Solid State Power Amp Reviews

I stuck with the USB-A and Ethernet inputs for digital playback. While I couldn't play DSD128 via USB-A—when I tried, the ACI 600 went silent—I could use Roon to convert DSD128 to PCM 24/176.4 before transmitting it via Ethernet.

Seek and ye shall find
Bel Canto's downloadable Seek iOS app lets you stream music from Tidal to a networked Bel Canto Black system, create and manage playlists, control volume via an iOS smartphone or tablet, and play MQA, DSD, and PCM files from a computer, external drive, or USB stick. Alas, Bel Canto offers but a single page of instructions remarkable for their lack of detail. If you can figure out how to build a playlist from it, your psychic powers surpass my own.

When I wrote McCormick for help with playlists, I received 15 sentences of instruction and this assurance: "We are expanding the user manual for Seek. We decided to first start with the basic information so that someone could start using Seek without be[ing] burdened with details. We'll see how we live with Seek so that we can focus the user on what is important." I can only assume that this much-needed manual will clarify how to avoid the initial display of titles in reverse alphabetical order, which happened every time I opened the app, and how to play a previous track without being told "Delay in streaming or unsupported file format" and, frequently, then having to close Seek entirely and start anew.

Nonetheless, once I mastered Seek, it worked well. It can't do half the things Roon can, and displays only vertically on an iOS device's screen even if you rotate the device 90ß—but it does supply track titles, resolution, and other vital information.

Setup
I'd hoped to isolate the Black ACI 600's digital and power functions from each other so that I could compare its digital playback with my far costlier dCS combo of Vivaldi DAC ($35,999), Scarlatti clock (no longer available), and Network Bridge network player ($4250), and with the Mytek HiFi Manhattan II DAC ($6000), as well as pit its power-amp section against my Pass Laboratories XA200.8 class-A monoblocks. Unfortunately, the ACI 600 lacks the outputs to make this possible. The only way to perform comparisons was to keep my reference components on standby and switch between entire systems. But since my Nordost cables take 24 hours to sound their best after being bent, and a lot of cable bending was required for the switch, rapid A/B comparisons were impossible.

I placed the ACI 600 on one of the middle shelves of my Grand Prix Monaco rack, atop the Grand Prix Formula Platform that usually supports the dCS Scarlatti clock. Although both platform and ACI 600 were prone to sliding, I retained the Formula because I've gotten used to the more transparent sound my reference system produces with it.

The rest of the setup was simple: Attach network-connected Ethernet, speaker, and power cables. Add my source(s) of choice and use the remote to select the desired input (eg, Ethernet, USB, phono, etc.) and volume level. The only difficulty I experienced was when I inserted USB sticks in the USB-A input and selected USB: I heard nothing. As it turns out, selecting USB activates only the USB-B input. To access the USB-A input you must select Ethernet, since the contents of USB-A–connected devices are accessible only by using Seek via Ethernet. No, this is not explained in the manual.

Before doing any serious listening, I ran break-in and demagnetizing tones through the Bel Canto for a good 120 hours. The ACI 600 appears to require a good 48 to 72 hours of warm-up to sound its best.

Listening
Once fully warmed up, the Black ACI 600 shattered all notions of class-D sounding colorless and uninviting. When I played, from a USB stick, the first movement of Iván Fischer and the Budapest Festival Orchestra's recording of Mahler's Symphony 5 (DSD64, Channel Classics 34213/NativeDSD.com), the bass was excellent, and the sound was warm and filled with vitality. I also loved the range of color and astounding bass in Michael Tilson Thomas and the San Francisco Symphony's digital-only release of Berg's Three Pieces for Orchestra (24/192 WAV, SFS Media/HDtracks). Treble extension was a bit toned down from what I'm accustomed to, which in music this emotionally ravaging is not necessarily a bad thing.

Switching to choral music, I was left almost breathless at the enrapturing warmth of the remarkable acoustic captured by engineer John Atkinson for The First Tears, the 14-minute opening track of Ethan Sperry and the superb Portland State Chamber Choir's recording of the music of Eriks Ešenvalds, The Doors of Heaven (24/88.2 WAV, Naxos 8.579008). With space, air, and midrange beauty the sound's strong points, I hung on every note. Amazing.

