What's the point of reviewing a pair of monoblock amplifiers that costs more than most people spend on two or even several cars and far more than most audiophiles spend on an entire music system? That's a good question. Another is: Why should I write this review when, just seven years ago, I reviewed a pair of darTZeel monoblocks that looklike this new pair?

I realize that products such as the darTZeel NHB-468 ($170,000/pair) are for the very few, but the very few include far more people throughout the world than you may realize people who can afford such costly audio products and who do buy them. I know, because in my travels I've met a lot of them, and they deserve to read reviews of products they're considering buyingthings most of us can only dream of owning.

What also makes this product worth reading about is that the man who created it admits to being directed more by sound than by measurementswithin the boundaries of competent electronic engineering, of course. That takes him to some unusual, thought-provoking places where the measurements-oriented people often don't go . . .

Description

Last spring, before the High End 2019 show in Munich, I made a side trip to Switzerland to tour darTZeel's factory and machine shop, and to visit the listening room of founder and chief designer Hervé Delétraz. His speakers of choice are Klipschorns: not at all what I was expecting I'm not sure what I did expect from the man behind this quirky brandnor was the sound what I thought I'd hear.

In that system, driven by either the darTZeel NHB-108 Model 2 amplifier or the new 468 monoblocks, those corner horns delivered a steady stream of "I've never before heard that!" revelations from every high-resolution file and vinyl record we played. I came away understanding Delétraz's transducer choice. On the other hand, the obviously bumpy frequency balance, lack of deep bass, and "last century" spatial presentationlike listening through an open windowleft a great deal to be desired compared to what I hear at home. "You can't have everythingeither at home or in the concert hall," I thought while listening at Hervé's, though what these speakers did correctly kept me seated for quite a long time!

Soon after my return to the US, a pair of NHB-468s arrived at my house. They look very similar to the NHB-458 monoblocks, which I've owned and enjoyed since I reviewed them in 2012, and which I had no desire to replace. As I like to say about my similarly quirky 2008 Saab 9-3 Turbo X, "I'll drive this car to the nursing home!" (hopefully no time soon), and I have felt the same way about the 458s. (Unlike Saab, at least darTZeel is still in business!)

Outwardly, the 468 differs from the 458 only in the richer and more gleaming gold finish on its front panelthat and, on its rear panel, a 20A IEC jack that replaces the 458's 15A one. (According to Delétraz, 458 owners can get the guts upgraded and keep the old chassis.) The new amplifier's operating system, feature set, and front-panel controls and display screen remain unchanged from those of the 458I describe these in detail in that 2012 reviewalthough much of what's inside (though not all) has been completely redesigned and is very different. The biggest change to the audio circuitry involves a patent-pending technology/circuit design darTZeel calls CP2C, an acronym for Constant Power to Current. According to Delétraz, he and his design team have invented a "physically contradictory" circuit that "mimics a current source with the output impedance of a voltage source." The circuit design is said to deliver appropriate power to the loudspeaker on demand, with the proper speed and timing, while correcting in real time the loudspeaker drivers' counter electromotive force (back EMF).

The instruction manual avers that the amplifier should not be considered as either a theoretically ideal voltage source or an ideal current source, both of which, in addition to many flaws, have good qualities. To paraphrase the manual's "Franglish," a perfect voltage source has zero output impedance and in practical terms requires 100% negative and global feedback, which produces temporal (phase) distortion. An ideal current source has infinite source impedance, which is good for controlling a speaker's back EMF, but produces poorer measured performance and sound quality that is overly speaker-dependent. The company claims that the CP2C circuit produces an amplifier-to-speaker connection equivalent to the matched impedance linksthe 50 ohm "transmission line" Zeel connectionthat darTZeel uses between their NHB-18S preamp and its amplifiers.

The amp is biased into class-B "as much as possible," allowing for cooler operation, longer component life, and no negative sonic impact, because darTZeel claims faster rise/fall times than with higher-bias designs, suggesting that class-A is only useful in asymmetrical circuits, not push-pull.

The 468 is rated as outputting 475W RMS (550W peak) into 8 ohms, 625W (700W peak) into 4 ohms, and 222W RMS (300W peak) into 2 ohms. That's bound to raise some eyebrows, especially considering the massive power supply, with 750 joules of available energy. Power measurements for its predecessor, the NHB-458, are 530W into 8 ohms, 800W into 4 ohms, and 1025W into 2 ohms. Note that both amps use the same massive power supply, featuring one of the biggest power transformers I've encountered in an audio product, which floats on a suspended platform that's tuned to absorb frequencies between 40Hz and 70Hz.

The 468 retains the 458's zero global negative feedback, fully open-loop output stage, which means no output impedance compensation (Zobel network, etc.), so if you accidentally short the amplifier it's likely to blow up in a cloud of smoke. That's the price paid for purity.

According to Delétraz, the NHB-468 can drive "any speaker and you can go down to 1 ohm and even below (and the amp will still produce great, undistorted music)," and he suggests the amp "was not designed for electric welding but for hysteric goose bumping."

More hysteric goose bumping

In my room, driving the Wilson Audio Specialties Alexx speakers, my darTZeel NHB-458 monoblocks were rarely required to produce more than a few wattsalthough when called on to reproduce some bass-heavy material, and where I wanted it loud, they peaked out at a few hundred watts.

But I prefer having the wattage on reserve for reviewing other speakers. For instance, I was happy to have the added current when I reviewed the Sonus Faber Aidas.

For seven years, the 458s have kept me sonically satisfied. Sure, the recently reviewed CH Precision M1.1 amp, configured as a monoblock pair, definitely produced more bottom-end weight and punch, just as the Ypsilon Hyperion hybrid monoblock amps demonstrated that greater harmonic expression and bloom could be produced in the mids and upper-mids (and being a hybrid design with solid-state output, they were no slouches on bottom). While I wouldn't mind having me some of that, I found more to my liking the rest of what the 458s do.

Then the 468s arrived.

A friend helped me install the new monoblocks, and I ran them with my NHB 18NS preamp for many weeks, using the original darTZeel 50 ohm cables I'd received with the 458s in 2012 (although a new and supposedly improved set of 50 ohm cables came packed with the 468s).