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ATI AT522NC Stereo Amplifier Review

amirm

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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:
ATI AT522NC Stereo Class D Amplifier Audio Review.jpg

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:

ATI AT522NC Stereo Class D Amplifier Back Panel Connectors Audio Review.jpg

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.

Amplifier Audio Measurements
As usual, we start with our dashboard of 1 kHz tone, at 5 watt into a 4 ohm load:
ATI AT522NC Stereo Class D Amplifier Audio Measurements.png


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:
Best Stereo Amplifiers Class D Reviewed.png


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:
ATI AT522NC Stereo Class D Amplifier SNR Audio Measurements.png


Frequency response is orderly for a class D amplifier which needs an output filter to truncate switching frequency:
ATI AT522NC Stereo Class D Amplifier Frequency Response Audio Measurements.png


32-tone source trying to simulate "music" shows very low levels of intermodulation distortion:

ATI AT522NC Stereo Class D Amplifier Multitone Audio Measurements.png


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:

ATI AT522NC Stereo Class D Amplifier Crosstalk Audio Measurements.png


Most important data is power into 4 and 8 ohm versus distortion and noise. So let's start with 4 ohm:
ATI AT522NC Stereo Class D Amplifier Power into 4 ohm Audio Measurements.png


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:
ATI AT522NC Stereo Class D Amplifier Power into 8 ohm Audio Measurements.png


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:

ATI AT522NC Stereo Class D Amplifier Power into 4 Ohm from 20 to 20 kHz Audio Measurements.png


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:

ATI AT522NC Stereo Class D Amplifier Spectrum FFT Audio Measurements.png


The noise doesn't scale with power though. Plotting the same at max power with source frequency of 40 Hz gives us this:
ATI AT522NC Stereo Class D Amplifier Spectrum full Power Audio Measurements.png


And intermodulation distortion using 19+20 kHz at 5 watts output into 4 ohm:

ATI AT522NC Stereo Class D Amplifier 19+20 kHz IMD Audio Measurements.png


Conclusions
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, I can recommend the ATI AT522NC stereo amplifier.

--------
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. Please donate if you can using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
 
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RayDunzl

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Curious:

1574272245896.png


Maybe not specific to this amp, but...

I'm used to seeing power harmonics, and test tone harmonics, but not used to seeing spikes in other frequencies.

They are low enough, but where are they coming from? Is it a Class D thing?

Compare to

1574272856656.png
 
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VintageFlanker

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Maybe not specific to this amp, but...

I'm used to seeing power harmonics, and test tone harmonics, but not used to seeing spikes in other frequencies.

They are low enough, but where are they coming from? Is it a Class D thing?
649410281_large_755cfd3499fc78674d218522f47ea775.jpg


1771072-ati-at500nc-at522nc-hypex-nc500-amplifier.jpg


Use of linear power supply defeats the size and weight advantages of common class D amplifiers.

The use of linear power supply has its price in the form of power supply spurious tones (60, 18, 300 Hz, etc.).
 
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confucius_zero

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chassis isn't much of a looker but dat performance seems legit :D
 

FAST

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How many AMPs of current does this unit provide??
 
D

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What happens in the audible range if you feed it a 420khz signal at various power levels? :)
You can go on the air with your own radio station??? :)

Actually, I'm sure you know the answer to your question. This "design oversight" was extensively discussed over on DIYaudio a few years ago.

Dave.
 
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amirm

amirm

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What happens in the audible range if you feed it a 420khz signal at various power levels? :)
Highest frequency I can generate with APx555 is 204 kHz. To go up to 420 kHz I have to use my dedicated function generator which doesn't remotely have the same quality.
 

garbulky

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I know some of the hypex with their switching supplies were only able to sustain their "RMS" figures for very short periods of time and the true figure of continuous output is closer to 40 watts even though they are able to provide short busts for much higher output. Since this one has a torroid can it sustain its power output much longer than the ones with switching before shutting down?
 

blueone

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Amir, you're defining the state of the art when it comes to measuring and reviewing Class D amps. I especially appreciated the FFT of full spectrum noise relative to a 40Hz signal. It puts things into perspective, and I've never seen a similar graph published before. Nicely done.
 
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amirm

amirm

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Since this one has a torroid can it sustain its power output much longer than the ones with switching before shutting down?
The power sweep vs frequency took a while and it did not cause a problem. AVRs for example routinely shut down during that testing.

I am worried about super stressing amplifiers this expensive beyond that kind of testing.
 

audimus

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Amir, you're defining the state of the art when it comes to measuring and reviewing Class D amps. I especially appreciated the FFT of full spectrum noise relative to a 40Hz signal. It puts things into perspective, and I've never seen a similar graph published before. Nicely done.

I have been listening to the testimony in DC, and the above reminded me of what they always say when a witness is confirming their position. :p

Ok, I’ll bite. What is the significance of a full spectrum to 1mhz sweep with a 40hz signal? Why 40hz and what perspective does this provide? I truly don’t know and I would guess most here don’t. I yield the floor to you...
 

audimus

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Is it possible to verify if it has a RF filter on inputs as good practice for amps, in particular for designs that may be affected by it?
 

KMN

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"What is the significance of a full spectrum to 1mhz sweep with a 40hz signal? Why 40hz and what perspective does this provide? " Amir explained his justification of the 40Hz frequency. Perhaps having an idea about signal strength of unwanted (inaudible) signals being sent out onto IO connectors can give you a sense of how easily this piece might be integrated with another piece of gear with susceptibility in that frequency range. For example you might argue it will be difficult to efficiently radiate 20K off a speaker wire. As you start talking frequencies approaching a Meg or more, that is a significantly different story. Without the broadband scan you don't know whatis there or not. Personally I like seeing that info. I just wish there was also an accompanying immunity result, of some sort, to go with it. Obviously this broadband test is not the perfect complete characterization of all things, even raising new questions, but I'd argue this info quite a bit better than no info at all. I hope Amir keeps fine tuning these sorts of tests, personally.
 
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