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Audio Note Cobra Integrated Tube Amplifier

05-02-2022 | By Wojciech Pacuła | Issue 121

Audio Note is a company founded in 1976 (company site says October 1979) in Japan by Hiroyasu Kondo. The first products hit the world markets in 1989. In 1997, the company split into two parts, Kondo Labs, focused on the Asian market, and Audio Note UK, responsible for Europe and the rest of the world. A year later, the British branch was taken over by Peter Qvortrup, which over time also took over the rights to the Audio Note UK brand and turned from a distributor into a manufacturer. 

Peter Qvortrup's involvement in the brand resulted in the introduction to the Audio Note UK's lineup of devices of his idea, such as: turntables, CD players, digital-to-analog converters and loudspeakers. The Kondo, on the other hand, remained faithful to the original ideas of Hiroyasu Kondo, i.e. focused on amplifiers. With time, also a turntable and loudspeakers were included in its offer, but it seems that these were only temporal and not a permanent changes. One thing has not changed—these devices have been and still are expensive or very expensive.

Audio Note UK, on the other hand, offers a wider range of devices, both in terms of their type and price. To maintain the latter, some time ago Qvortrup decided to establish cooperation with two factories outside the UK—in Austria, and in Lithuania. The latter, called Serva Se, is a particularly interesting case. Its owner is Tadas Motiejunas, who has been a distributor of Audio Note products in the Baltic countries since 1998.

Tadas also owns a factory in Vilnius. It manufactures various devices in the OEM system for many well-known audio brands, and since 2004, so for a very long time, it has been producing many of Audio Note components, including all CD players and transports, the entire Zero series, as well as power supplies for TT1 / TT2 / TT3 turntables. And it is in this Lithuanian company that the reviewed Cobra amplifier is manufactured.

A few simple words…

ANDY GROVE, AN Chief designer, AN R&D engineer

The Audio Note Cobra is a simple and straightforward design, using a triode connected 6AU6 as the input valve, and a 5670 double triode as cathode-coupled phase splitter. It's a common topology in valve amplifiers as it's elegant, works well electrically and sounds excellent if implemented properly. Coupling between the two stages incorporates both LF and HF compensation networks to ensure a clean, crisp transient response at both ends of the audio spectrum.

Both the 6AU6 and 5670 are valves that we have many years of experience with, and, importantly, plenty of NOS stock, so servicing is not going to become an issue. The EL34s are operated as straight pentodes, approximating the condition recommended by Philips (Class A/B, 3k5 anode-anode load), which brings out the character of the EL34 superbly. The output transformers are in-house designed and manufactured and fitted with our IHiB C Cores. C Cores provide higher primary inductance and power handling, together with lower distortion compared to a stack of laminations of the same size.

The mains transformer is also an in-house design, optimized for the Cobra's circuitry, and is fitted with our heat-treated, non-oriented silicon steel laminations. This material has a warm, natural character which complements the valves and topology. The power supply uses silicon diodes in a voltage doubler configuration and is choke filtered, using another in-house designed and produced component specific to the Cobra.


The optimization task for the AN Cobra was important because the design represents a new generation of AN products, it was a part of a today's better known Tonmeister Strategy. The AN Cobra is, in its essence, a typical, traditional and musical AN product but one that should also reach a broader spectrum of customers and it will successfully drive a wider range of speakers than just our own, so we've focused on that whilst it controls speakers well, it still flows well, it possesses a refined feel.

In order to achieve that, we continually revised the PCB design with careful trace width optimization, choice of parts blending using Beyschlag Metal Films, Audio Note Tantalum Resistors, selected Carbon film in anodes of the 6AU6s, ERO NOS, Silver Mica and AN Standard Electrolytics throughout. The dynamic behaviour was carefully tuned and harmonized with a specially waxed output transformers and to available EL34s- the Zobel, lag and lead compensation were optimized for a maximal system robustness, minimal overshoot and a fast settling time which always gives a natural response and a minimal sound of its own. 


When the current owner of Audio Note started working with Hiroyasu Kondo, the company designed push-pull amplifiers. Only his suggestions made the company turn its attention to single-ended designs. He was one of the consultants, and in time even the initiators of such models as Kagura, GakuOn and OnGaku; Qvortrup is, moreover, the "father" of the name of the latter. Which is very interesting because the owner of Audio Note has no electronic education. On the other hand, he has amazing experience in selling expensive equipment and thus knows the needs of his customers.

