Military intelligence. Jumbo shrimp. Microsoft works. These are called oxymorons, words that seem to contradict each other, put together for the amusement of those who know better. Many audio enthusiasts might even consider "Audiophile Full-Spectrum Class D" an oxymoron; until now, that is. The engineering team at Xtant, world famous for doing things differently, has dared to introduce the first full-range Class D designed for sound quality in the new Xtant 1.1i Mono Amplifier.
Xtant rates the output power of the 1.1i at 100 watts into 4 ohms and 100 watts into 2 ohms. That's not a typo, and it grabbed my attention, too. Current limiting to keep the amplifier operating within its optimum range? Perhaps. Rail switching, like some other Xtant models? Maybe. Xtant also claims a frequency response of 17Hz to 20kHz, +2dB. Seems a bit ambitious for a Class D.
Cosmetics
As I mentioned, the 1.1i is a 100-watt mono amplifier, so I received a pair of them for testing. They arrived in boxes seeming to contain more air than amplifier. Of course you probably think I am kidding, yet at 6.5"L x 5.81"W x 1.63"H (about the size of a 20-disc CD wallet), these amps are unbelievably small. However small, though, these guys are surprisingly weighty.
On one side of the amplifier are three fins, part of the black, extruded aluminum heatsink. Atop the black heatsink is a polished stainless steel cover that immediately proclaims, "I am Xtant!" The power, speaker and input connections are all on one side, accompanied by the gain control and an electric blue power indicator LED. It is clean, classic and unapologetic in appearance. When you take the cover off to gain access to the mounting screw holes (wrenches and screws provided), the "Black Box" mystery continues inside, where Xtant purposely removed part number markings on the ICs.
Design and Layout
All the connections are on the front of this amplifier and the flow from the power side to the signal side makes good sense. Battery power enters through terminal blocks large enough for 4awg cable on the right-hand side of the panel and runs straight back to the primary caps (3000uF total) and on to the switching devices mounted to the heatsink. The transformer is a little forward and to the right, while the rectifiers and thermal sensor occupy the center area of the heatsink. Signal input is front and to the left of center, adjacent to the speaker outputs, which are capable of handling 8-gauge cable. The output FETs are on the left side of the heatsink and feed the output filter directly forward and on to the output terminals.
Xtant uses little standard circuitry in the 1.1i. The input section isolates the ground (or shield) side sufficiently to treat it as the negative side of a balanced line input. The negative side ultimately runs into a differential amplifier (op amp). Even if you don't have a balanced line source unit there are at least three advantages to using it with standard RCAs. First, the signal clarity and definition is much greater. Second, most of the noise that can be radiated into your signal cables will be eliminated through common mode rejection. Third, you can connect virtually any head unit directly to the amplifier.
Nearly all of the components are surface mount, even some of the large resistors, lending itself to tighter tolerances. The board itself is double sided, and there are no jumpers or buss bars present. All of the components are on one side of the board, and everything is tight and neatly laid out. We applaud Xtant for using 105-degree C rated capacitors in both the primary and secondary portions of the power supply rather than the more common 85-degree C caps that, because of heat, degrade more quickly over time.
The power supply in the Xtant 1.1i is self-oscillating, switching at 1.06MHz. This is one of the keys to the extended frequency range for any class D amplifier. To have a high frequency range the switching frequency must be at least twice the highest frequency that you want to reproduce. Additionally, you have to have a low-pass filter on the output to get rid of the switching frequency (noise) itself. If you want frequency response out to 20kHz, and the switching frequency is 40kHz, you now have to come up with a sharp enough filter (crossover if you prefer) to reduce the 40kHz noise sufficiently in a single octave. To have a signal (20kHz) to noise (40kHz) ratio of 100dB, you would need a 100dB/octave low-pass crossover, and it has to be passive! Obviously this is not going to work at 40kHz, because a practical passive filter will only attenuate 6-, 12-, 18-, or 24dB (that's really pushing it). In fact, we want to use a lower slope if possible to preserve the sound quality. So we need more octaves, and therefore a higher switching frequency so that we can use a passive output filter to eliminate the noise. As an example: using a 6dB/octave filter, a switching frequency of 1.28MHz would result in a mere 60dB attenuation from a 20kHz signal.
"But wait," you say, "the 1.1i shows a 105dB signal-to-noise ratio and response out to 34kHz!" They're obviously using a higher slope for their output filter, but, alas, there's more going on here than they want known at this point. (They'd have to kill me if I told you.)
The output section uses two N-channel Phillips devices rated at 150-watts dissipation each, more than adequate per this amplifier's rating. Looking at the layout, it is notable that this is a true mono amplifier. This is not your typical mono amplifier that consists of two individual channels that have been bridged internally, but with only a single channel output. And, these amplifiers are designed to be used on your full-range speakers.
