Expert review and test of the Bel Canto ACI 600 Integrated Amplifier & DAC by Australian Hi-Fi Magazine. Free pdf download included.


Is Bel Canto’s new ACI 600 ‘the ultimate integrated amplifier’, as the famous US manufacturer claims? It could very well be, except that it’s not just an integrated amplifier, it’s also a DAC, a streamer, a pre-amplifier, a power amplifier, and a headphone amplifier, all in the one compact chassis.

The Equipment

It’s a bit spooky once you’ve unpacked the ACI 600 because in spite of the extent of its capabilities, there isn’t a single knob—or button—to be seen on the expansive front panel (and it’s certainly expansive: the ACI 600 is 483mm wide… though it’s only 97mm high). There’s just that huge front panel display and that’s it. Thanks to its size, the display is super-easy to read, which is handy because it not only shows system status, source, volume level and all programming functions, but also serves as the streaming display for track, album, and other metadata.

My guess is that Bel Canto expects that most ACI 600 owners will load the Bel Canto Black iOS app onto their phone or tablet and control the ACI 600 with that. Not having an Android version of this app could therefore be a bit of a downer for Android owners, which is no doubt why Bel Canto also provides a conventional infra-red remote control. And if you misplace this, there always the ‘jog wheel’ on the top of the ACI 600 that can be used instead.

If you do own an iDevice, and you’re in a downloading state of mind, you might care to also download the Bel Canto ‘Seek’ app. This app controls music streaming up to 24/192, and can link to your Tidal account for MQA and full CD resolution and to your Qobuz account for CD and High Res Streaming. It also allows you to stream music from an attached USB drive, from your iOS device, from Dropbox or from your OneDrive account. It will also find any DLNA-compatible servers on your network after which you can use it to control any music library on the network. (And here’s a handy tip: If you’re looking for this app, make sure you search for ‘Bel Canto Seek’. If you just type the word ‘Seek’ into the app search you’ll be presented with half-a-gazillion apps for finding anything from a job to a soul-mate.)

But if the front panel is featureless, you’ll find the rear panel is anything but, so if you want to get an idea of what the ACI 600 can do, you’d be advised to take a long, hard look at it. And the first thing you’ll see is the American flag alongside the words ‘Made in the USA’. Yep, the Bel Canto ACI 600 is one of the few hi-fi products that’s made entirely in the USA… at least it is except for the Hypex amplifier modules, about which more later. It was designed by John Stronzer who, before he founded Bel Canto more than a quarter of a century ago, was an engineer at Honeywell where he developed GaAS integrated circuits and a 10GHz optical network receiver amplifier.

The next thing you’ll notice is that the ACI 600 has a phono input. Yep, a modern high-powered Class-D amplifier with a phono input. What’s more, the two gold-plated RCA sockets don’t show that you can adjust the sensitivity of the phono input through four different gain settings (0.25mV, 0.5mV, 2.5mV and 5.0mV) and through five different load impedances (50Ω, 100Ω, 500Ω, 1kΩ and 47kΩ) to suit both moving-magnet and moving-coil cartridges. Alongside the phono input are two line-level inputs, labelled L1 and L2, plus a line-level output.

One strange feature of the rear panel that you should notice straight away is the 6.35mm headphone socket. This is driven by its own dedicated headphone amplifier, which can drive loads down to 32Ω. All good, except why put the socket on the rear panel, where it will be difficult to access? The most obvious reason is that putting a socket on the front panel would ruin the sleek ‘look’ of that panel, which kind of puts form before function… something engineers are supposed to eschew. A not-quite so obvious reason would be that it’s cheaper to put the socket on the rear rather than the front—which seems unlikely given the asking price of the ACI 600. Neither of which reasons address the issue that it makes using the socket inconvenient unless you leave your headphones plugged in all the time. You could do this, because you have to turn the headphone socket ‘On’ via the ACI 600’s menuing system. However, turning the headphones on also turns your speakers off and deactivates the line output.

