Bel Canto Design Black ACI 600 integrated amplifier

When Michael McCormick, president of Bel Canto Design, suggested that I review their Black ACI 600 integrated amplifier, I accepted without hesitation. As wonderful as my reference system may sound, its dCS digital front end alone comprises four boxes and a web of cables complex enough to send many a spider spinning. Given the choice between connecting that front end to a pair of expensive, enormous monoblocks—with their similarly expensive AC cords and equipment racks/isolation platforms—or to a single, visually elegant, 45-lb box that costs $25,000, produces 300Wpc into 8 ohms, and requires only a single power cord and shelf, I think many an audiophile, even those with lots of money, might gravitate toward the latter.

Launched in January 2017, the Black ACI 600 offers several features my system lacks: an MM/MC phono stage with two gain settings for MM, two others for MC, and five for cartridge loading; a headphone jack; adjustments for tonal balance; and the provision for adding a subwoofer. And not only can it play PCM files of resolutions up to 24-bit/192kHz and DSD up to 128Fs (depending on input)—it can also decode MQA.

For some, learning that the ACI 600's power-amp section is class-D will be enough to send them running in horror. That it converts all DSD signals to 24/176.4 PCM before sending them to its final DAC and analog stages may be another turnoff. As for MQA . . . well, just look at how some people react to the very sound of those initials.

What have we here?
Bel Canto Design's founder and design architect, electrical engineer John Stronczer, told me in an e-mail that "the ACI 600 diverges from traditional architectures starting in the amp/preamp section. There aren't separate analog preamp and power amp sections per se; it's really a highly integrated and optimized DAC/amplifier. Our unique digital architecture leverages advances in ultra-low noise master clocks and asynchronous retiming to keep analog signal paths short and optimized . . . and get the best analog audio performance from the system."

The Black ACI 600 has an NCore class-D output stage that operates as a low-gain (13dB) power buffer. This stage is driven by Bel Canto's proprietary single-stage, high-current, class-A amplifier section, which is configured as a differential amplifier with 14dB of gain.

Immediately preceding and connected to these gain and power stages is Bel Canto's HDRII (for High Dynamic Resolution, Second Generation), 126dB, fully differential DAC. This combination of power amp and HDRII DAC is driven, through a proprietary digital interface, by the ACI 600's separate, galvanically isolated, asynchronous multi-input platform (AMiP) processor. Tasked with processing all analog and digital inputs, the AMiP contains Bel Canto's 32-bit/800MHz module/network interface and USB-2 audio input processor.

The ACI 600's analog line and MC/MM phono inputs use a 24/192 analog-to-digital converter whose master clocks reside on the AMiP processor board. These clocks retime all digital inputs, and convert and render DSD to PCM. After extensive listening tests, Bel Canto chose to use its custom-optimized, MQA-based FIR filters for both standard PCM input and final MQA rendering.

A double-precision, 64-bit audio processor in the HDRII DAC-amp section, driven by its own dedicated AC power supply and equipped with its own master clock, handles lossless bass management, Tilt (which changes the overall shape of the response curve, boosting bass and cutting treble or vice versa), Bass EQ, and volume control. Bel Canto claims that this processor maintains a resolution of 126dB, regardless of setting. Ultimately, the digital signal goes through two stages of asynchronous retiming before being converted to analog.

To create a highly damped environment, the ACI 600's case is machined from billet aluminum, except for the bottom and rear panels. Each of the latter comprises two 1/8"-thick layers of aluminum bonded with a 3M material in a constrained-layer-damped construction. The result is a handsome enclosure with beveled edges and horizontal striations.

The long LED display on the ACI 600's front panel is filled with big red letters and numbers indicating the inputs, the volume level, and programmable functions—it was a cinch to read from across the room. Everything can be controlled by rotating and pushing the large aluminum wheel near the front right corner of the top plate—but who wants to play with the wheel when you can do everything from your listening seat with the uncluttered, metal remote handset?


The ACI 600's rear panel is hardly spare. Vying for space there are connectors and ports for every function: speaker cables; 2V line-level analog inputs for phono and two other devices (with optional home-theater bypass); another set of analog outputs, for a powered subwoofer; a ground lug; a headphone jack; two high-speed, ST fiber-optic digital outs for use with Bel Canto's MPS1 monoblocks; RS132, AES, S/PDIF, TosLink, USB-B, USB-A, and Ethernet inputs; and the power-cord inlet and main power rocker switch.

Because the ACI 600's Ethernet jack sits directly above its USB-A port, you may need to unplug the Ethernet cable every time you want to change USB sticks or cables. Similarly, headphone listeners may be dismayed to learn that the headphone jack is on the rear panel, and requires a cable of considerable length.

Worse, the ACI 600's Delrin footers easily slide across platforms and shelves at the least provocation. To avoid disaster any time I plugged in or unplugged anything, I had to steady the front of the ACI 600 with my other hand. Unless you have headphone cable to spare, nodding your head to the beat is fraught with peril.

The ACI 600's digital inputs are not equally abled. The AES/XLR, S/PDIF, and TosLink accept digital signals of PCM data rates up to only 24/192; the USB-A and Ethernet accept up to 24/192 plus DSD64 and MQA, and can serve as Roon-Ready endpoints; and the USB-B accepts PCM up to 24/384, MQA, and DSD64/128 in DoP format. At the time of writing, some of this information had not been updated in the online and print manuals.

Bel Canto Design
221 N. First Street
Minneapolis, MN 55401
(612) 317-4550

DH's picture

One choice of filter, the MQA? No chance to use another type of filter if you tend to prefer it, like a linear phase one?

rom661's picture

Leaving the filter issue aside, you get a streamer, a phono stage, a DAC, preamp and power amp. I'm listening to one now on evaluation, comparing it to a 30K stack and it's quite good. Plus the savings in great cables is immense. Just sayin'...

mrkaic's picture

Not exactly affordable, is it? And not a stellar performer either. The THD is way too high.

rom661's picture

I've been doing this for 30 years and designing for longer. Maybe, just maybe, you can tell more about a piece by listening than by looking at one spec. I love what I do/I hate what I do...

Jason P Jackson's picture

Why is there a picture of a standard power amplifier on page 2? This could not be the internals of the class d Bell Canto.