ingle-ended triode (SET): an ultimate dream of many music lovers, usually these more experienced ones, people who consider a fall of the Berlin Wall an event of quite recent history, and not something that happened when they were kids. An age of said dreaming person is a key to understanding this phenomenon. It is not about deterioration of human hearing, but about something else. Some loss of hearing especially of high frequencies is inevitable for each of us, so unless our brain is able to compensate for that, our perception of sound has to degrade over time. But time, or age gives us also an advantage – an experience that we gain over time. As long as our experience doesn't become a routine, we can benefit from it. What does experience tell us about amplifiers? It tells us that unless our favorite music is heavy metal, or some other alike, or we need some ultra-low bass, or we have a huge room, than the best possible option is a SET amplifier, preferably with a single power tube per channel offering output power as low as 5-8W. The only distortions such a device produces are even harmonics, that are interpreted by our hearing as elements of basic sound. On the other hand push-pull amplifiers, that are a majority of products available on the market, deliver mostly odd harmonics, that result in some harshness and nervousness of the sound.
Obviously, it is not a golden rule, it works but not in each and every case. I can easily name some examples like Lavardin IT-15, Jeff Rowland 625, or Dan D’Agostino Momentum Stereo, all of them not fitting to solid-state push-pull stereotype at all. On the other hand there are some tube amplifiers, some from Ayon Audio for example, that offer very open, dynamic sound despite having only a single triode in the output stage. Anyway this stereotype I described above is still quite popular and followed by a lot of people.

SET means single-ended triode, which is a tube amplifier that sports a single triode tube in the output stage. This is quite a popular solution still today. There are also some variations of SET design like parallel single-ended triode, with two (or more) triodes working in parallel in each channel, or even a penthode single-ended, with a penthode or tetrode working in single-ended setup. All these designs are quite popular today and we've already got used to them.
Solid-state amplifiers are a different story. Almost all of them are push-pull designs. The variations base on different types of transistors – bipolar, or field-polar ones (MOSFET, HEXFET, etc.) and on different types of push-pull configurations. But the solid-state amps working in single-ended setup come extremely rare.
To understand why we need to realize why solid-state amplifiers were introduced in the first place. Obviously transistors were supposed to replace tubes. A transistor was smaller, more effective, less noisy and more reliable. There was one downside though - transistors required more complex circuits, but for a long time designers weren't bothered by that. Over time designers learned of more issues of such designs but it was too late for history to make another turn and for majority of designers to go back to tubes. But fortunately tubes haven't died off, they found their niche on the market making many people quite happy about it.
One the main advantages of transistors was a high output power they were able to deliver. To push the power boundary even further designers use transistors in push-pull setup. Such setup offers also some other than high power advantages, like much lower harmonic distortion. So in fact designing a low-power, single-ended solid-state amplifier goes against currently effective canon.

So where the idea of building a single-ended solid-state amplifier came from? Why such a serious engineers like for example Nelson Pass (Pass Labs) or Colin Wonfor (Tellurium Q) decided to build SE transistor amps? The answer is very simple: it's about the sound such amps deliver.

Srajan Ebaen, a man who loves his single driver speakers is also one of the biggest admirers of single-ended solid-state amplifiers. He's been using FirstWatt (a second brand of Nelson Pass) for a long time, and also a while ago he sold off all his tube amplifiers to „make a room” for SIT-1 monos (see HERE). Those use a single silicon-carbide power JFet output device exclusive to Pass which at its operating point exhibits true triode curves. This amp provides 10 W per channel. Nelson Pass is not the only one who prefers this type of design. A British company renown for its unique cables, Tellurium Q offers now also electronics, including a power amplifier called Iridium 20 (now in its second incarnation, which is easy to differentiate from a previous one because of its red power button). It's a large device working in a single-ended mode with MOSFETs in the output stage, without any capacitors in signals path (DC-coupled), that utilizes an interesting way of bias setting. This amplifier offers 18W per channel for 8 Ω loading, but only 9 W for 4 Ω (which is quite the opposite proportion to what we know from classic amplifiers, working as a perfect current source – I'm quoting „HiFi World” here). It's output impedance is very low which resulted in quite high damping factor: 93. To achieve such a high damping factor designers used two, working in parallel, N-channel transistors. These operate with a high voltage (100 VDS) and impressive threshold working temperature of 175ºC. The supply voltage is regulated by a pair of bipolar transistors.