The Audio Note's lineup includes only tube devices, with single-ended designs being a dominant solution. The Cobra, however, belongs to the "cheaper" devices of this manufacturer, therefore it is a push-pull design. It is based on EL34 power pentodes operating in class A. They are driven using the rare JAN-5670W double triodes (JAN = Joint Army & Navy), and in the input one finds miniature JAN-6AU6WC pentodes; these tubes were manufactured by the General Electric, and the output pentodes come from Electro-Harmonix.

By the way, a word about tubes from Russia—I think it is advisable for manufacturers to stop buying tubes from this country and instead choose products offered by JJ Electronics, PSVANE, KR Audio and other manufacturers. Mere human decency demands it.


I like EL34 tubes very much. I appreciate the KT-series beam tetrodes like the KT88, KT120 and KT150, but the EL34s occupy a special place in my heart. The development of this pentode was initiated by Philips in order to improve the operation of the tetrodes previously used to amplify an audio signal. These had an unpleasant discontinuity in the gain; this discontinuity is called "kink."

In a "corrected" tube an additional suppressor grid (or "brake") was introduced. Its originator was Bernard Tellegen, and the invention dates back to 1926 - the patent was owned by Philips & Co. In 1933, two EMI engineers, Cabott Bull and Sidney Roda, decided to circumvent the Dutch patent. They proposed a tetrode that did not have an additional mesh, and the "kink" referred to was eliminated by redirecting the electron beam to a special element called "beam"—hence their technical name: beam tube; in the Polish nomenclature they are called the beam tetrode.

Initially, the new type of tubes were considered better than Philips pentodes. In response to these claims around 1937, the company's engineers introduced innovative changes to their designs, thanks to which a year later these pentodes had offered parameters than the KT series tubes, i.e. beam tetrodes. Developed in 1950, EL34 tubes were the pinnacle of the entire series. 

source: The Absolute Sound's Illustrated History of High-End Audio, Volume 2: Electronics, ed. Robert Harley, Austin 2015.

Cobra Integrated Amplifier

It offers 27 W output per channel from two pairs of EL34 tubes, working in a push-pull mode, in class A. The use of pentodes in the input stage allowed to obtain a high input sensitivity of 300 mV. The amplifier features three line inputs with RCA connectors and three digital ones: RCA, Toslink, and USB.

The D/A converter is the same as in the CD 2.1 player from this manufacturer—it is a NOS chip, Philips TDA1543. This is one of the most famous 16-bit chips, loved by DIY people because it can work without oversampling. Each of the digital inputs accepts signals with different limit parameters. And so the RCA accepts signal up to 24-bits and 176.4kHz, Toslink up to 24/96, and USB up to 16/48; clearly it is an auxiliary input. Regardless of what digital signal we send to the amplifier, it is converted to 16/44.1, "understandable" for the TDA1543.

The volume control is carried out in a rotary potentiometer integrated with a motor, and the input switching is indicated on the front, inclined baffle using red LEDs. There is no information about whether the a source and converter achieve synch. The amplifier is controlled using a remote control - an ugly, plastic one, straight from the 1990s. Its only advantage is that you can also use it to control the company's CD player.

The amplifier looks very ordinary, there are no subtleties and impressive technical solutions in its design. However, this is not the only such case, just look at the Grandinote Supremo amplifier, costing over PLN 100,000, featured on our cover (review in Polish HERE) . On the other hand, nowhere else we can get the same "double C" type output transformers and power transformers, produced by Audio Note.

It is thanks to them, as the manufacturer says, that such a wide frequency response was achieved with low distortions. The noise is also negligible, because even when putting my ear to the Harbeth tweeter I did not hear a loud hissing, but a slight hum. This is a great achievement for a tube amplifier. In turn, there was a slight hum in the woofer, but audible only when I bent down to them and held my breath.

When buying Cobra amplifier, we do not buy a "luxury product," as it is understood in the high-end world in other industries. Instead, we buy a know-how and become part of the legend of this brand and its unique sound—we'll get to it in a moment.



The Audio Note Cobra amplifier was tested in the High Fidelity reference system, in which it was compared to a system consisting of the Ayon Audio Spheris III tube preamplifier and the Soulution 710 transistor power amplifier. In a separate listening session I compared it also to the Octave Jubilee PRE SE & MONO SE pre + power set. Thus, the reference points cost PLN 300,000 and PLN 480,000, respectively. So to be sure, I also listened to the LEBEN CS-600X amplifier, one also using EL34 tubes.

