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Several conditions will trigger the monitoring circuit:
  • Mismatched impedance
  • Short circuit at the speaker terminals
  • DC output voltage drift greater than two volts
  • Powering up the amp without a connected load

The output stages are protected by a smart current-sensing system outside the signal path using Hall Effect electromagnetic sensors with magnetic coupling. This circuit allows for essentially unlimited peak currents while providing safety in the case of a catastrophic event - a simple but expensive solution.

Setup is relatively straightforward. How complex can it get for connection of a solid-state amp? One unique feature is a three-way input selector on the rear that sets for RCA, balanced or the unique 50Ω BNC Zeel input. DarTZeel claims this input will produce the mot accurate sound and will be incorporated as an output option of their new pre-amp. The balanced inputs are truly balanced via floating transformers.

Although not complex, the amp probably has a few idiosyncratic tendencies above average. Even in the world of today's esoteric pieces, most amps are more or less plug & play. With the power off, connect the speaker and preamp cables, select the input and you're pretty much done. A couple of extra considerations for the 108: There are two internal impedance settings [below]. Factory default is 3Ω to 8Ω. A low impedance setting accommodates speakers less than 3Ω.

Unlike most conventional designs, the 108 uses only one pair of bipolar output transistors per channel. Most will use dozens of output transistors, which paralleled will yield high output current into low impedance loads. According to Hervé, such massively paralleled output devices become a sonic liability causing, among other sins, temporal distortion " degradation of propagation time delay uniformity, each path not being of identical length to the others." The impedance switch selects the output windings of the power transformer in parallel or series, delivering high voltage or high current depending on switch position yet keeping the output transistors in their safe operating range.

DarTZeel claims 90% of the "musical magic" after five minutes of
playing (the amp never shuts off totally unless it is unplugged). The 108 is powered by two volts all of the time to extend the life of internal components, especially the capacitors. It also allows the amp to warm up in just five minutes after defeating what is really standby. Another factor is that of sustained bias power. A given component, being active or passive, lasts longer if some bias voltage, however low, remains applied to its terminals. For electrolytic capacitors, this small polarizing voltage keeps their internal electrolytes in perfect chemical working condition. An interesting side effect of this "sustaining bias power" is that the amp will still quietly play even when you turn the power off, an indication of the audio circuit's simplicity according to darTZeel.

While most are familiar with RCA and balanced inputs, how about the firm's 50Ω BNC connection? Hervé's research concluded that "...the one and only means for transmitting an electrical musical signal with no alterations and losses over a long distance is with impedance-matched lines from end to end." Coaxial cable impedance matching is used in applications like radars, microwaves and computers to eliminate loss regardless of cable length. With no 50Ω output source in my audio arsenal, this feature was a merely academic curiosity for me.

There are three internal jumper options with three small jumper sleeves for each that connect to two of three metal posts. These in effect act as a single-pole double-throw switch. Two jumpers affect the grounding and are used to eliminate hum. Since I had no problems, these remained in factory default mode. The third one does have an audible effect. Quick history recap: the original design by darTZeel eschewed any type of monitoring or control circuit including DC compensation. The original circuit demanded that if excess DC at the output was detected, the user would have to manually trim out DC with a multi-meter. The problem from a practical standpoint was that certain users would resist sticking a screwdriver into the innards of a live amp especially with the following warning from the manual: "Do not forget that the amplifier will be powered on! Never touch the copper bus bars, which are at a potential difference of 115 Volts DC. A short circuit induced by the screwdriver will partially evaporate the blade by instantaneous melting of the metal of the latter!"

For the electrophobes or generally cautious among us, darTZeel (in the B version of the amp) installed an automatic DC compensation circuit which can be switched in or out by the placement of that third jumper. Being bold and fearless, I set the jumper to "off", turned the amp on and waited for the eyes to flicker to warn of excess DC at the speaker terminals. There was no flicker. This setting reduced the noise floor for blacker backgrounds and slightly improved micro detail. All of my listening comments reflect the DC compensation circuit defeated. If required, the trimming procedure is really not that difficult and similar to my Walker Reference phono preamp. Like Delétraz, Walker believes in the deleterious side effects of DC compensation circuits and avoids them like the plague. Overall, this is really no more complex than adjusting the bias on a tube amp. No big deal.

The NHB-108 was in good company in my reference system. Speakers are the incomparable Wilson Alexandrias, preamp the VTL Reference 7.5, with the Walker Proscenium Gold and Reference phono preamp as source. Cabling includes Transparent Opus, Omega Mikro and Silent Source. A Furman Balanced Power feeds the Walker Velocitor S for all line-level equipment.

Warning - danger, Will Robinson
The NHB-108 has a few rules. Mess with them and the crowbar circuit will terminate the internal fuses with extreme prejudice. Given the design choice of convenience or sonics, Hervé chose sonics. Not that the amp is difficult to use but it's not a Sony receiver either.

  • Rules:
  • Never turn on the amp without speakers connected
  • Never short the terminals
  • Don't switch the input selector with the power on
  • Don't disconnect the input source with the power on
  • Don't connect low impedance speaker without first selecting the internal switch

Slip up and the sensing circuit will, in the words of darTZeel, "vaporize the fuse". That's infinitely better than vaporizing your tweeter. The fuse is located next to the transformers. Fuse changes require use of the supplied needle nose pliers. It's not brain surgery but certainly not as convenient as rear-mounted fuses on other amps. Eventually you will slip and have to replace fuses. Big deal? Not - but roaming inside an amp might not be your cuppa tea.

