ATI AT543NC Amplifier Review

Stereophile
by Kalman Rubinson

The power-amp saga continues. For months, I've been plowing through the market, searching for something to drive my three front speakers (I use a two-channel amp for the surrounds). It can be a three-channel amp or three monoblocks-it just has to sound great with my speakers, and be light enough that I can lift it by myself when I need to rearrange my system. I'd finally settled on Classé's Sigma Monos for their transparency, and because I can manage their weight, one at a time. At the CEDIA Expo in September 2016, I saw two more candidates worthy of consideration. Review samples of both arrived here almost simultaneously.

ATI AT543NC 3 Channel 500 watts/channel Power Amplifier

Amplifier Technologies, Inc. is a well-established company founded by Morris Kessler, who first made a splash in audio in 1967 when he co-founded SAE. ATI makes a wide range of audio devices under its own name, as well as under brand names it has acquired, including B&K and Theta Digital, as well as SAE. In addition, it has produced amplifiers for many other companies which I will tactfully not list. Until recently, ATI expended all of its efforts on producing class-A/B amplifiers of solid engineering and performance, but Kessler and Theta Digital's Dave Reich have now stepped boldly into class-D with Theta's impressive Prometheus monoblock and Dreadnaught D modular multichannel amp, both based on Hypex's NCore technology. Larry Greenhill raved about the Prometheus, and I was as enthusiastic about the Dreadnaught D.

Class-D amplifiers, particularly those based on Hypex NCore modules, come in many flavors, depending on the manufacturer's choice of power supply, input stage, and bridging, and careful listeners have found that these can sound as different from each other as can conventional class-A/B amps. The use of conventional linear power supplies based on heavy-duty toroidal transformers results in amplifiers that are nearly as heavy as class-A/B designs of equivalent power. For example, the three-channel, 225 watts/channel Dreadnaught D weighs about 90 lbs-too much for me.

So when I heard about ATI's new line of NCore amps, I figured that the same expertise was behind them, and that there should be some cost savings, particularly as they lacked anything like the Thetas' elegant cases. ATI's new line borrows from the flexible modular designs of the Dreadnaught D and so is quite comprehensive, comprising 200 watts/channel models of two through eight channels (ATI AT522NC, ATI AT523NC, ATI AT524C, ATI AT525NC, ATI AT526NC, ATI AT527NC, ATI AT528NC) and 500 watts/channel models of two, three, and four channels (ATI AT542NC, ATI AT543NC, ATI AT544NC). I opted for the three-channel, 500 watts/channel ATI AT543NC, which weighs 57.5 lbs-no lightweight, but I can move it when I must.

Description

Inside the ATI AT543NC are two modules, one with two amplifiers and the other with only one. Each amp has a pair of NCore NC500 boards that are fed via ATI's custom input/gain stage and bridged for the high rated output. These are powered by a conventional linear power supply with a 950kVA toroidal transformer to power the two-channel module (with separate windings for each amp), and another 650kVA toroid for the single-channel module. This results in a theoretical difference between the single channel connected to the smaller transformer and the two that share the larger one; with that in mind, I used the latter to power the left and right speakers and the former to run the center-though it's unlikely that this strategy is of practical significance.

At the left of the ATI AT543NC's interior is a self-configuring power supply, which first ascertains the available supply voltage within a range of 90-135V or 200-260V, then configures the transformer taps for local usage. Only the fuse and the power cord might then need to be changed. This power supply also senses and protects against over/undervoltage, overheating, DC faults, and overload. The three small input boards are affixed to the rear panel, the amp modules to the bottom plate, and its two large toroidal transformers near the front panel.

On the right rear of the ATI AT543NC are, from bottom to top: an IEC AC input, an AC fuse post, a ground terminal, and a 3-24V remote trigger input. The rest of the panel has, for each channel, unbalanced (RCA) and balanced (XLR) input jacks and a pair of multiway speaker terminals. Next to each RCA input jack is a little toggle for switching that input between unbalanced and balanced operation. Into the XLR inputs I plugged AudioQuest Earth/DBS and connected the speakers with AudioQuest Oak/DBS biwire, but none of my standard power cords fit the ATI AT543NC. ATI uses a standard 20 amp IEC power inlet, type C20, with three heavy, flat, parallel blades, to ensure that no user substitutes an unsuitably lightweight cord. The provided cord, with matching C19 connector, was barely long enough for me.

On the front panel, from top to bottom at center, are the ATI logo, LEDs for Peak level and Standby, and an illuminated power switch. When the ATI AT543NC is plugged in, the power switch pulses dimly blue to indicate standby mode. Push the button, the Standby LED lights up, and a few seconds later goes dark as the blue power LED glows full: the amp is ready. After 10 minutes without an input signal, the ATI AT543NC shuts down all its high-power functions and goes into Sleep mode; when it senses a signal, it powers itself back up. It has a soft start to minimize power surges, but was so efficient that I found no need to use my 12V trigger wire.

Listening

The ATI AT543NC's sound was immediately appealing in terms of both balance and clarity. In orchestral recordings, the individuation of lower strings and winds and their melodic lines was notable. There was plenty of weight at the bottom end, but it was taut, with no loss of control at any volume level. The same was true of the upper midrange and treble, which were open and airy, but with no glint of brightness.

I played all the reference recordings I've used to evaluate other amps, but the ATI AT543NC was not to be faulted. Every recording of solo voice-male or female, high or low-was reproduced as I would want it, and as I have heard it through this system with the very best amps I've tried. As I played recordings of larger and still larger ensembles, I heard no diminution in the ATI AT543NC's clarity and balance. Power was generous; I got the Peak LED to wink only once, and it was my own fault: with the gain set very high, I switched sources. The ATI AT543NC's recovery was instantaneous and without consequence.

That brief paragraph characterizes the ATI AT543NC's sound quality but is insufficient to distinguish it from other amps. With my Bowers & Wilkins 802 D3 speakers, the ATI AT543NC ran toe to toe with the Classé Sigma Monos, except that the ATI was marginally less detailed in the upper midrange. This was not a significant problem except with some high-resolution files, such as the Color Field Quartet's recording of James Matheson's String Quartet (multichannel DSD256 download, Yarlung YAR25670/NativeDSD), and even those sounded startlingly transparent through the ATI AT543NC.

In direct comparisons with Parasound's Halo A 31 three-channel amp, the ATI AT543NC seemed to offer more lower midrange, less treble, marginally more richness, and a slight tilt in balance. The ATI seemed to present a bit less air and space, but I can't say whether the Parasound is any more accurate in this regard. However, all that was noted only briefly upon switching from one amp to the other. The ATI and Parasound both had smoother, more enjoyable sound with Focal's Sopra No3 speakers than did the Classé Sigma Monos, which unsympathetically revealed some upper-midrange glare in some recordings.

I also tried the ATI AT543NC's unbalanced inputs, using Cardas Cross 10m cables. The overall level was down about 6dB, as expected, but the sound quality was unchanged.

Conclusion

I am completely taken with ATI's AT543NC. It seemed to do everything right, and didn't get in the way while communicating the elements and the spirit of the music. I wish I'd had on hand its older cousin, the Theta Dreadnaught D, for direct comparisons, but given the Theta's higher price ($7,099) and weight (90 lbs), any outcome could not possibly blemish the $3,295, 57.5-lb ATI AT543NC.