Audio Note Cobra was placed on the top shelf made of carbon fiber of the Finite Elemente Master Reference Pagode Edition SE rack. It powered the Harbeth M.40.1 speakers, difficult-to-drive monitors from the BBC's "school," via the Siltech Triple Crown cables or—alternately—Western Electric WE16GA (NOS). The latter turned out to be an excellent choice. The source of the analog and digital signal was the Ayon Audio CD-35 HF Edition SACD player. The signal was transmitted via Siltech Triple Crown cables.

Recordings used for the test | a selection

  •    Pink Floyd, The Dark Side Of The Moon, Harvest SHVL 804, 1E 064 o 05249/Analogue Productions CAPP 81033 SA, SACD/CD (1973/2021).
  •    Led Zeppelin, Led Zeppelin (I), Atlantic/Warner Music 8122796439, “Super Deluxe Box Set", 2 x CD + 2 x LP (1961/2014).
  •    LANG LANG Goldberg Variations Deluxe Edition, Deutsche Grammophon 481 9701, 4 x CD (2020).
  •    Dave Grusin, Discovered Again! Plus, Sheffield Lab/Lasting Impression Music LIM XR 002, XRCD24 (1976/2003).
  •    Vangelis, Blade Runner, Atlantic Records/Audio Fidelity AFZ 154, "Limited Edition | No. 2398", SACD/CD (1998/2013).

There are a few strategies applied by audio manufacturers to build devices and achieve a desired result. They can approach such goal based solely on engineering knowledge and measurements—a technical approach, if you will. Others prefer a "passionate" approach, so their decisions and choices are based on endless auditions and corrections based on these. In between, however, there is a wide range of possibilities and the Cobra amplifier explores one of them.

If I am not mistaken, this device was created as a result of engineering efforts to achieve the best possible parameters, and later it was almost completely reworked as a result of listening sessions. This initial DNA is still present in its sound, it is actually its basis. However, it was brought to the point where one can no longer just say that it is a technically correct device, but rather that it is refined—musically and sonically.

There is no other way to explain the fact that, on the one hand, it is an extremely well-ordered, perfectly resolving sound, and on the other, a clearly “tube-like" sound. Not just "tube-like" but even "tube-tube-like." There is no pretending, but a clear declaration: "I'm a tube amplifier and I'm fine with it."

The sound of the Audio Note amplifier is therefore unequivocally warm. Not "a tiny bit," not "a little," or even "quite"—it is a warm, full sound with strong mid-bass and sweet, withdrawn treble. When I heard the first sounds, i.e. the pulse of the beating heart, from the Speak To Me track, opening the Pink Floyd's album, The Dark Side Of The Moon, I immediately sat down intrigued, because it sounded so interesting.

This album was recorded with a lot of hum from analog keyboards and guitar stoves. The Audio Note amplifier did not emphasize it, but also clearly showed that fact. Immediately a large, clear image was thrown in front of me, with guitars, piano and vocals. Cobra builds large phantom images, offering me a large scale of instruments and soundstage.

On the one hand, the space was focused around the sound sources, there were no long reverbs here, and on the other hand, it was fantastically coherent. This is why in the "On The Run," when at 0:52 and 1:41, and then in the second minute, we are surrounded by sounds as they move around us, Cobra showed it brilliantly, in a dense, full, coherent way, without flattening the perspective and without holes in the movement of sounds.

It is a unique achievement, one rarely encountered, because it is usually sacrificed for precision and clarity. The sound of the Audio Note amplifier is not clear, at least in the classic understanding of this feature. So it's not very selective. Rather, it shows the whole element, not its "component parts," and more strongly indicates the sustain of the sound, not its attack or decay. And yet ... And yet everything in this presentation is absolutely clear.

I know this sounds schizophrenic, but it isn't. This simultaneity of two opposing features happens in high-end, just like here. Because when I listened to the Lang Lang's album with his version of the Goldberg Variations I got carried away by the mood and the beauty, I didn't have to listen to the details, because those were there, but they offered something more as a whole. They were combined into the smooth music, connectivity of its elements, instead of creating a boundary between the show and me.

