequency, which amplifies them. Then the carrier is removed and the remaining audio is delivered to the speaker. This changes the impedance plane to match the impedance of the speaker, so the audio output transformer is no longer required to match the tube to the speaker. An impedance converter accomplishes this match up. This super linear amplification process allows the amplifier to have a flat frequency response from 8 Hz to 50 Khz, which is near impossible with an output transformer. Audio amplifier design engineers have long sought to eliminate audio output transformers because of the restrictions they impose on amp performance. Leakage inductance and inter-winding capacitance limit the high-frequency response of transformers while core saturation and magnetizing current limit their low-frequency response. Transformer-core hysteresis causes specific distortions of non-symmetric and transient waveforms that are characteristic of musical reproduction. Audio output transformers simply cannot achieve the correct turns ratios as they only max out at 25:1 due to saturation and hysteresis effects. This leaves much to be desired, which creates and issue that needs to be resolved. Zero hysteresis Transformer-Less (ZOTL) achieves effective turns ratios of nearly 300:1 and resolves all issues involving audio output transformers by eliminating them entirely.”

 

The promise of Berning’s amplifiers is offering us the transparency of the tubes without the limits imposed by the transformers to the high and low pitches of the audio range. Such feature has been often defined with a negative meaning as “valve-like sound”. In a positive meaning, instead, we can speak of “euphonic” behaviors that are typical of the traditional tube amplifications.

The ZOTL should extend the transparency of the tubes on the frequency extremes, adding the velocity and the capability of driving that is typical of the solid state amps.

 

Lots of stuff, indeed….

 

Our technicians inform me that the story of the tube circuitry has been smooth and that our collaborator Alberto Pepe will be deal with this project very soon.

For now, here you can find some interesting links: at this link you will find the page of David Berning’s website where he introduces his own technologies and here a PDF folder with his patents in the audio field made available to Mark Schneider.

 

Oh, sorry. I still have to introduce Mark, an American engineer owner of the Linear Tube Audio company. He boasts a long career in designs and products in different industrial sectors. After a period at home for illness, he dedicated himself to the audio field. A passion started a long time ago with a Thorens turntable, a SME tonearm, SAE amplifiers and a pair of Magnepan Magneplanar Tympani loudspeakers.

Since its introduction, Berning’s technology has never been implemented in series productions nor cloned. See to believe. On David Berning’s website you can find costs and specs of his devices. A personal production and unique pieces…

This mix of performance qualities and difficulties in the realization has intrigued Schneider, who has persuaded Berning to give him the manufacturing patent for a series of devices provided with his patented layout.

Mark and David have worked together to the development of a line of amps with such performances but really less expensive.

From this agreement, last year was founded the Linear Tube Audio company based in Washington D.C., USA. Berning supervises every starting device that has to respond to strict productive standards, while Mark and his team deal with the strict production and everything else.

Besides the microZOTL, they manufacture the ZOTL10 and ZOTL40 amps, which mount the EL-84 and EL-34 tube, respectively.

 

Manufacture

The microZOTL was already manufactured from 2000 to 2007 by David Berning himself. It represents his headphones amp by all means. Now it is also the first product by Linear Tube Audio to use one of his original designs. In a sense is a Linear Tube Audio.

 

The Linear Tube Audio’s revision consists in a refinement and, why not, a slimming of designs. With the triode circuitry in class A and no counter-reaction, with the impedance converter and separate ZOTL power supply, with a handmade and accurate manufacture, this device is really something.

 

Light but strong cabinet, blue or black, airy and with a lexan top so to enjoy the internal layout and the light of the tubes. I like this solution, but only who has nothing to fear can adopt it. You can also buy separately the top with the holes placed in correspondence of the power tubes so to try the tube rolling.

On the front side are the on/off switch, the selector for the two inputs, the volume control and the headphone jack input with silvery contacts.

On the back, the golden binding post for the speakers, WBT type, the RCA preamp output, two RCA inputs and the 12 volt power in input.

Inside are 12AT7 driver tubes and double 6SN7 Tung-Sol Russian power tubes. Besides, an Alps Blue Velvet potentiometer, silver-coated teflon copper insulated wiring, hand-wound coils and impedance converters, gold connectors throughout signal path and high quality metal film resistors.