I have no idea where my 24/96 FLAC file of Ella Fitzgerald and Louis Armstrong performing "Summertime" came from, but the sound was delicious. Then again, this duo's recordings are so resplendent in the midrange that it takes really poor equipment to make them sound less than great—so I was even more convinced of the ACI 600's excellence by a recording of baritone Matthias Goerne singing "Meine Rose," from Schumann's Six Poems, Op.90, accompanied by pianist Markus Hinterhäuser on their recital Einsamkeit (24/96 WAV, Harmonia Mundi HMM 902243/HDtracks). Though I heard less transparency and detail in Goerne's voice than I do through the far more expensive dCS rig, it was mesmerizingly sweet, warm, and expressive, and the piano sounded convincingly natural.

I compared the sound of files from USB sticks, inserted first in an Intel NUC computer connected to the ACI 600 via Ethernet and played using Roon, and then inserted directly in the ACI 600's USB-A port and played via Bel Canto's Seek app. Consistent with the results of my December 2017 review of the dCS Network Bridge, the files accessed via Roon sounded a bit softer, with less vibrant highs. Regardless, Roon's extra features—I could even read pdfs of liner notes—kept me returning to it for more.

I was eager to compare the sound of MQA-encoded files with their high-resolution, non-MQA equivalents. With Bach Trios, by mandolinist Chris Thile, cellist Yo-Yo Ma, and double bassist Edgar Meyer (Nonesuch 558933), I used Seek to first listen to the 24/48 MQA track of J.S. Bach's "Wachet auf, ruft uns die Stimme" (Sleepers Awake) from Tidal HiFi, and then to a non-MQA 24/96 WAV file from USB. Although I've heard major differences between MQA and non-MQA versions of the same files with other gear, here the differences were subtle. Meyer's fingering and pitches were better delineated via MQA, but it was hardly a stop-the-presses improvement. Perhaps this had to do with the limits of the ACI 600's digital resolution, or the fact that its strengths lay more in bass power and midrange warmth than in brilliance and transparency.

I love Camille Thurman's delectable performance of Ray Noble's "Cherokee" on her Inside the Moment (CD, Chesky JD397), but the differences between 24/192 WAV and 16/44.1 MQA were slight. A bit of haze was removed when I switched from the 24/88.2 ALAC file to the 24/44.1 MQA-encoded FLAC file of John Atkinson's recording of Eric Whitacre's Lux Aurumque, from Cantus's While You Are Alive (CD, CD Baby 5637240534). Ditto for the MQA versions of Ešenvalds's Northern Lights and his arrangement of "Amazing Grace," both from JA's first recording of the Portland State Chamber Choir, Into Unknown Worlds (CD, CD Baby 888295153546).

I had to ask myself if music actually sounds this warm in real life. While my answer was ultimately No, I'd rather have warm and smooth over harsh and hollow any day. Because the ACI 600's top end was less bright than my reference, I could listen at higher volumes without pain, and thus enjoy more bass impact and a richer midrange. That greater percussive slam really hit home with Ludovic Morlot and the Seattle Symphony's recording of Stravinsky's The Rite of Spring (24/96 WAV, Seattle Symphony Media SSM1005), and with "Electrified II," from Yello's Toy (24/48 WAV, Polydor 4782160/HDtracks).

Headphones & LPs
While listening through Audeze LCD-X headphones equipped with Nordost Heimdall 2 cabling yielded disappointingly flat sound—in retrospect, I wonder if the ACI 600's headphone jack needed its own break-in—connecting the Bel Canto to the analog rig of my physician, Gary Forbes, revealed that it held its own against an EAR 834P phono stage and VTL's TP2.1 preamplifier and ST-150 stereo power amplifier. It was a snap to choose the right cartridge settings for Gary's Pro-Ject RPM 1 Carbon turntable with SME 309 tonearm and over-bright Lyra Delos cartridge. With Avalon Acoustics Arcus loudspeakers and a JL Audio Fathom f112 subwoofer the sounds of "Walking on a Wire," from Richard & Linda Thompson's Shoot Out the Lights (LP, Hannibal/Rhino 71303), and of Taj Mahal singing "Jes' Grew" on Conjure: Music for the Texts of Ishmael Reed (LP, American Clave 1006), didn't sound as liquid, air-filled, and tube-like as through Gary's reference—but the bass was notably better.

"I'm a rhythm guy," said my doc, who plays bass guitar and percussion in brass and rock bands. "The ACI 600's bass is very tight and well defined, with notable ease and lack of strain. The separation of instruments is also better than my reference." In short, the Black ACI 600 gave Gary more of what he most values.

Conclusions
Dismiss all preconceptions: Bel Canto Design's Black ACI 600 integrated class-D amplifier is a major contender whose warmth of sound, versatility, and relative ease of use—firmware- and software-fixable hiccups aside—deserve a solid Class A recommendation. That this single box can do so much so well, while looking so simple and elegant, should earn it pride of place in many a system.