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.
PerformanceThe 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 testThe 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.