The company assures an optimized tube life expectancy of ten/twenty years based on use, considered that the tubes are not particularly “squeezed” by the design. Also at full power, the microZOTL can easily drive demanding headphones like the magnetoplanar HiFiMan HD-1000. The low impedance output is compatible with a wide range of headphones and drivers. I have my own best result with the Sennheiser HD 800. By the way: have I spoken of “full power”? Have I already told you that the microZOTL brings out one (1) watt into four (4) ohms?

 

LOL

 

Yes, I know, now you think that I have been driven mad. No, I have not. At all effects, the microZOTL is a class A amp of low power, like…like…like many preamps that we love and appreciate! This is why the Linear Tube Audio states that it is capable of driving speakers with 89 or more dB of sensitivity at moderate levels but “in an amazing way”.

The intelligence of the product is having made a headphone amp with a unique and innovative tube circuitry, but AS IT WERE a small and pure tube power amp.

 

Listening test

As I do with every apparatus, the first listening test has been made after some days of run-in with pink noise and tracks chosen for frequency extension, variety of genres and instruments, male and female voices, etc.

 

Here comes again the wonder I told you before.

 

At the beginning, the microZOTL does not exist, does not play. You have to turn up the volume, since the distortion is so low and so your own sensation of volume. As soon as the bass takes power, you have to change your mind and…turn off the volume!

The bass range without unnatural swelling, the difference among the instruments, the lightness and the high pitches, the diversity and richness of the voices, from record to record and from performer to performer… The microZOTL “digs” up in the music naturally and without efforts. This is why is so difficult to understand it at the first listening test. Then the fluidity and immediacy of the music and its simplicity and easiness in playback capture you unconditionally.

Compared to other headphones amps, the microZOTL prevails for its capacity of “doing” the headphone amp. Period. As a preamp its features are neutrality and expressivity. New paragraph. As integrated amp, with high efficiency speakers and high module, it has driven them with rare introspection. Period.

Since the outcome is univocal, the listening session is very short.

 

Pro

It depends. What do you ask to a headphone amplifier or to a preamplifier? I prefer the minimalism with results, not idle. Here we get it. There is what we need. Music, its articulation, the fullness without swelling, the precision with the passion. Whether you consider it just a headphone amp or also a preamp, the microZOTL is small and fair, it does not heat, it comes quickly to the right temperature, and it suits easily with the room. I also consider the chance to use it as pure low power amp. The microZOTL answers the long-standing question: how does my system perform with a shorter and performing signal path? Put all that in the price and you will understand my enthusiasm.

 

Con

It depends. What do you ask to a headphone amp or to a preamp? If you want to drive more headphones, if you want the remote control, balance, tone control, DAC stage, many inputs and outputs, maybe also RCA and XLR, a charming device and other functions and gadgets, well, here, you cannot find them. Used as an amplifier, the microZOTL is amazing, but it cannot and does not want making miracles.

 

Maybe

If it performs like that with just one watt, how could perform the real amps of David & Mark? I cannot even imagine how the Berning’s amps could play. The ones made in few exemplars and at skyrocket prices.

 

Quick metaphors and final recap

The microZOTL has become my reference headphone amp. I find innovative and unique the way it highlights the sound fabric.

As a preamp, the microZOTL has become one of my reference preamps. A preamp that you know because you have its sound in “your head”. I can quote many of them, different ones, but with significant meanings. Alphabetically: Audible Illusions, Audio Note, Audio Research, Audio Tekne, Conrad-Johnson, Counterpoint, Klyne, Leben, Spectral, Threshold, Zanden…Different times, real brands with results always at the state of the art.

As integrated amp, the microZOTL is like a joke. I mean, it plays tricks on amps with much more current disposal…

If it were a car, the microZOTL would be an Ariel Atom or a Lotus Seven. Feeling the road at a high speed, low cost, no compromises, no gimmick or comfort? With the microZOTL you get the same result. With music of course.

 

Who it's for

If you REALLY want to hear how the vacuum tubes perform, you have no choice. The microZOTL is unique and it seems succeeding in the purpose of eliminating the distortions of the traditional tube amps.