Performance
The frequency response was an astounding 3Hz to 34kHz at -1dB, better than many class A/B amplifiers and far greater than any class D I've come across. How did Xyant do this? I can see why they painted out the marks on all of the ICs inside! Power was higher than rated, even at 12.5V, and did in fact increase substantially from 4 ohms to 2 ohms. Apparently the current limiting isn't too restrictive, but seriously, wouldn't you really rather have the extra power? The owner's manual indicates a regulated power supply, but again it's not very tight as the power definitely increases from 12.5VDC to 14.4VDC.
Very curious was the relationship between the THD and SNR (signal-to-noise ratio). With a THD of 0.7 percent, I would not have expected the SNR to measure much over 85dB, and yet it was actually over 105dB (double checked and verified by another lab nearby). The Output Power to THD was not the typical gentle rise towards maximum power, then rising sharply as the amplifier begins to clip. Instead it rises to about 75 percent power, then drops off before rising sharply at clipping, never exceeding 0.7 percent throughout its range.
The thermal protection circuit consists of progressively reducing the power until the amplifier begins cooling down. My tests showed two steps of power reduction and then eventual shutdown under continuous extreme thermal stress. I am a big fan of the power-reduction format because the music virtually never stops. Chances are very good that in a system your amplifier will recover from heat build-up without you ever noticing the power reduction. And on those long trips nobody wants their music shutting off!
The over-current protection works in the same manner -- that is, the power is reduced if the amplifier is playing into a lower than allowed impedance or short circuit.
The owner's manual does a great job of taking you step-by step through the installation. The "Connections and Controls" page makes use of small black and white illustrations that are hard to distinguish, but with only four items to contend with you could always look at the amplifier itself. Xtant offers a four-year materials and workmanship warranty when the unit is installed by an authorized dealer, otherwise it's one year. As always, keep your sales receipt, and Xtant offers on-line registration to keep things easier if you ever need warranty service.
Listening test
The two amplifiers were first installed in my car to power the front separates that utilize prototype passive crossovers. I am happy to announce that there was no noticeable engine noise, so I went straight for the volume knob for some fun. The big item of concern was the high frequency range, which is the weak link for a Class D. I have to admit to being a little dismayed at finding no issues here, so I moved on to imaging, tonal balance and the like.
After the in-car audition, the Xtant duo was connected to a pair of Clements Audio prototypes in the Redrock theater room. This round was geared for slightly more serious listening. My initial instinct again was to audition the high frequencies. As in the car, I was met with exceptional performance. One item to note: there is a very slight turn-on pop due to a minute DC offset. However, this pop was detectable only in the Redrock theater room, not in the car.
The following is a cumulation of both in-car and in-theater listening experiences: from my trusty Car Audio Nationals competition CD (from the dawn of car stereo civilization), I listened to Jennifer Warnes' "Bird on a Wire." The vocals, which can sound thin and out of phase on some systems, sounded full and rich, and the triangle could be heard clearly. The fanfare's dynamic range and the snare hits sounded as real as I've ever heard.
Robben Ford's "Talk To Your Daughter" and "Eyes Of Love" by Michael Ruff have the ability to reveal low frequency equalization or imbalance in an amplifier's performance, but both tracks were pleasantly uncolored when played through the Xtant 1.1is.
Finally, I consider "Tomorrow's Girls" from Donald Fagen's Kamakiriad to contain some of the most well played and recorded cymbal work of all time. Each tap and slip was clearly evident, crisp and smooth in all the right places. When pushed too far, though, there was a distinct graininess affecting the upper ranges. With a typical class A/B amp you notice a muddiness in the lower mid-range section of this track, revealing that the clipping signature is very different on this amplifier.
Conclusion
The Xtant 1.1i met or exceeded all of the claims in the owner's manual. The pair of amplifiers reproduced all of the tracks as well or better than most 12V amplifiers that I have heard, regardless of whether they're Class A/B, D, H, or anything else. With a suggested retail price of $350, the dollar-to-watts ratio comes to $1.57 at 2 ohms, which is a bit pricey considering the absence of frivolous bells and whistles. But the Xtant 1.1i was built to run with the best, so don't buy these amplifiers to run cheap or mid-fi speakers in a noisy car, 'cause you won't do them justice.
In my mind, here's what you get for your money: a true Audiophile Full-Spectrum Class D amplifier that is a perfect match for top-shelf, full-range speakers. Add a big subwoofer and some sound deadening and you're good to go. The Xtant1.1i is small enough to mount anywhere, including under your seat, in the glove compartment, into a custom dash, or even behind your speakers in many cases, and is up to 80 percent power efficient.
Even with its diminutive frame, the Xtant1.1i is a mighty mite with the heart and soul of a giant. Maybe jumbo shrimp isn't an oxymoron after all.