Country of Origin


EHT monos

Reviewer: Srajan Ebaen
Financial interests: click here
Sources: Retina 5K 27" iMac (4GHz quad-core with Turbo boost, 32GB RAM, 3TB FusionDrive, OSX Yosemite. iTunes 14.4), PureMusic 3.02, Qobuz Hifi, Tidal Hifi, Fore Audio DAISy1, COS Engineering D1, Denafrips Terminator, AURALiC Vega, Soundaware D300Ref
Preamplifier: Nagra Classic, Wyred4Sound STP-SE Stage II, COS Engineering D1, Vinnie Rossi LIO (AVT module)
Power & integrated amplifiers: Pass Labs XA30.8; FirstWatt SIT1 monos, F5, F6, F7; Crayon Audio CFA-1.2; Goldmund/Job 225; Aura Note Premier; Wyred4Sound mINT; Nord Acoustics NC500 monos; LinnenberG Audio Liszt monos
Loudspeakers: Audio Physic Codex; Albedo Audio Aptica; EnigmAcoustics Mythology 1; Boenicke Audio W5se; Zu Audio Druid V, VI & Submission; German Physiks HRS-120; Eversound Essence; Cube Audio Nenuphar [on loan]
Cables: Complete loom of Allnic Audio ZL 3000; Zu Event; KingRex uArt, Zu and LightHarmonic LightSpeed double-header USB cables; Tombo Trøn S/PDIF; van den Hul AES/EBU; AudioQuest Diamond glass-fibre Toslink; Black Cat Cable redlevel Lupo; Ocellia OCC Silver
Power delivery: Vibex Granada/Alhambra on all source components, Vibex One 11R on amps/sub
Equipment rack: Artesania Audio Exoteryc double-wide 3-tier with optional glass shelves, Exoteryc Krion and glass amp stands [on loan]
Sundry accessories: Acoustic System resonators
Room: 4 x 6m with high gabled beam ceiling opening into 4 x 8m kitchen and 5 x 8m living room, hence no wall behind the listening chairs
Review component retail: €14'000/pr incl. VAT recommended global price

At Munich HighEnd 2018

Collusion. "I'm not even sure that's a crime." (Rudy Giuliani on the Trump election's Russian interference angle.) Today's review is about collusion of a different sort. Had you been hacking my email account, you'd have read that "I have an idea/proposal for a review I'd like to do – your 300B monos. I very recently reviewed an exceptional pair of speakers made in Poland.

"They use their own 10" widebander with triple whizzer and 2.4 Tesla neodymium motor with 81 magnet slugs and a very unusual phenolic spider. Though 'only' 93dB sensitive, they behave exactly like 100dB types. By that I mean, they really prefer high output impedance to generate an old-fashioned ratio between output and input impedance. I had an amp of 0.25Ω which still was very good. By the time I got to a damping factor of 100, things started to overdamp. Once I hit 0.00x output impedances, things got very bright and a good octave of bass just disappeared. In short, this is the perfect type speaker to show off the special virtues of low-power no-feedback triode amps.

"It's not so insanely efficient as to sniff out the most subliminal noise. Unlike most 100dB types, it's got real bandwidth to 30Hz! I reviewed it with transistor amps which have 4Ω output impedance (single stage, single-ended, no feedback, static induction transistor types) and the results were stunning. I've owned a few valve amps over the years like Art Audio, Octave, Yamamoto and Trafomatic. But when I no longer had speakers which play to and not against them, I divested myself of these amps. Now I'm back in this sector. If you are interested, let's talk." The email recipient was Reinhard Thöress, owner of the eponymous valve audio firm in Germany.

Monitoring my account, you'd have been privy to his response a day later. "Amazing FR drivers those guys from Poland make. Phenolic spider, hybrid magnet, underhung(!) voice coil. Whoa. I am not a FR person myself. Never was, actually. That's despite the fact that years ago I created the FR20MkII based on a rather unique principle which one might call 'one-way speakers with three voice coils'.