I am not a fan of this mode of operation, because I rather like listening to my system whilst wearing headphones and with the speakers playing at the same time. This gives me that unique ‘headphone’ experience, while at the same time enjoying better bass response than I’d ever get from any pair of headphones. If you haven’t tried this, I think you should… you’ll probably like it, though you do need to use open-back or ‘on-ear’ headphones—it doesn’t work at all with IEDs and not very well with ‘around ear’ headphones. If you need to plug in and unplug your headphones, I’d recommend investing in a short extension lead with a 6.35mm stereo phone jack on one end, and a 6.35mm stereo phone socket at the other. This would enable easy access… maybe even easier access than if there was a socket on the front panel.

There are two digital outputs which look like ordinary Toslink digital outputs, but they’re not, and because they are actually used to drive a pair of Bel Canto MPS1 monobloc amplifiers, their output varies depending on the setting of the Tilt, Bass EQ and Main HP settings… settings which I’ll get around to explaining later in this review.

The digital inputs are more conventional, with AES XLR, Toslink and SPDIF inputs that accept up to 24-bit/192kHz data and a USB input that accepts up to 24/384kHz PCM and DSD64/128 in DoP format and is MQA-capable. The USB-A input is intended to accommodate FAT32 USB devices. It works with all externally-powered devices, but can also drive ‘self-powered’ USB devices so long as they draw one amp (or less) of current. All inputs are asynchronously re-clocked into a fully-balanced advanced-segment DAC that Bel Canto says ‘has less than 0.0005% distortion and 127dB of dynamic range’, with the stated purpose, according to Stronzer, ‘of significantly reducing both jitter and distortion, preserving the delicate audio signals from all sources.’

The ACI 600 also has a line output that can be used to drive a powered subwoofer. It’s sensible that Bel Canto has provided such an output, firstly because it allows sophisticated filtering to be applied to both this output and the speaker output, about which more later, but perhaps even more importantly because you should not connect the speaker outputs of the ACI 600 to the ‘high-level’ or ‘speaker’ inputs of any subwoofer whose negative terminal speaker terminal is referenced to ground. Since most subwoofer manufacturers don’t specify this, it’s safer to not try it at all.

Bel Canto provides quite a nice on-line ‘Owners’ Manual’ that is also downloadable as a pdf. However, the type is white (and colours) on a black background (no doubt to tie in with Bel Canto’s ‘black’ marketing concept). This means that if you print it out, you’d better make sure you have spare black ink in your printer. And, if you try to economise by printing the manual in black and white, all those pretty coloured parts of the manual become very hard to read indeed… which is one of the reasons that virtually all the printed matter in the world involves using black type on a white background. So here’s a plea for anyone at Bel Canto who’s listening: By all means present the on-line Owner’s Manual in whatever pretty colours you like, but please, please, make the downloadable pdf version of the Owner’s Manual so that it prints in standard ‘black on white’. (And while you’re at it, it would be great if you could also make sure that all the versions of the pdf on the site are the same. At the time of this review, the downloadable version was v1.3 and that of the online version v1.7.)


As with most Bel Canto products, the model number hints at the ACI 600’s rated power output, which is 600-watts per channel continuous into 4Ω and 300-watts per channel continuous into 8Ω. This power is provided by N-Core modules manufactured by Dutch Class-D specialist Hypex, but Stronzer says the N-Core Class-D modules used in the ACI 600 are custom designs manufactured exclusively for Bel Canto.

The Bel Canto ACI 600 measures 483×356×97mm (WDH) and weighs a substantial 20.5kg.

In Use and Listening Sessions

If you want to adjust the tonal balance of your system using the Bel Canto ACI, your first port of call would be Bel Canto’s ‘Tilt’ control… except that it’s not really Bel Canto’s, because the ‘Tilt’ control made its first appearance on Quad components made in the UK back in the 70s and 80s, most famously on the Quad 34. The idea behind a ‘Tilt’ control is that unlike separate bass and treble controls, it ‘tilts’ the whole audio spectrum, so bass and treble regions are both affected at the same time, in exactly equal (but opposite) amounts depending on frequency. The concept is to change the overall balance of the sound without affecting either the perceived volume level, or the ‘colour’ of the sound. However whereas Quad chose an axis point of 800Hz, Bel Canto uses an axis point of 775Hz. As to why this approach might be advantageous, we have no less an authority than J. Gordon Holt on record as saying of its efficacy: ‘it’s far better able than most to correct for many of the worst and most commonly-encountered sonic flaws in recordings.’

