Solid State Power Amp Reviews

Like the proverbial pot of gold, darTZeel's golden equipment beckons. That, at least, is how it felt in January 2010, when John Atkinson and I ended our coverage of T.H.E. Show Las Vegas in the room shared by darTZeel, Evolution Acoustics, and Playback Designs. Listening to darTZeel's discontinued NHB-458 monoblocks (footnote 1) and NHB-18NS reference preamplifier (now updated), I was transfixed by the fullness of the system's midrange and overall beauty of the sound. "It was as though the system was opening its heart and welcoming us in," I wrote. "That's how warm and nurturing the sound was."

The high point of the show came when I offered up the CD While You Are Alive by the male vocal ensemble Cantus, which John recorded. "The sound was little short of heavenly," I wrote of an experience that left us glowing and feeling at one with the music. What remains ensconced in my memory is the system's disarming naturalness of sound and generosity of spirit.

Now, almost 12 years later, I was preparing to revisit darTZeel, this time in my own home (footnote 2). The product—darTZeel calls their components "instruments"—was the "model two" version of darTZeel's original Swiss-made stereo amplifier, the NHB-108 (footnote 3). When it was first released, that looker of an amp cost $18,000; the model two costs $53,000. It's likely that some readers will look at the price and think that they need to stumble upon a pot of gold in order to afford one.

The NHB-108 model two's designer, company founder Hervé Delétraz, clearly intended to attract attention when his first product emerged with a gold faceplate, red heatsink fins, and a see-through glass top that revealed one-of-a-kind gold crescents, gold-topped capacitors, and (of course) gold-topped transformers. Blue and red also figure prominently in the visual display. As with certain Swiss watches that purposely expose their gears, the NHB-108 puts on quite the show.

"The first amp took 16 years to design because it wasn't a full-time job and I needed to eat," Delétraz explained in a conversation via Skype. "After I patented the electronic circuit [in 2000], which is different than others, I wanted everything else to be different, including the look and parts inside and out. I replaced straight bus bars with gold crescents because I thought they looked better. The internal finishings are also hand-brushed and anodized. We are the only company to do this; even the other Swiss manufacturers don't do it."

Although one might expect that glass would prove worse protetion from EMI/RMF than heavy aluminum plates, Delétraz told me that in going topless, he was after more than just good looks. He chose glass, in part, because he wanted electromagnetic fields to escape through the cover so that there would be less interference between the transformer and the housing. He also made the glass 8mm thick to help isolate the amp from vibration. "There are different resonant frequencies between the aluminum and the glass, which makes the cabinet quite inert," he said. "The only downside is that you can't stack the preamp on top because you will receive stray magnetic fields, especially in the phono section. ... As long as they're a shelf apart, you're fine." (footnote 4)

Design basics, darTZeel style
The NHB-108 model two remains a class-AB, single-ended design with symmetrical power supplies and three interconnect choices: RCA, true balanced, and the company's proprietary, 50 ohm Zeel BNC inputs and cables. Nominal output power has increased to 150W RMS into 8 ohms, 225W RMS into 4 ohms. With two pairs of transistors at the output—the previous version had only one—bass grip has increased to "pretty close" (quoting the designer) to that of the NHB-486 monoblocks. "Model two is a down-scaled design from the NHB-468 and offers wider bandwidth, no phase shift down to a few hertz, tighter bass, and 50% more output power than the NHB-108 model one," Delétraz said.

Delétraz's sense of humor extends to the amp's front. In addition to an engraved serial number plate that includes the owner's name, it has two yellow "eyes" and, lower down and midway between them, a "power nose." The eyes blink or "shut" when something goes wrong. (The face thing is mild compared to the window on the preamp's front, which reads "Foreplay" as it's warming up and "Climaxed" when you turn it off. Don't think of donating an NHB-18NS preamp to your local public school. Not that you'd be so inclined.)

Handles on the front and rear make positioning easy. The only tricky thing about the rear panel is that, depending upon the angle at which you connect them, the speaker-cable terminations may cover the little switch that toggles between Zeel, RCA, and XLR inputs. This won't be an issue for users who prefer to spend their time listening to music rather than, like any Stereophile reviewer, switching interconnects several times a week.

The original amp's fuses have been replaced with a magnetic tripper/breaker that offers better contact over a much larger area. The internal impedance switch—used in the earlier model when connecting low-impedance speakers—is no longer necessary and has been removed. Similarly, darTZeel's stop-you-in-your-tracks warning that the world may come to an end if you turn on the amp before you connect it to speakers has been jettisoned.

"The rest of the difference is in the listening," Delétraz said. "When you listen to the model two, you hear a global improvement. As you know, measurements don't tell everything. Listening is the ultimate test .... If you would put a magnifier on the different models, the original 108 was perhaps a little bit softer and more romantic than the others because it didn't have the same grip on the bass. But it did go very low in frequency and could render the bass with very detailed and textured sound. Model two has much more precise, tighter, and deeper bass, but the same delicacy and grace in the mids and highs. But when you measure the two things, the specs are exactly the same, which is crazy. A measurement is always useful, but it's not the last word. The last word is the listening, of course."

The detailed manual for the original model one—the manual for model two was in process while I was carrying out this review—includes a 27-page technical section filled with assertions that some will consider against the grain. In an interview transcript that's longer than this review, Delétraz explains some of his potentially controversial design decisions.

