This review page is supported in part by the sponsors whose ad is displayed above
However according to Hervé, this universally accepted ratio of ten ignores the inherent electrical properties of the interconnect cable. Let's look at a simplified diagram of an electrical circuit with a low impedance source and a high impedance receptor. (Figure 1)
Nice in theory but the real-world electrical equivalent (Figure 2) of the simplified circuit is far more complex.
Every cable has intrinsic parasitic inductance and capacitance in addition to internal resistance, giving it intrinsic impedance. The other important factor is that energy is not flowing continually through the line. Energy is stored and then released in steps, based on the nature of the dielectric where losses can occur. Interestingly, the intrinsic bandwidth of the typical audio cable can extend to several megahertz (MHz) which should in theory minimize any problems. But if all cables can handle these high frequencies, why do we care about matching impedance provided that the input-output ratio exceeds ten? According to Hervé, "...the intrinsic bandwidth is key here. When the cable behaves as a transmission line, it has practically unlimited bandwidth only when correctly matched in impedance."


Most audiophiles are familiar with the 75Ω S/PDIF cables used between CD transports and D/A converters. The referenced 75 ohms is the cable's intrinsic impedance. In the digital world, impedance matching is crucial as frequencies extend into the MHz range. Even small errors create data problems. An impedance mismatch will cause signal echoes which smear the data. Since not all data cables are manufactured to the same tolerance, not all data cables sound the same. According to Hervé, what you are hearing is data error caused by impedance mismatch. Logically, if impedance matching is so important in the digital domain, why not analog? With a frequency range that is up to 1,000 times lower and due to the fact that analog, unlike digital, is not transmitting discrete values, the resultant echoes due to impedance mismatch might not be audible. For those interested, any cable's intrinsic impedance can be calculated from the simple equation below.
In reality, very few cable manufacturers give out their capacitance and inductance values per meter. So for your particular brand, calculations might not be easy. However, even without the capacitance and inductance values, it is possible to directly calculate the cable impedance using the conductor radius, shield radius and type of dielectric. This leads to the intrinsic impedance of some typical cable shown in the following chart.


We are now at the core of the issue as to why virtually all audio cables distort the sound. "When the impedance is mismatched, a part of the outgoing signal from the source is reflected by the receptor." Mathematically, the reflection ratio is represented by the following equation:
Typically 75ohm cables are manufactured to a +/- 2Ω variance. Using the above equation, the reflection factor calculates to approximately 1.35%. In the analog realm, severely mismatched components and cables will yield reflective ratios close to 1.0 meaning that virtually 100% of the signal is reflected. Ironically, this is the norm yet no one seems to be complaining. This is how cables become tone controls between the components.


Figure 5 shows an example of a completely matched system source, cable and load, all of which have an impedance of 75 ohms. The top and bottom traces are virtually identical. The bottom trace is slightly delayed due to the propagation time of the cable. The roughness visible in the traces is due to the oscilloscope and the D/A converter, not cable-induced noise.
At the other end of the spectrum, Figure 6 shows the receptor having much higher impedance than the source, producing virtually 100% reflection. Also, the signal at the receptor is dramatically altered with the rising plateau top and slowed rise time.
Now let's look at a current audio example of a 150 ohms source, a 10k receptor and a 75 ohm cable (Figure 7). The initial pulse is distorted at the receptor with a noticeably slower rise time and rising plateau. Additionally, the reflection back to the source is so severe that it causes a subsequent reflection in the receptor and then again back to the source and to the load – on and on.
Now let's take a look at a 10kHz audio signal in a perfectly matched system (Figure 8) – it's a perfect match.
Figure 9 is a real-world audio signal: 250 ohms impedance, 10k receptor through 100 meters of cable.
The slow rise time and rounded corner of the pulse's leading edge indicate what is pretty standard audio theory: that parasitic capacitance in the cable acts as high frequency filter. Except Hervé asks "what if the rounded leading edge of the pulse is not due to reduced bandwidth but in reality multiple echoes induced by mismatched impedance?" Let's look at Figure 10. What was assumed to be high frequency attenuation is in fact a super imposition of multiple echoes spaced apart by 500ns, which correspond to the propagation time of the cable. As each echo is delayed by the propagation time they add together forming steps..." While it is true that shorter cables would lessen the effect, the key word is 'lessen', not 'eliminate'.
(Data and graphs reproduced with permission courtesy of Stereophile magazine. © 2001 Primedia Corporation. Originally published in an article entitled "Reflections, Echoes, and Impedances" in the November 2001 issue of Stereophile)