Your second port of call when adjusting the tonal balance of your system using the Bel Canto ACI 600 would be the Bass Eq control, which is a bass trim control that, in the words of the manual, ‘operates over a 3dB range with 0.6dB increments (–5, Off, +5) providing subtle optimisation of your bass energy.’ Curiously—though I am sure it’s just a coincidence—exactly the same control was fitted to the Quad 34… and for precisely the same reason. However, whereas the Quad 34 applied Tilt and Bass Eq via analogue circuitry, it’s applied in the digital domain by the Bel Canto ACI 600.

In fact all user adjustments on the ACI 600 are made in the digital domain: the programmable phono gain, the programmable cartridge loading, the programmable line level input, the programmable maximum volume settings, and the programmable high- and low-pass filters discussed in the following paragraph…

If you’re using a subwoofer in conjunction with the ACI 600 (which is really easy, thanks the subwoofer output on the rear panel) you should make sure that you activate and calibrate the ACI 600’s ‘High Pass’ circuitry, which inserts a high-pass filter into the speaker outputs. You can separately insert a low-pass filter into the subwoofer output. You can set the turnovers of these filters anywhere from 40Hz to 120Hz (in increments of 10Hz), depending on the low-frequency extension of your main speakers and the size of your subwoofer. The provision of these high-pass and low-pass filters is far-sighted, as it enables far superior integration of your main speakers with your subwoofer than would otherwise be possible. Although this feature is quite common on home theatre receivers, it’s rare to find it on two-channel amplifiers… particularly high-end two-channel amplifiers!

One interesting feature on the Bel Canto ACI 600 is the ability to set a ‘maximum volume’… except there are only three settings, one of which is 0dB, effectively leaving only two (–10dB and –20dB). You could use this feature to ensure the ACI 600’s not-inconsiderable power output will not overdrive your speakers, or so other people using your system cannot play music at levels you’d consider inappropriate. I would, however, have found it more useful if a greater range of options was available. The good news is that because the whole ACI 600 runs on an XMOS-based processor, it would be easy for Bel Canto to change the programming to permit this. You could then update your amplifier’s software via the Ethernet connection on the rear panel. In the same fashion, if Bel Canto decides to change any other operating parameters, or add new features, it can do so at any time, after which owners can, if they wish, apply such upgrades via Ethernet.

The provision of extensive programming also means you can ‘hide’ inputs that you are not using, so you don’t have to waste time scrolling through inputs. As for those inputs that you are using, you can program these with names of your own choosing, though the somewhat miserly four-character limit means you’ll have to be very creative! (Though to be fair, this limit is mostly a function of the sheer size of the letters on the front panel display, which are 15mm high.) I did like the fact that when you first switch the ACI 600 on, all the LEDs on the front panel glow simultaneously, so you can see if you’ve lost one… though given the reliability of LEDs this is so unlikely that I think Bel Canto has just done it just for the visual effect, which is also fine by me!

I have to say that I found it a little hard to come to grips (literally!) with the top-mounted jog wheel, which involves pressing right, left, up, down and rotating in various subtle combinations in order to get it to do what it’s supposed to do. It required more than a little degree of familiarisation before I could quickly and easily get the ACI 600 to do what I wanted… so much so that I found it faster and easier to just use Bel Canto’s remote control.

It certainly wasn’t hard to come to grips with the sound quality of the Bel Canto ACI 600, which was outstandingly good. Perhaps most noticeable is the absolute lack of background noise, which I discovered when I thought I’d initiated music playback, but actually hadn’t. Thinking I should be hearing something, I cranked the ACI 600’s volume to the max, yet still heard nothing—no hiss, no hum, zero… nada.

It was lucky I wound back the volume to zero before discovering that I hadn’t pressed the ‘play’ button hard enough in the first place, because this time when I advanced the volume, I realised exactly how powerful the ACI 600 is… which is extraordinarily powerful! Actually, I didn’t quite get to find how powerful the ACI 600 really is, because it was powerful enough to have my fairly insensitive reference speakers asking for mercy well before I reached the ACI 600’s maximum power output… at which volume level I should also say that I would have stopped cranking up the volume anyway, for fear of hearing damage. Trust me, unless your home is the size of the Sydney Opera House’s concert hall, you will never—ever!—need more power than the Bel Canto ACI 600 can deliver. And what power it is! Nary a hint of distortion, no matter whether I was listening late at night, with the speakers playing at barely above a whisper (at which times I put those Tilt and Bass Eq functions to good use), or whether I’d cranked the volume right up, trying to drown out the sound of heavy rain on the tin roof.