No capacitors in the output stage and low distortion in an open-loop came from the philosophy of Tellurium Q designers. According to them what „kills” music is a phase distortion. So their main aim when designing both, cables and electronics, is to minimize it. After a short test of a Ultra Black speaker cable I conducted I was simply amazed with what these guys had achieved with this not so complexed design. So when I agreed with a distributor a review of this amp I requested also Tellurium Q cables to be delivered with it, because I assumed only then Iridium would really shine. I received a Black Diamond IC (preamp-amp), Black Diamond speaker cable and Black Power power cord. You should treat this test as a review of a whole Tellurium Q system – amplifier + cables, although you can check for our separate review of cable set in January. During this test Iridium worked in a system with Ayon Audio Polaris III preamplifier and Dan D’Agostini Momentum Preamplifier. It drove my own Harbeth M40.1 speakers, but also JBL S3900.

Recordings used during audition (a selection)

• Antonio Caldara, Maddalena ai piedi di Cristo, dyr. René Jacobs, wyk. Schola Cantorum Basiliensis, Harmonia Mundi France HMC 905221.22, 2 x CD (1996/2002).
• Black Sabbath, 13, Vertigo/Universal Music LLC (Japan) UICN-1034/5, 2 x SHM-CD (2013).
• Can, Tago Mago. 40th Anniversary Edition, Spoon Records/Hostess K.K. (Japan) 40SPOON6/7J, 2 x Blu-Spec CD (1971/2011).
• Carmen McRae, Carmen McRae, Bethlehem/JVC VICJ-61458, K2HD CD (1955/2007).
• Charlie Haden, The Private Collection, The Naim Label naimcd108, 2 x CD (2007).
• Clifford Jordan Quartet, Glass Bead Games, Strata-East/Bomba Records BOM24104, CD (1973/2006).
• Massive Attack, Heligoland, Virgin Records 996094662, CD (2010).
• Mel Tormé, Mel Tormé sings Fred Astaire, Bethlehem/JVC VICJ-61457, K2HD CD (1956/2007).
• Mike Oldfield, Tubular Bells, Mercury Records/Universal Music LLC (Japan) UICY-40016, Platinum SHM-CD (1973/2013).
• Mills Brothers, Spectacular, Going for a Song GFS275, CD (?).
• Nat “King” Cole, Welcome to the Club, Columbia/Audio Fidelity AFZ 153, SACD/CD (1959/2013).
• Ornette Coleman, The Shape of Jazz to Come, Atlantic Records/ORG Music ORGM-1081, SACD/CD (1959/2013).
• Sting, All This Time, A&M Records 212354-2, SP CD (1991).
• Vangelis, Blade Runner, soundtrack, reż. Ridley Scott, Atlantic Records/Audio Fidelity AFZ 154, “Limited Numbered Edition No. 2398”, SACD/CD (1982/2013).

Japanese editions of CDs and SACDs are available from

One of the ground rules of any scientific development or experiment is gathering as much information that is already available on the subject as possible. Our business, audio, is no different, as what we are trying to do is to assess products qualities using our hearing. So when I got such an interesting product as Iridium 20 for a review/assessment I simply had no choice but to look for any and every information I could find, or get on it, from the net, or via email. There is quite a lot of information in previous tests, there are some on manufacturer's website too. Not all technical details are revealed though, but what we have is still more than what's given about Tellurium Q cables as manufacturer share no information about their design at all. I found some things very interesting like a comparison Srajan Ebaen used in his review. He wrote that while class A, especially SE solid-states amp could be compared to SET amplifiers, Tellurium Q reminded him more of a SE, class A amp but based on a single penthode per channel rather then triode. Another thing that caught my attention was Marek Dyba mentioning in his integrated amplifier Sugden Masterclass IA-4 review, a 33W, class A machine, that the way it sounded reminded him somehow of Audeze LCD-3 headphones.
Both Srajan and Marek refer to a very specific group of products, class A solid-state amplifiers. And it seems that both of them stick to a stereotype of a class A amplifier, even if they did some deconstruction of it, and refer it to it indirectly.
This stereotype associates this type of amplifier with a warm, deep sound, with a delicate treble and poorly controlled bass. As any stereotype also this one has some real bases for it – it is true that a lot of solid-state amplifiers with a bias set to rather high values sounded like that – Sugden A21, Pass Aleph 0, or Musical Fidelity A1 to name just a few. But a lot has changed since then. Modern, nicely build class A amplifiers offer different sound today, like, for example, Accuphase A65 and A200, or tube amplifiers like most SETs Ayon Audio offer. Iridium 20 is an example of yet another approach to the topic.