Hervé's experiments concluded that mismatched components and cables change and distort the sound. While at audio frequencies this distortion might be low, it is distortion and degradation nonetheless. If there is a better solution, why induce distortion to begin with? Enter the Zeel connection. Precisely matched inputs, outputs and cables offer pure distortion-free signal transfer. As we move from theory to application, the first question is whether it works. Is there a difference between a Zeel connection and a high quality interconnect and could the relatively inexpensive 50-ohm Swiss cable be a giant killer? Well, the sound through the Zeel connections and cables closely matches the theoretical predictions of Hervé. At the low end, the bass seems slightly less forceful through the 50-ohm cable but far more delineated and focused. The other end offers an even more dramatic change. The extension, detail and purity of the highs through the Zeel connection yields a far clearer and cleaner overall presentation.
Donald Fagan's Morph the Cat [Reprise Records] is a superb reference disc. Although I find the music to be highly derivative of his earlier work, sonically this disc soars. Dynamics, deep clean bass, extended highs and vocals - it's everything that you need to give your system a test drive and the Zeel connections delivered it all. A delicate chime on side 4 is clearly present, with just a wonderful presence and decay. Yet with the VTL, it was far more deeply buried in the mix. When the 18NS and the 108 amp are connected with the balanced Transparent cables, the overall sound of the DarTZeel system sounds -- that is the amp and preamp -- slightly more "traditional" than with the Zeel connections. I cannot overstate this. If there never was a Zeel connection, I could be happy with the 18 forever using traditional interconnect. The Zeel connection simply raises the bar with a touch of extra purity, clarity and definition.
Drum roll please: The darTZeel NHB-18NS vs. the VTL 7.5
My source is of course the Walker Proscenium Gold turntable recently upgraded to Black Diamond status. The signal starts with the Clearaudio Goldfinger cartridge through Walker's custom pickup arm wiring on to the Walker phono reference preamp. I bounced between Walker's Silent Source interconnects and Brian Kyles' Xtreme Cables from the phono preamp to the darTZeel. The combo is certainly among the top source components around. With this high definition source material, the sonic comparisons between the darTZeel and the VTL 7.5 were actually rather easy. At first I wondered whether two such exalted pieces might have similar sonic footprints, making the comparison difficult. Boy was I wrong. Any non-audiophile raised on Bose would have no difficulty distinguishing between the two.


Having lived with the VTL for several years, I know its sound intimately. The ability to switch back and forth between the VTL and darTZeel provided a fascinating comparison. The VTL's soundstage blooms farther out into the room. The 18 exhibits a slightly smaller 3-D soundstage but improves upon the clarity within that soundstage. When listening in the nearfield, the VTL immerses you into the music whereas the darTZeel presents the music to you. The VTL has slightly better macrodynamics, the darT the better microdynamics. The darTZeel extends moderately in front of the speakers, approximately 50% of the forwardness of the VTL, but extends very deep behind the speakers, farther than the VTL. Both preamps present a wide soundstage, with the VTL's ever so slightly broader. The VTL is warmer, with a subtle mid-bass emphasis compared to the 18. Here it gets tricky: while the VTL has apparently deeper bass with more visceral slam, the darT is more defined, tighter and has a clear edge in clarity and air. Although the VTL sound appears deeper, rounder and fuller, I refer you to Hervé's caution in comparing the 18's bass response. Do not get tricked into an initial impression that the 18 lacks in low-end. It doesn't.

The crucial midrange in each preamp is smooth, neutral and emotionally satisfying. Again these preamps sound different. The darTZeel wins slightly with its extended highs and that special air. The VTL has a tube-like emotion to the sound while the 18 offers unbelievable resolution. The VTL has a full-featured audio control center while the darTZeel is - well, less endowed. That is assuming six inputs, an integrated phono preamp, recording outputs and three types of primary outputs qualify as minimalist. Do not take these comparative comments out of context. Just because the darTZeel resolves like no other preamp does not mean that the VTL is lacking. And just because the VTL has a luscious 3-D soundstage does not mean that the darTZeel is lacking. Unless you had the ability to switch back and forth between these two in an ultra-high resolution system, you might not draw any of these conclusions. If you are looking for the final pronouncement as to the superior unit, you're not going to get to it from me. At this stratum, both are world-class contenders. When I listen to the darTZeel for an extended period, I am positive it is the one that I want and could live with for the rest of my life. Switching back to the VTL a week later and it could be my long term reference preamp. If you are looking for a definitive verdict, I can tell you that the darTZeel does things especially in the area of resolution and definition combined with musicality that I've never heard before in any component – ever.


Summary
In my review of the 108 amp, I described the sound as a superb mix of tube and solid-state, probably leaning slightly more toward tube than that of solid-state. It presents music with slightly more tube-like liquidity than even my VTL S400. Interestingly, the 18NS, which has virtually the same circuit design as the 108, would not be mistaken for tube preamp. It really has no attributable sound. It's pretty close to the sound of the Placette Passive Preamp in neutrality but has far more slam, dynamics and that exciting sense of aliveness. Again don't read more into these comparisons than intended. I'm not suggesting that the Placette -- a great passive preamp -- is in the same sonic ballpark as the darTZeel. It's not. However, both are extraordinarily neutral.


In the end, if you're going to plunk down $20,000 for a preamp, you better listen to it. If one is not available locally, get on a plane and find one. When you've reached this level of excellence, you move into the arena of personal taste. If you choose to ignore this advice and your system has the resolving power, you cannot go wrong with the 18NS. In fact, if you don't need complex control functionality, you'll be in heaven with the darTZeel. I bet that for many, it will be a lifetime purchase. It is that rarest of breeds, a revolutionary state-of-the-art preamp that breaks new ground with its design, execution and performance. If you're in the market for a state-of-the-art preamp or just want to hear the results of one man's lifelong passion and brilliance, track one down and listen to it. Highly recommended. Interview on the next page.