ATI AT4002 Class AB and AT522NC Class D Amplifier Review Shootout

ATI is a well-known and highly-regarded amplifier manufacturer building amps under their own brand and for other manufacturers that include Lexicon, Outlaw, SAE, Theta, and most recently, Monoprice.

ATI now has two completely different technology amps occupying the 200-Watt sweet spot. The AT4000 class A/B and the AT522NC Class-D amplifiers, both with surprisingly similar specifications. The AT52x series is 25% cheaper and up to 50% lighter than the AT4000’s.

The AT4002 (bottom and AT522NC (top).

The AT4000 is the slightly lower powered (200 versus 300 WPC) version in their top-of-the-line Signature Series amplifiers. In reality, the difference is about 1 dB. Like the AT6000, it is a fully-balanced class A/B design using two toroidal transformers and independent secondary windings to provide separate power supplies for each channel. The AT4000 amps can be configured from 2 to 7 channels. The 2-channel model is a dual-mono configuration.

The AT522NC is ATI’s first Class-D amplifier that uses a Hypex N-Core NC-500 module for each channel that can be configured with 2 to 8 channels, which is very appealing for folks moving to Atmos and DTS-X 3D configurations. Small, lighter and with equal power -- what’s not to like?

So here are two wildly different ATI built amps with very similar specs… how do they compare? Let’s find out.

Design

ATI amplifiers are all about performance, build quality, and utility. These do not have the design aesthetics for showcasing with dedicated stands and cable evaluators.

The AT4000 looks like a stouter version of the AT6000 sporting the same flat aluminum faceplate, handles, and 5-way binding posts but with a single power cord and power breaker. All the markings are present including the “Signature Pure Balanced Amplifier” and signed “M” for ATI’s designer and founder, Morris Kessler. The AT4000 is a fully-balanced Class-A/B amplifier with linear power supply. The Signature series represents ATI’s top-of-the-line performance that includes extremely low distortion and very low (>128dB) noise floor. The front power button/ light is an elegant touch-capacitive button. The ATI logo backlit can be touch-toggled on and off.

AT4002 Class A/B Dual Mono Amplifier

The AT500NC is an entirely different Hypex N-Core amplifier that uses ATI’s chiseled faceplate, 5-way binding posts (though not as nice as the AT4000), power cord, and fuse. The cosmetics match the announced ATI ATP-16 Home theater processor.

ATI has been using breakers for years, so the presence of a fuse was unexpected. Consulting the manual, it is understandable since different module counts require different fuses. There is a chrome-faced ATI logo and dual power button and light. When in stand-by, the power button light very-slowly pulses from off to dim. It’s nice that we live in a time where such things can be done, but whether they should be done is worth considering.

AT522NC Hypex N-Core 2 Channel Amplifier

Both amps include balanced and single-ended connections controlled by a toggle that can be simultaneously connected, a useful feature for connecting a separate 2-channel system. Outputs are numbered, and I recommend using the outer channels for the mains (for optimal heat distribution). An optional ground thumb screw is provided as well as a single trigger input. STANDBY and PEAK indicators are located on the front panel. All amplifiers should include peak/clip indicators.

EDIITORIAL NOTE ON AT500NC Auto-Power Off
My AT522NC auto-power off features was not functioning properly. Whether manually or trigger power-on, the amp powers off after 10 minutes. Apparently, an EU feature gone awry. Thankfully, ATI has a procedure that involves pressing the power button while plugging-in the power cable that disables the inactivity power-off function. Once disabled, the manual and trigger power-on works properly.

Soft-startup avoids overloading the circuits during power up. It takes a few seconds for sound to emerge while the amplifier completes the power-up process.

Both amps are standard-sized, about 17” wide, but that is where the similarities end. The AT522NC is 2 inches shorter and only 11 inches deep, which is approximately six inches less than the AT4002.  Weighing in at 39 lbs., the AT522NC is not one of those one-hand-carry class-D amplifiers. It has a linear, transformer-based power supply. Still, it is considerably lighter than the 51 pound AT4002. That AT500NC weight advantage increases with the channel count.

AT522NC Class D (top) and AT4002 (bottom) Class AB rear panels

Listening Tests

Single-Blind Testing

Stacking the gear provided the opportunity to perform some single-blind testing (SBT). Double-blind testing is the gold standard but SBT can be done “at home”. A friend and I took turns and, while not perfect, we both were able to pick the amps consistently. Preference is another matter, that varied with source material. With the M20’s, we were both taken with the AT522NC, giving it the nod.