Before we move to the actual sound description I need to mention few other things about this amplifier and its everyday's usage. I'd say that this is a device for so called „hardcore” audiophiles. Simply put there are things about it that many people will find unacceptable. First of all there is a loud „puff” you will hear in your speakers each time when you turn amplifier on or off. That was pretty loud already with not so sensitive Harbeths, even louder with JBLs, so I can't even imagine how loud does it get with high-sensitivity speakers Iridium is in fact designed to work with. It happens because there are no capacitors nor coils, that were not used in this design because they introduced phase distortions. To be clear – this „puff” no matter how loud should not damage speakers, but it will get on user's nerves so many of them will go for another amplifier, just not hear that sound. Another bit problematic issue is how hot radiators, hence enclose get.

All class A amplifiers get hot, some, like above mentioned A200, get even hotter. But in this particular case every 3 hours amplifier overheated and turned off. The safety measures put in place for situations like this worked like a charm so each time, after cooling down, I could play music again. To speed up the process I used hair dryer. It looks like to total surface of radiator is not big enough, should be bigger to ensure that amplifier wouldn't overheat. Using some coolers is an alternative but this always create some level of noise. It is possible that with different speakers of higher impedance problem will be gone. But listening to this amplifier provoked me to argue with above mentioned stereotype, and that's obviously a good thing.

The sound of Iridium 20 is quite characteristic and can't be really confused with any other amp I know. It offers huge, expansive sound. I takes you by surprise with powerful range extremes and impressive micro-dynamics At the same time nicely extended, powerful bass lacks some control. Neither with Harbeths, nor with JBLs I managed to overcome that – I couldn't get tight, punchy bass I was used to. Well, I could probably solve that issue using speakers that didn't play lowest bass at all, or with lowest tone seriously rolled off. So this is where Iridium 20 fits into the above mentioned stereotype of class A, s-s amp. When I listened to Charlie Haden from The Private Collection his double bass sounds were sustained too long, or extended for a longer time they should have been. I was still fun to listen to but it was also clear that there was no true „fidelity” to the recording.
The other extreme was quite impressive though. Its openness and great resolution reminded me of the best SET amplifiers. I have no doubts that this is what Iridium's designers were after, as the quality of treble that this amp delivers is so much better than anything most of solid-state competitors offer. Maybe not in all aspects, but surely in majority of them that „overwhelm” those slightly less impressive ones. Considering how well it fit into the whole context I was truly amazed.
The point is that most solid-state amplifiers present treble that is at least slightly muffled and lack bit of dynamics. Usually you can't really hear that as speakers cover it up, but if you know exactly what to look for you will notice that each time. Only the top, very expensive devices are able to get rid (mostly) of that indigenous feature of solid-state technology. But even these can't offer such an openness and resolution that would properly deliver all the correlations between particular sounds, bigger structures and then sound within these structures. In fact solid-states do the opposite – try to enhance resolution and selectivity to achieve more detailed sound and basing on that build something more. So they try to achieve something that's indigenous to high quality SET amps. Iridium 20 seems to come close to those SETs.

