This is a review and detailed measurements of the ATI AT522NC Stereo power amplifier. It is on kind loan from a member. The 522NC costs US $2,195 but I see it on Amazon for US $1,995. The AT522NC is part of a series which starts with 2 channels of class D amplification and goes up to 4 channels in the same enclosure.The industrial design of the AT522NC has some unique touches, some of which work, some that don't:The part that detracts is ironically the ATI logo.The power button has a very nice feel and gives one impression of quality.Looking at the back, I was surprised by choice of input power connection:It is a 20 amp IEC connection rather than the most common, 15 amp variety. This is partially due to use of linear power supply and massive/heavy toroidal transformer. But the most common reason is that the same setup is used in higher channel count amplification.The amplifier is on a vertically mounted hypex NC-500 class D amplifiers with their own heatsinks. The "motherboard" has the power supply components. Overall, it seems like a very neat and logical design.The speaker connections are of course very secure and high quality as can be expected in this price range. Great to see balanced XLR inputs which is how I tested the unit.In use, there was barely a heat rise in the enclosure and the unit never shut down or complain even in the face of severe clipping. The large feet allow better convection cooling which is appreciated.As usual, we start with our dashboard of 1 kHz tone, at 5 watt into a 4 ohm load:The use of linear power supply has its price in the form of power supply spurious tones (60, 18, 300 Hz, etc.). The dominant distortion peak is third harmonic at a bit less than 100 dB. Add some noise to it and SINAD (THD+N inverted) becomes around 98 dB. This places the ATI522NC well above average of all the amplifiers we have tested:It sits just to the level of NC500 from Nord. Those of you who keep track of such things, I am sure will compare and contrast how it does relative to other Hypex based implementations.Signal to noise ratio is good reaching excellence at full power:Frequency response is orderly for a class D amplifier which needs an output filter to truncate switching frequency:32-tone source trying to simulate "music" shows very low levels of intermodulation distortion:But for a peak at 60 Hz (typo in the graph), the distortion-free range is 110 dB or about 18 bits.Crosstalk is also very good:Most important data is power into 4 and 8 ohm versus distortion and noise. So let's start with 4 ohm:So not class leading but low distortion while providing 300 or so watts before sudden rise in distortion. Spec is higher but one can have more data points at the limit and gradually walk up the curve, and hence squeeze out a few more watts. I will show this later.For 4 ohm load, here is the comparison to our state-of-the-art in class D amplification in the form of Purifi reference design:Purifi maintains the lead in distortion but produces a bit less power.I was able to run my tough test of THD+N at 0.1% versus frequencies. Some amplifiers have died trying to run this test. Others are had to poinpoint as their distortion jumps all over the place:We see that at 1 kHz we normally measure, power is almost near max. Above that we reach nearly 360 watts. At 20 kHz though, distortion shoots way up and we are able to only produce 22 watts before THD+N of 0.1% is hit. Fortunately there is not much music energy there so practical audible concern is low.I made a survey of a few of the tracks in my music library. Peak energy seems to hover around 40 Hz. For that reason, I have started to measure amount of power at 40 Hz and here, we have 316 watts which is good.Finishing off, switching frequency is around 400 kHz:The noise doesn't scale with power though. Plotting the same at max power with source frequency of 40 Hz gives us this:And intermodulation distortion using 19+20 kHz at 5 watts output into 4 ohm:The ATI AT522NC seems like a solidly built amplifier based on Hypex NC500 modules. ATI builds amplifiers for many other companies and this competence shows up in this review. Performance is of course limited by what the NC500 module can do. Use of linear power supply defeats the size and weight advantages of common class D amplifiers. It may provide better peak power rating -- something I wanted to test but forgot.Overall,--------As always, questions, comments, corrections, etc. are welcome.Watching important news on TV as I type this so no positive energy to think of a lame joke. Pleaseusing: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/