Game Room System

I began testing with my game room system where the amps were conveniently stacked for quick-switching. FLAC files were played via Roon playing to the Oppo UDP-205 connected via balanced inputs to the Oppo HA-1. The HA-1 provides fully balanced preamplification. The speakers are a pair of Revel M20 bookshelf speakers. The M20’s dip to 4 ohms between 2 and 2kHz so it is best driven by an amplifier that can handle these loads.

Main System

The final listening tests were performed on my main system using Roon playing to the Oppo UPD-205 DAC. Balanced connections were used with the Emotiva XMC-1 Reference Stereo mode providing volume control. The amplifiers are driving the Revel Salon2 speakers. They can handle power and need current, dipping below 4 ohms and hovering in that range from 20 to 500 Hz.

Although the AT4002 voltage gain is specified at 28 dB and the AT522NC at 27.5 dB, volt-meter measurements showed the AT522NC had approximately 1.5 dB more gain when driven by the HA-1. This gain difference was confirmed using near-field measurements and a calibrated UMIK-1 microphone. The level-matched near field measurement of the right M20 is shown below. The dips are due to room interaction but both amps track one another with near perfection.

AT4002 (red) and AT522NC (blue) level matched REW Sweeps.

Quiet – No problem

These amps are both very quiet at the speaker. Technically, the AT4002 has 5 dB signal-to-noise advantage but both were virtually silent with my ear pressed to the tweeter. Transformer hum on both was undetectable at 2 feet.

Two-channel Listening

Since these are both two-channel amps listening tests were performed exclusively with 2-channel FLAC files.

Alison Krauss and the Union Station “Paper Airplane” (HD Tracks 94/24) is an excellent recording that combines acoustic instruments, strong bass lines, and female and male vocals. “Dust Bowl Children” banjo has a deeper cast on the AT4002 than on the AT522NC. The AT522NC draws attention to the sibilants in Alison Krauss vocals on “Lie Awake” and the strings, with great attack, whereas the AT4002 brought my focus toward instrument decay. This was a common theme in all listening sessions.

The dynamic and hard-mic’ed vocals on Sarah Jorosz “Undercurrent” (HDTracks 96/24) are not forgiving on any system. Driving the Salon2s, the AT522NC drum strikes hit hard, demanding attention and the vocals were a bit more relaxed with the AT4002. “House of Mercy” bass tracks were pounded out by the AT522NC with more authority than the AT4002. There is no telling which one is more accurate, though.

Above and Beyond “Acoustic” is a busy recording that feels more like a live recording than a studio production. The AT4002 shines here separating instruments which leads a subjectively more authentic presentation of the violin section. The AT522NC, in contrast seemed to elevate the strings.

Lyle Lovett’s North Dakota is a truly fantastic recording. Piano and guitar work once again demonstrates the AT4002 fullness and the AT522NC’s dynamics. The AT522NC brings forward the snap of the drum and the AT4002 presents richer male vocals.

Editorial Note about Amplifier “Sound”
Ideally, amplifiers should be utterly transparent, adding no character of their own. When differences exist, they should be measurable and explainable with the proper test and equipment. Unfortunately, I don’t have access to this gear. To delve further into the subject, check out the Audioholics article “Why Audio Amplifiers Can Sound Different”.

Conclusion

I have experienced several amps and generally the differences are small when operating within their limits. However, I did notice differences during side-by-side listening sessions. The AT522NC woke up the Revel M20’s, adding fun to many familiar recordings. Female vocals lifted, presenting a great emphasis on dynamics and sibilants. The AT4002 excels in the presentation of strings and vocals with familiar heft and balance provided by class A/B amplifiers. The AT4002 encouraged a greater turn of the volume knob.

Class-D has been described as dark and often lacking bass when driving difficult loads. This is not the case with the AT522NC. It is dynamic, if not bright, and clearly capable of delivering room-shaking bass from the Salon2’s woofers. The AT522NC is no lightweight amplifier but the weight and heat reduction are benefits worth considering. ATI’s NCore amps are popular for Home Theater. Judging from ATI’s styling, they are clearly targeting that market.

The AT4002 is a refined amplifier that is every bit as good as its big brother the AT6002. It has all the power needed to drive a pair of Revel Salon2’s ($22,000) loud -- very loud. It provides a tonal balance that I find particularly suited to female vocals and provides excellent resolution.

These amps are sturdy, powerful, and have no trouble driving even the power-hungry Salon2s. Speaker pairing, cost, or personal preference all weigh in selecting the proper amplifier. Here, we have two worthy contenders; the more compact energetic AT500NC and the signature performance of ATI’s fully-balanced AT4000 amps. I selected the AT522NC for my game room system for its performance driving the Revel M20s, relatively light weight, and demur size.

Which amplifier topology do you prefer, traditional Class AB or high-efficient Class D? Please share your comments in the related forum thread below.