For example, he does not consider pure class-A necessary. He prefers single-ended connections because balanced interconnects "depart from simplicity. ... To me, balanced is very nice for studio use because with very long lengths of cable, it provides immunity against external parasites. But you don't need balanced in day-to-day home use because you never run hundreds of meters of cables. The problem with full balanced from in to out is that you need to use double the components—double the parts—because the global signal path is doubled. To me, the best is simplicity. The less components and parts you use, the less the sound is affected. If you get closer to a single wire, you are closer to the original sound."

In place of balanced interconnects, Delétraz prefers his Zeel 50 ohm BNC interconnects between his preamp and amp. To explain why, he sent me the white paper on impedance published in Stereophile two decades ago. In summary, he said, "Matching impedance links remove all echoes back and forth in the cable that make the sound less focused. More than 100 years ago, when you used a simple telephone, you had to match impedances on the line; otherwise, the sound was so smeared and filled with echoes that you couldn't hear your neighbors. The impedance then was 600 ohms. There's an analogy with a jump-rope. If you attach one end to the wall and shake at the right speed/frequency, you can find the point where the rope seems immobile because you have a single unmoving sinewave. It's the same with matching impedance cabling; it removes parasites and echoes in the lines.

"Twenty years ago, people told me that with one- or two-meter interconnects, the echoes were so short that they wouldn't matter. But when you listen, the difference is just amazing. This is because hearing is so sensitive. You hear more than frequency; you hear timing. We can hear things you cannot measure."

Delétraz doesn't consider watts as important as many people think, because they are not "the ultimate mean. Output power is, in the end, only a power supply that music will control and modulate. Around 100 watts is the optimal choice if you want high quality, high SPL levels, high resolution at low levels, compact design, and virtually affordable price. ... If you need a higher SPL level, don't change your amp; change your speakers and get more [sensitive] ones."

DarTZeel amplifiers eschew global negative feedback; there's only a small amount of local negative feedback to ensure voltage gain and overall stability. "I don't care about the load. We simply transfer the input signal to the output, and then we don't have to think about what kind of load we have at the end. This is a big factor in our design, because we are almost completely independent of the kind of speaker at the end. They can be ... pure coil or pure capacitors; it doesn't matter because we don't have any reaction with the counter electromagnetic force of the speakers.

"Most amplifiers use a lot of global negative feedback to correct all distortion. When your speakers present a very difficult load to the amplifier due to phase shift—phase shift is just a time shift between current and voltage—the amplifier does not deliver current at the same time as the voltage. When you apply negative feedback to correct this, sometimes it makes things worse because music itself is not a continuous sinewave and is always changing. With a high feedback ratio, if your amplifier can't cope with the phase shift, it becomes very disoriented. It doesn't know what it has to do, and control of loudspeakers suffers as a result. Even when there is no phase shift, global negative feedback always applies with a delay since music is continuously changing. Phase shift just adds more complexity for the amplifier to cope with. Ultimately, it's about how to drive the speakers in the right way without being affected by phase shift."

Because Delétraz believes as strongly in impedance matching as in his own designs, he asked me to audition the NHB-108 model two with both my reference D'Agostino Momentum HD preamp ($40,000), which only uses balanced interconnects, and his own NHB-18NS preamplifier. For the latter, he sent the same top-of-the-line 10mm Zeel 50 ohm BNC interconnects that he supplied to Michael Fremer.

Close and personal
When I asked Delétraz about his favorite music, he said that he preferred the pop-rock from when he was a kid—think Pink Floyd, Genesis, Led Zeppelin, Jethro Tull, Joe Jackson, Deep Purple, and Supertramp. He also loves jazz. "It's easiest for me to judge a system by listening to pop/rock music or jazz. Of course, classical music is very emotional; it's easy to have tears when you listen to opera. But it's difficult for me to know if the system is good. I always say I'm too young to appreciate classical music. Maybe in 20 years."

For Pink Floyd, he prefers three albums of the Roger Waters period: Wish You Were Here, Dark Side of the Moon, and Animals. He's also a major fan of Waters's 1992 antiwar proclamation, Amused to Death, and he loves its 270° Q-sound effects. "Your system has to deliver a lot of detail and phase accuracy to create a real ambience. To me, it's a nice test, and [it's] what I've been using lately because it really shows the quality of the system. There are 24/192 and DSD versions. Qobuz's quality at 24/192 is quite good. If you have time, just close your eyes and you'll be transported. It's like taking a small trip." Did I take a listen and love it? You betcha.

---

Footnote 1: Michael Fremer currently uses the NHB-458's successor, the NHB-468 ($195,000/pair), as his reference.

Footnote 2: Ours and the bank's.

Footnote 3: Delétraz considers the first commercial version of the model one—dubbed Version A—"preliminary." After its introduction at the New York Audio Show in 2002—the late Wes Phillips wrote the first review—only 20 units were produced before the introduction of Version B, whose rear board could now accommodate balanced inputs, which Delétraz told me he's "not a fan of." Model one, Version B's balanced inputs used transformers, while the model two's balanced inputs use electronic chips that Delétraz claims offer better accuracy and wider bandwidth.

Footnote 4: In a subsequent email, Hervé explained in detail: For total silence, 60cm spacing is recommended, but as long as the amplifier and preamp are on separate shelves, any hum will likely be so low in level as to not be objectionable.