"It had a 12" woofer in series with the 8" FR driver where the coil inductance of the 12er was used to form a LCR correction network to flatten the FR's response. I also had an indirectly radiating (140×60°!) super tweeter to compensate for the treble directivity of the FR. So I respect achievements in this field with strong interest. And you're right, an FR driver of this kind certainly does not need extra damping via electronics. Perhaps even more importantly, such speakers—no matter how good—by nature tend to sound slim with an emphasis on the upper mids and presence. That effect is obviously and naturally compensated for by an amp with high output impedance. So your point about the ideal driving amp is readily understood and does make SE triode amps a very reasonable choice indeed. Certainly I am willing to bring my 300B monos into play in this context."

"However… perhaps my freshly minted single-ended-triode/single-ended Mosfet EHT monos (EHT = Eintakt Hybrid Triode in German) would be an even better choice despite their moderate 0.5Ω impedance? This would combine particular loudspeakers with particular electronics. These monos feature an input attenuator switch (low, mid, high, full gain/sensitivity) implemented such as to allow for minimalist 'tone controls' by adding just one part per channel. We could share insights and create two or three tone registers for these new FR floorstanders. As it is conceptually forbidden to equalize the response between amp and FR driver, we do this at the input of the amp by adding just one part. This avoids clumsy sound-deteriorating compensation circuitry. You take your pen and make a thrilling review out of all that, creating something unusual and different in the process. Would that be an option?" [Reinhard's photo from Dawid Grzyb's review of his F2A11.]

Perusing Reinhard's PDF, I learnt that his EHT were zero feedback pure class A point-to-point wired 40W/6Ω designs. Unusually, they were "fully short-circuit proof at the outputs without any sound-degrading protection. Moreover, a switch-on delay too is omitted because switching on/off develops no disturbing noise or dangerous signal in this circuit." It consists of a single-ended triode gain stage followed by a unity-gain Mosfet buffer. "This fully exploits and mates the most desirable characteristics of tubes and transistors as determined by their very natures. In spite of its obvious advantages, elegance and extremely high sonic capability, the EHT concept has never before been commercialized in serial audio production as far as we know. This is likely due to its low efficiency and rather tricky implementation. Yet analysis easily shows this topology to represent the purest form of all possible single-ended triode power amp configurations.

Nenuphar set up with FirstWatt SIT-1 as 10-watt single-stage single-ended transistor amps with no degeneration but bona fide triode curves and bias adjustment for tailored load-line shifts.

"Specifically it shows that without compromising sound quality, the EHT more easily obtains higher output power and with lower distortion than does all-tube single-ended triode amplification. That's because the critical power-tube-driven step-down output transformer as the typical all-tube impedance matching output arrangement can be omitted.

"As a result, the voltage swing required from the driver stage is up to 10 x lower (!) for an EHT of equivalent power. This crucial insight was the basis for our pursuit of this hybrid concept."

The double-coil ultra low-noise C-core power transformers Reinhard winds in-house to be available at the EU's 230VAC, then at 100VAC (Japan), 220VAC (South Korea, Thailand, Russia, mainland China) and 245VAC (Australia). The tubes he uses here—the octal GJ5GT as one half of a 6SN7GT and the noval 12HG7—he calls "known for being amongst the most linear vaccum tubes ever made". At right, the integrated version of this circuit.

Weighing 24kg, each mono amp measures 15.4×43.4×43.4cm.  Besides Nenuphar, I had Audio Physic Codex, German Physik HRS-120, Zu Druid VI, Boenicke W5 and Albedo Audio Aptica speakers.

They'd make for more conventional alternates all within reach of 40 watts into 8Ω given our civilized habits. They'd report on less specialized more normal loads. Having thus obeyed the usual prenuptial mating rituals, I very happily committed to Reinhard's novel new amps.

Time to remove your surveillance software from my email account. It only takes the most Rudymentary appreciation to see that special speakers like Nenuphar need special amplifiers. Not colluding on that would be criminal to almost certainly set up compromised results.