The microZOTL is the best headphone amp I have listen to. For this reason, it deserves a Spark in the Dark. It is also one of the best preamp I have listen to. I confirm our award. Its way of raising the signal is so original, effective and convincing, that it deserves our award also as integrated amplifier, although with its limits.

I need an in-depth analysis, so I want to test very soon one of the power amps by Linear Tube Audio. Such devices seem to have one of the best audio amplifications in the audio world.

My final suggestion is: get in touch with Mark Schneider, order him this device, and send the Linear Tube Audio’s production sold out.

And put this pure black pearl in your audio system.

 

 

SCHEME SUMMARY

Timbric | frequency extension, ability of reproducing faithfully the instrument and its harmonious build up

A faithful interpreter of what is in the audio support and in the source. The frequency extension is undistorted and free from superfluity and colors that are typical of the tube devices. If you listen to tracks that you know very well, you can detach all the details, one by one.

Dynamics | micro (detail) and macro (absolute) extension and rate of the transients

Since the “resolution” is one of the strong points, this device can bring you from pianissimo to fortissimo seamlessly and with a wide excursion. Its expressive extension shows itself also in the articulation of the instruments. You can distinguish them, how they are played, energetic or aggressive, or light and hinted.

Image | ambiance, transparency, scene, soundstages, virtual stage, sense of presence, resolution

Here we have new standards. The sense of presence and the 3D extension of the scene are natural and impressive.

Tonality | general setting of the device, if present or characteristic: for instance warm, cold, toned, amber, shiny, opaque...

A neutral device but strongly expressive, without colorations but with a natural trend to exalt the differences among the recordings.

Emotion | emotional involvement, strongly depending from the matches, an absolutely anarchic and personal parameter

Its way of proposing the music, not very Hi-Fi, I mean far from the usual categories, creates addiction and lot of what we are used to listen to, becomes unavoidably “ artificial”.

Manufacture and Packaging | Correct the first, adequate the second. Cardboard and lots of foam. Better the result than the aesthetic choice.

Price/quality ratio | Very high. You bring home a headphone amp of top quality, a reference preamp and an amp that is paradoxical in its effective power/expressive quality ratio.

 

 

Official technical specifications:

Amplifier class: Push-pull Class A, no feedback

Sensitivity: 0.6V RMS [full output]

Output impedance: (measured at 0.5A, 60Hz) 2ohms

Input impedance: 50k

Power Supply Input: Auto-switching 100V / 120V / 220 / 240V operation

Power Supply Output: 12VDC 2.5 amps

Hum and noise: minimum 60mV RMS or 90dB below full output (20Hz-20kHz)

Power consumption from ac power source: 33W

Power output: with 4-ohm load: 1W, 1%THD, with 14-ohm load: 0.5W, 1% THD

Channel separation: (4-ohm loads) 46dB, 100Hz-10kHz, (14-ohm loads): 54 dB, 100Hz-10kHz

Frequency response full power: (4-ohm load) +0, -1dB 10Hz-20kHz, (14-ohm load): +0, -1dB 5Hz-50kHz,

Voltage gain: (4-ohm load): 10.3dB, (14-ohm load): 12.4dB

Size: 9 1/2 inches (24.1 cm) wide, 4 3/4 inches (12.0 cm) tall, 7 7/8 inches (19.7 cm) deep (including connectors)

Net weight: 5.35 lbs (2.5 kg) - amplifier and external power supply

Finish: Aluminum case w/ lexan top

Tube complement per channel: 12AT7 input, phase splitter, 6SN7 p-p output

 

Official current price: 1.100,00 USD

Direct sales worldwide: to Linear Tube Audio website

Associated equipment: to my system

 

 

RIGHT OF REPLY | MANUFACTURERS FEEDBACK

Giuseppe,
I used Google translate to read your review.
I was just like to say that it was not a review, it was poetry.
You were able to express the experience as noone else has. We struggle ourselves with describing it.
Maybe just because it's in Italian, it's automatically beautiful (my Italian friends insist on this).
Anyway, thank you for your great idea to call us, taking the time to really experience it, and of course, the work to write it all down.
Thank you again... I will have to send you my other amps when I have more...
Best regards,
Mark Schneider, Linear Tube Audio - Urban HiFi

04/01/2016

by Giuseppe Castelli
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