It was with this disc that I also heard more precisely what the focus of the Audio Note amplifier on building big sound is about. The album was recorded at Jesus-Christus-Kirche in Berlin, and this church has a long, dark reverb (I was there, I checked). The reviewed amplifier showed it just like that, closing the upper treble at the same time. Interestingly, the legibility of the instrument was not affected—it was still a resolving and informative presentation. I lacked nothing in it, despite the fact that the "own sound" of the amplifier was so obvious, even ostentatious.

Our albums 

LANG LANG Goldberg Variations Deluxe Edition. Deutsche Grammophon 481 9701. 4 x CD 2020

The Johann Sebastian Bach - Goldberg Variations album is the culmination of the 20 years of Chinese pianist Lang Lang's work on this piece and, according to his official biography, "the most personal project in his career so far." He was only 17 years old when he played it by heart for the conductor and pianist Christoph Eschenbach. As we read in the press materials, by recording Goldberg Variations, Lang Lang made his life's dream come true:

I am 38 now, and although I am still young, I think it is time for a new stage in my artistic development, says Lang Lang. I moved to a whole new area with Goldberg Variations and was completely immersed in this project. As an artist, my goal is to constantly deepen my self-awareness and knowledge and inspire others. It's an ongoing process, but this particular project took me much further along this path.

The album was released by the Deutsche Grammophon label in two versions: the basic one, with two CDs, and the "Deluxe" one, that includes four discs. The first two contain a recording session from four days between March 15 and 18th in Jesus-Christus-Kirche in Berlin. The other two were recorded with the participation of the audience. It is a live recital performed on March 5th. 2020 at St. Thomas in Leipzig, the place where Bach worked for almost 30 years and where he is buried.

Philip Krause was responsible for the recording, mixing and mastering prepared in the digital domain. He is a young engineer who works for the Deutsche Grammophon label. The album was released on September 4th 2020. In Japan, MQA-CD version is available.

The sound of the piano recorded in Jesus-Christus-Kirche is dark, dense and you can hear long, strong reverb accompanying the instrument. You could even say that it is a "velvety" sound, as it is reproduced so smoothly. The image of the piano is quite big. However, this is a recording which is not about placing us "at" the open lid of the instrument, but rather in the, say, fourteenth row in its best spot, but at a certain distance from the stage.

The sound is also perfectly legible. Both the left and right hands have adequate weight and sonority. Which is interesting because the recording from Leipzig is less weighted and the image is smaller. Of course, you can also hear the acoustics of the church, but it is brighter, and the whole has a higher center of gravity. This version is also not so velvety. It is more emotional, less "thought out." There is more nervousness in it and less planning, but maybe that's why it seems to be the better one.

The musicality pf this device is striking , even addictive. And you don't need any technically sophisticated recording, because the Floyds have already sounded great. I did not expect, however, that the debut of the Led Zeppelin group would sound so well, rocking me with so much fire. I listened to the version remastered by Robert Plant and although it is not a particularly good version in terms of timbre, its resolution is excellent, much better than of all previous digital versions.

The sound had a weight and gravitated towards the lower midrange, there was also a lot of bass. The latter, however, was not exaggerated, and in its own special way it was actually an excellent basis for the sound, not overshadowing the midrange. In fact, the sound had great dynamics and scale, and was still warm at the same time. How was it achieved by Lithuanians and Qvortrup—I have no idea. But people from Leben achieved something similar. Again, the sound stage was wide and full, and the sounds placed by the recording engineer behind me, as in the "Babe I'm Gonna Leave You," where they had a well-defined position, they were big-bodied, they didn't seem fleeting.

All these qualities will be clear no matter what music you listen to. After all, jazz also sounded great! The Dave Grusin's album, recorded within three days, entitled Discovered Again! Plus, released by Mr. Winston Ma on his Lasting Impression Music's release on the XRCD24 disc, offered equally impressive dynamics, resolution, fluidity and space that I was talking about. Once again, I was fascinated by how you can perfectly combine the departure from neutrality and "linear" presentation without losing anything, and gaining in terms of beautiful melody and timbre.

The album was recorded in the direct-to-disc technique by the Sheffield Lab label, but in parallel with the acetate cutting, the sound was recorded on a cassette tape as a backup. And it was from this tape that the version in question was prepared at the JVC Mastering Center in Tokyo. It offers excellent dynamics—it is a two-track recording made "live," but there is also fluidity of the sound there, which DtD recordings often lack. The reviewed amplifier seemed to agree with me in this matter—it showed a full, dense, slightly soft bass, withdrew the percussion cymbals, but also perfectly showed the sonority of the piano and vibraphone.