The way this amp presents both frequency range extremes has a large influence on how we perceive its sound. But it would be a mistake to describe a sound of Iridium 20 as a one of both extremes pushed forward. It's a sort of phenomena I come across in audio every now and then. The amount of bass and treble does not determine our perception of a midrange, midrange is not rolled off. After a while I realized that the whole range is very energetic, vigorous and demanding my full attention. But since bass and treble get our attention more easily that's what we perceive at first. At least until we play some vocals. Then you realize that all you thought you already knew about this amplifier is not so obvious any more.
To verify that I listened to many recordings coming from Bethlehem label, re-issued as K2HD by JVC. Regardless if it was Mel Tormé from Mel Tormé sings Fred Astair, or Carmen McRae, or any other vocalist, each and every time Tellurium Q delivered a large, rich voice right in front of me. There was a slight emphasis on upper part of each voice, right below sibilants. That opened sound right in front of a listener, so to speak. And it felt like it complemented very well everything that was shown in frequencies below this one. And this sounded very... „mature”, I don't know how else I could call that. This emphasis on an upper midrange is a part of both above mentioned recordings, and Iridium 20 did nothing about it – just delivered it as it was recorded. When it came to some „smoother” recordings like Nat King Cole's Welcome to the Club this emphasis was gone which proved the Iridium's ability of differentiating recordings really nicely. It was not about simple „brighter or darker” presentation, but about de-constructing presentation in such a way that would allow brighter recording sound bit bright, a smoother ones smooth. In both cases it was clear that this amazingly open treble „ruled”, but it „ruled” in a different way depending on the recording. But when it came to some recordings with no treble at all, like additional pieces on McRae's album, Iridium 20 offered me a warm, sensual voice, without even as much as a trace of brightness, harshness or anything else that could spoil this amazing performance.

Jazz wasn't the only music that sounded spectacular with Tellurium Q. Although, as I mentioned, bass could be better defined, but that might be an issue only with the recording, that were taken with microphones placed very closely to instrument (Haden's recording is a good example here). When I played jazz from 1950ties there was no problem at all, as at that time bass was recorded with different microphone placement. Also when I listened to Vangelis I didn't have a problem with bass as I simply had no idea how it was supposed to sound. I mean I had my vision of it gained by listening to this recording on different systems, but it was about my vision, or preference and not the „real” sound, as I couldn't possible knew it. When playing Blade Runner, on Audio Fidelity CD , Iridium 20 delivered large, deep soundstage, and was able to create an unforgettable ambiance of that movie. This ambiance was build up with details, that Tellurium Q show in a pretty intense way, but they don't just exist on their own, but rather are a part of something bigger, of some point this kind of presentation is trying to make.

Summary

What Tellurium Q is offering is so different from anything available today on the market that it is really difficult to say who's gonna like it and who's not. It should be used with Tellurium cables as they help to shape the final sound. It gets warm as hell so I would recommend to use speakers with way above average sensibility and impedance. It won't offer crazy volume levels so most likely it's not the best possible partner for heavy rock and heavy metal fans. It should attract attention of people who look for sound that is different from anything else, who know exactly what they want. Jazz, vocals, electronic music (the last one was quite a surprise) – that's what you need to play to witness this amplifier to „come to life”, to really shine. Instruments have true bodies, sound is amazingly open, and treble resolution is remarkable. It reminded me of best performers among SET amps, although rather these with 845 and 211 tubes, than 300B or 2A3. Bass that is not so well controlled won't be an issue with properly chosen speakers. With the right ones it will still be rather on the „soft” side, but it shouldn't be a real issue. And there will be plenty of bass. That is an amplifier for certain group of individuals with certain expectations, and in proper setup it will be unmistakable for its remarkable sound.

This is a large amplifier that radiates a lot of heat, which means it needs a lot of space around it. The casing is quite simple, surely not that level of sophistication as Soulution 710, or even inexpensive, made in China casings of Cambridge Audio or Arcam. One can clearly see Iridium 20 was made in a small shop. It's casing is made of aluminum. The front baffle is a thick piece of that material. There are three LEDs there, that change color from red to green after circuit stabilizes from stand-by mode to operation mode, and a large, red on/off button. At the back there is a pair of speaker bindings, a pair of RCA inputs, and power inlet with a mechanical switch above it. All sockets seem solid, although surely are not fancy ones.
The power circuits are bolted to aluminum plates and these to large radiators. The latter have a quite rare shape of a herringbone. The power stage is proprietary Tellurium Q design. It's a parallel single-ended setup with two N-type MOSFETs per channel, with also single-ended controlling circuit. The voltage is regulated via bipolar transistors. Voltage regulation is supposed to help eliminate large part of distortion that is a large issue in most single-ended designs. Current source is based on transistors in a Darlington array, controlled by a precise integrated circuits. This is a dual-mono design with separate power supply for right and left channel. Two toroidal transformers are mounted next to the front baffle.
Build quality of both, the casing and electronic circuits inside seemed to me like a very well done, but still DIY product, something that was made by hand in a small shop. So one might consider it's look and finish as a downside, but a certainty that each piece has been built by hand and carefully checked surely comes as an upside.