The 108 needs break-in. Let me rephrase that. It requires a metric ton of break-in. My break-in procedure is very simple. Place product in the equipment chain, listen for a short time, leave it running and leave for days. Come back, listen and repeat the process until the sound stops changing. My unit was not new. It was a review sample forwarded from another industry reviewer. For this piece, it took 2.5 weeks of continuous playing in addition to whatever the previous reviewer had put on it to arrive at sonic stability. It would have been an unforgivable mistake to evaluate this amp before then.

Some components reveal their true character right out of the box and offer incremental refinements over time. Other components change character. The darTZeel belongs to the latter group. The amp starts off sounding like a nice little tube amp - sweet, fairly dimensional, with a polite bottom end - nothing to get exited about. Soundstaging at that point is average for a high-end amp - good but not world class. A few hundred hours into the process? Zowie! The authority and majesty of the low end explodes out of nowhere. What started off polite ends up powerful. A previously slightly recessed soundstage now blooms out into the room. The amp transforms from "really nice" to "holy shit!" Let's get past the break-in debate nonsense of some who claim the amp does not change but it's actually the listener's growing familiarity and adjustment to the sound. No doubt about it, this amp changed big time. Listen to the opening drum whacks of Jennifer Warnes' Famous Blue Raincoat [Rock the House Records] out of the box and then thirty days later.

A stranger off the street raised on Bose™ could hear the difference in one second. It's that real. From warm, sweet and polite to ballsy and three-dimensional in 30 days à la Jules Verne. I hear your questions. "Is it right for an amp costing almost $20,000 to require a month's continuous break-in to perform to potential?" Frankly, I don't care about the break-in time. For those who are bothered by this phenomenon, I respect your opinion. You could argue that for this money, one should demand close-to-optimal performance out of the box. It's a respectable argument. But since I have never owned a high-end product that performed without break-in yet, I am less offended than most. To me it's part of the game. In a strange way, listening to a product that sounds good out of the box and gets better and better as time goes on is perversely fun.

Listening experience
The NBH-108 is a tube amp clothed in transistor circuitry. I have never heard a solid-state amp with this combination of warmth and harmonic richness. It is the polar opposite of the cool analytical Halcro sound, for example. I'm not saying that one is right or wrong. At this level, personal preference plays a huge role.

Clarity, no congestion
Throughout my many review sessions, I continually noted three points: simplicity, clarity and harmonic richness. The 108 has the ability to take the most complex and congested music and present it in a beautiful, rich, musical and simplified way. Carol King's Tapestry [Classic Records] is filled with an immediate connection to her voice. This amp is able to unravel complexity and present a startling picture. This clarity is not the same as detail. Although I love detail in the music, never at the expense of emotional presentation. The imaging here is spectacular. You can reach out and touch the performers but again without excessive analytical detail. You get focus and clarity without the slightest hint of grain, hardness or stridency. Dull? No way. DarTZeel offers the compelling immediacy of low-power tube amps with the speed and PraT of solid state. This is a solid-state tube amp - warmth without bloat. The amp is spectacular in how it reveals the harmonic structure of instruments. It combines solid-state attack and tube decay, allowing you to hear/feel the essential emotion of the music.

Some amps tend to get forward, hard and congested when pushed. Not the d'Arts. Everything remains in place at all levels - subtle micro-details of the best SETs with the power of transistors. Add the harmonic richness of tubes and holy Christmas - this brings music to life. With Janis Ian on Breaking Silence [Analog Productions], I don't know what is more interesting - the warm and melodic voice or the presentation of the complete and rhythmic background presented with a new simplicity and clarity. It's possibly a touch too heavy in the bass, which I never thought I'd say with my current room and acoustics.

Having played the high-end game for many years with a high-resolution system, I understand the phenomenon of spotlighting. Detail becomes the end all and be all, each instrument stands out individually and unnaturally as though cut and pasted into the acoustic picture. The coherent waveform of the entire sound plus the ability to focus on specific sounds is to me one of the key distinguishing elements of the live music event versus reproduction. The darTZeel brings reproduced music a step closer to that natural coherence. The musical waveform emanating from the speaker forms an extraordinarily coherent sound that naturally blooms into the room. Yet with that continuity, the individual instruments are still there for your sonic viewing pleasure. Some amps give you spectacular individual notes but no emotion. With the dartz you get both.

Quincy Jones' Hip Hits [Mercury] is 60's fluff pop set to big band jazz sound. The arrangements span from serious and sophisticated to fanciful, with Quincy having obvious fun in a multi-miked hi-fi kind of way. Interestingly, the darTZeel's coherence brings it all together. I've seen the effect before with tubes - a wonderful midrange and dimensionally articulate soundstage with great harmonics. Unfortunately, valves sometimes dulled the leading edge of transients, softening the brass to, contrary to its natural sound, become transformed to warm and melodic. The darTZeel doesn't blunt transients nor glazes things over. As your listening mind begins to wander down the warm beautiful tubey path, the ferocious blat of trumpets cuts through the room in startling relief to the luscious sound for the best of both worlds.