The DAC featured in the Cobra is more than good. It is not an "accessory," but a full-fledged piece of equipment. You can easily choose a good CD transport, for example from CEC or Cyrus, you do not need an integrated CD player, because the Cobra's DAC will do a really fine job. Also a file transport will benefit from it.

The DAC in the amplifier does not offer such a saturated midrange as the reference player and is also not as resolving. Interestingly, however, it retains its full sound and excellent bass extension. I could hear it very well in the bass guitar solo played by Ron Carter on "The Colorado Trail" from the aforementioned Grusin's album. The Rhodes electric piano also had exceptional fullness and velvety.

You can achieve an even better sound with expensive CD players that will add even better vividness and selectivity to what the Cobra's DAC presents. However, the timbre and dynamics are excellent with it, they are really impressive. Therefore, I would not hesitate and buy just a good transport to pair with it—it will do a very good job.


The Cobra amplifier by Audio Note is not inexpensive. At least if we look at its make and finish. There is nothing special about his physical appearance. However, when we connect it to good loudspeakers and feed it with a signal from a quality source, we will hear magic—real magic with it. Seemingly inconspicuous, musically extremely musical. Only Audio Note can do that.


The Cobra is an amplifier by the British, West Sussex-based company Audio Note. It was designed and built in the Lithuanian Lyra factory, cooperating with AN. The design, and then the final product, were evaluated and then approved by Peter Qvortrup.

It is an integrated tube amplifier with a semiconductor power supply. It features NOS (New Old Stock) tubes by General Electric in the input stage, i.e. ones produced some time ago but never used. These are special versions of tubes, intended for the American army - JAN (Joint Army & Navy): two miniature GE 6AU6 pentodes in the input and two double, tiny 5670 triodes per channel, as drivers for the output tubes. And the latter are EL34 pentodes, bought from the Russian company Electro-Harmonix.


The amplifier's chassis is a solid, rigid design made of thick, bent steel plates. However, it is very simple to make. The transformer cover is integrated with the top panel. The tubes are placed classically for this class of products, i.e. on the upper surface of the low chassis. On the sloped front panel, characteristic for Audio Note, there are two knobs—input selector and volume control.

The former works with an encoder, and the selected input is indicated using a red LED. The potentiometer, on the other hand, is a classic design and it is integrated with a motor—hence the amplifier is controlled by a remote control. The indicator on the volume knob is almost invisible and it should clearly show its position. What's more, its indications are in no way scaled, there are no dots nor dashes, not even a "zero" (infinity) point, and they should be clearly marked.

On the back there are input sockets, loudspeaker terminals and an IEC power socket integrated with a mechanical switch. The connectors seem to be silver plated and are manufactured for the Audio Note. There are single loudspeaker terminals for 4 Ω load.


The electronic circuit is assembled on a single printed circuit board, with components soldered on both its sides. There are separate PCBs for an AC supply voltage filter and the receiver for the remote control.

On the back you can see beautiful transformers with Audio Note stickers attached. The power transformer is exceptionally large, made of EI type sheets. On the other hand, the loudspeaker transformers are unusual, because they are "double C" designs. There is also a large choke that works with capacitors in the network ripple suppression circuit. All these elements have been thickly covered with a colorless varnish to reduce vibrations.

The input signal is switched in relays and led to the PCB with silver Audio Note cables; the same interconnects lead the signal to and from the potentiometer. The tubes are coupled with ERO Vishay capacitors. EL tube sockets are ceramic ones, but the rest of components look less expensive.

The whole circuit makes a very good impression, and the the greatest value of the device are the transformers. The chassis is solid, but very simple in appearance and workmanship.

Technical specifications (according to the manufacturer)

  • Nominal output power: 2 x 28 W
  • Tubes: 4 x EL34, 2 x 6AU6, 2 x 5670
  • Input impedance: 100 kΩ
  • Input sensitivity: 300 mV
  • Max power consumption: 175 W
  • Digital inputs: USB – up to 16/48, RCA – up to 24/176,4, Toslink – up to 24/96
  • Dimensions: 340 x 199 x 451 mm (W x H x D)
  • Weight: 13.6 kg

Price (in Poland): 23 700 PLN

Audio Note (UK) Ltd,

Viscount House, Units C, D & E, Star Road,

Star Trading Estate, Partridge Green,

West Sussex, RH13 8RA | UNITED KINGDOM





translation Marek Dyba

images Wojciech Pacuła

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