Bass delivery was incredibly quick: So fast that it was almost as if the Bel Canto was able to anticipate that a drummer was about to stomp the pedal on his kick drum, or that the fuse on the cannon in Tchaikovsky’s 1812 Overture was just about to light the gunpowder. And the way it reproduced the sonic mayhem on Asking Alexandria’s ‘From Death to Destiny’ brought the music to life. When you hear drummer James Cassells’ stick attacks and kick drum frenzies on the intros to the tracks Run Free or Moving On, followed by Sam Bettley chiming-in with his powerful crunching electric bass, you immediately know you’re hearing the real thing.

Yet for all the power and fire of the bass, the midrange was gloriously rich, superbly linear and wonderfully delicate. The tone of Angela Hewitt’s piano on her wonderful album (on Hyperion) of arrangements of various of Bach’s works (famous and otherwise) was unerringly delivered by the Bel Canto ACI 600, and the clarity of the amplifier’s delivery and its ability to differentiate the most subtle differences in volume levels also illuminated Hewitt’s wonderful ability to differently weight her piano keys in order to extract different tonal shadings. This is a wonderful album also because we get to hear on the one album transcriptions of Bach’s work not only by Hewitt herself, but also by such musical luminaries as Wilhelm Kempff, Myra Hess, William Walton, John Ireland and Herbert Howells.

Listening to the DSD version of the Aeon Trio’s ‘Elegy’ (on trptk) dispelled any thoughts I may have harboured of the high-frequency sound of Class-D amplification being at all brittle, because the treble of both Maya Fridman’s cello and Atzko Kohashi’s piano (a Fazioli Modello F228 Grand) shimmered like threads of gossamer in the moonlight. Further auditioning to music played by even higher-pitched instruments, whose harmonics reached higher in the audio spectrum, simply confirmed my opinion of the Bel Canto ACI 600’s high-frequency sound quality, and that opinion can be summed up in one word: perfect.


Essentially what Bel Canto has done is take three of its top-of-the-line individual components, put them all in the one sleek chassis, under the control of a sophisticated electronic controller. This is a really smart move by Bel Canto, because many people don’t like ‘stacks’ of components, whilst some others don’t like the small form-factor chassis the company uses for components in its other product lines. It’s also really smart because buyers get a single, great-looking component that ‘does it all’ and can be upgraded to accommodate future developments in audio. In short, the Bel Canto ACI 600 is an ‘I can do everything’ masterpiece!  # Scott Campbell


Laboratory Test Results

Newport Test Labs’ measurements proved beyond any shadow of a doubt that the Bel Canto ACI 600 is a powerful amplifier. It delivered more than 300-watts per channel into 8Ω, irrespective of whether one or both channels was driven, and more than 600-watts per channel into 4Ω loads, again irrespective of whether one or both channels was driven. It also was also able to deliver this above-specification power right across the frequency band, from 20Hz to 20kHz. The results are tabulated in the test result table, and shown graphically as a bar graph. Note that we haven’t shown the usual two bar graphs (one for single-channel-driven and the other for both-channels-driven) because the two graphs were identical.

Channel separation was superb, besting 100dB at low and midrange frequencies and still 91dB at 20kHz. Channel balance was also excellent, with Newport Test Labs measuring 0.116dB at 1kHz. Interchannel phase was excellent at low and midrange frequencies, as you can see from the tabulated results, and only 1.94° out at 20kHz.

Distortion was very, very low, measured either into 8Ω or 4Ω loads and also either at low output levels or at rated output. Graph 1 shows harmonic distortion levels when the amplifier is delivering one watt into an 8Ω load. You can see a single third harmonic component at –110dB (0.00031%) and then fifth and sixth harmonic components at around –115dB (0.00017%), but that’s about it. Equally remarkable is the low level of the noise floor—down close to –130dB—and the lack of low-frequency noise.

Performance was almost as good when the Bel Canto ACI 600 was driving a 4Ω load. This time there’s a second harmonic at –105dB (0.00056%) and the third harmonic is a little higher in level, at around –108dB (0.00039%). There’s also a fourth harmonic at –122dB (0.00007%), plus the fifth and sixth harmonics are fractionally higher than they were when the amplifier was driving an 8Ω load, at around –113dB (0.00022%). Overall wideband distortion at one watt was just 0.005%, as you can see in the tabulated figures.

As I noted earlier, distortion was also very low at rated output, as you can see from the measurements made by Newport Test Labs that are shown in Graph 3 and Graph 4. At an output of 300-watts into 8Ω, apart from the second harmonic at –90dB (0.00316%) and the third harmonic at –92dB (0.00251%), all other distortion components were more than 100dB down (0.001%). Distortion increased slightly when the ACI 600 was delivering 600-watts into 4Ω (Graph 4) but apart from the second and third harmonics, all other distortion components were more than 100dB down as well. In both graphs you can see the noise floor has dropped even lower—down close to –140dB. The extra distortion components visible in Graph 4 appear to be the result of an increased level of 100Hz signal from the power supply. Overall wideband distortion at rated output was measured at 0.004%.

Frequency response was measured as extending from 7Hz to 21kHz –1dB, and from 4Hz to 33kHz –3dB. This response is shown in Graph 5 and was measured with a laboratory standard non-inductive 8Ω load.

Intermodulation distortion was exceptionally low, as you can see from Graph 6. The 18kHz and 21kHz sidebands alongside the two test signals are at –95dB (0.00177%) and –97dB (0.00141%), while the 17kHz and 22kHz sidebands are both around –114dB (0.00019%). The unwanted regenerated signal at 1kHz is nearly 120dB down. Interestingly, there’s an obvious signal at 2kHz, though as it’s 111dB down (0.00028%) it would not be audible. 

The effect of the Bass Eq control is shown in Graph 7. You can see that it delivers a 3dB boost or cut from 20Hz up to 100Hz, after which the boost or cut rapidly diminishes, so there’s basically no effect on the response above 300Hz.

The effect of the Tilt controls is shown in Graph 8. Although at first glance the boost and cut effect seems to be the same as standard bass and treble controls, if you look closely you can see quite a few differences.

Firstly, maximum boost and cut is restricted to around 3dB, whereas tone controls usually offer around 8–12dB. Second, the frequency where there’s no boost or cut at all when either of the controls is used is somewhat lower than usual, at 780Hz (it’s usually at 1kHz). Finally, the range of frequencies that is boosted (or cut) is somewhat wider than is usual with tone controls, both in the bass and treble regions.

Signal-to-noise ratios were measured at 81dB unweighted and 91dB A-weighted referenced to one-watt output, and at 94dB unweighted and 100dB A-weighted referenced to rated output. Although these are good results, the reason they’re not better is because of the presence of high-frequency switching noise from the Class-D output stage. Although this noise is so high in frequency that it is not audible, it affects the measurements.

The switching noise certainly affects the square wave measurements, so for the square waves shown with this report, Newport Test Labs used a low-pass filter to remove it. The 100Hz wave shows a degree of tilt, indicating that the Bel Canto ACI 600’s low-frequency response rolls off at low frequencies and does not extend to d.c., but there’s no bending, so there’s no phase shift at low frequencies.

The 1kHz square wave shows a slight rise-time limitation, plus an overshoot that would suggest a lift in the frequency response at some ultrasonic frequency.

The 10kHz square wave is of course affected by the presence of the low-pass filter used by the lab, but you can see the rise-time limitation, the high-frequency response roll-off, and the ultrasonic lift. I have included a square wave of a the same 10kHz wave without the filter in place, and you can see that the high-frequency switching noise obscures the information the square wave test could otherwise reveal.

The square wave showing the Bel Canto ACI 600’s performance into a highly capacitative load shows that the amplifier it will be unconditionally stable when driving highly reactive loads, such as electrostatic loudspeakers.

The performance of Class-D amplifiers continues to improve apace and now mostly rivals or exceeds that of linear amplifiers in almost all areas. The amplifiers inside the ACI 600 are exceptionally good examples of Class-D engineering.  # Steve Holding