Stack Audio Link II Network Bridge/Streamer/Reclocker

The world of streaming audio is a fast-moving place to be; what was competitive last year is old hat this year so it helps to be young and quick to adapt. Theo Stack is young by audio industry standards and has proved himself to be very adaptable. He originally hit the scene in 2015 when the first Stack Audio prototype was unveiled at the Bristol show. Called the Onset, it was a streaming bridge with dCS-specific connectivity in a very stylish machined aluminium case. Sadly that project never came to fruition and Theo had to diversify, creating the Serene LP12 upgrade package of high-grade metalwork for the venerable Linn Sondek that provided him with sufficient funds to continue working on a streaming product. The original Stack Link was launched in 2019, it’s a network bridge which is essentially a streamer without an onboard DAC that connects to either a network (wired and wireless) or a PC via USB where it acts as a re-clocker, it sends out a digital signal on USB to the digital to analogue converter of your choice.

The Link II followed a year later – I told you it’s a fast-moving sector – and inhabits the same casework but with upgraded electronics and totally new software. The first Link ran on an open-source OS made by Italian company Volumio, it worked well enough but the control interface was via a browser and a little clunky by the standards of modern apps. It was possible to use third party UPnP apps such as Bubble but Theo decided that the software side could be improved and incorporated an OS that’s designed to work with Roon and Audirvana alongside UPnP/DLNA controllers. The benefit of the first two options is that they do the lion’s share of the processing leaving the Link II with a relatively easy job and that means higher performance. In both Roon and Audirvana the control software is run on an external processor, often in a PC albeit Roon cores can be found in Innuos servers and the Nucleus device that Roon produces. In both instances the Link II is seen as an endpoint and can be controlled by an appropriate app.

The physical change in Link II is a new clock that produces half the phase noise of its predecessor, you might not think that is all that significant but that would be to forget that noise is the enemy when it comes to digital audio. Less noise means more signal, a situation where less really is more.

The Link II is a compact device that’s under seven inches wide and weighs a kilo, but it is beautifully formed in machined aluminium. The casework is of a quality you don’t usually encounter in products costing less than a few thousand Pounds, Dollars or Euros. It has a good range of connectivity given its inch thick back panel, with USB in- and outputs, an RJ45 socket for ethernet and a second USB A, I had imagined that the latter was for accessing a USB drive but that’s not the case, at present its earmarked for ‘future use’. There is a USB for thumb drives on the front however, this is hidden behind a lozenge shaped panel that is magnetically held in place. There’s also an HDMI connector on the Link II, this is for the intrepid enthusiast who want to flash (install) different OS software onto the device.

There is only one control on the back of the Link II in the form of a switch between the USB A sockets and the power inlet, this cuts off the 5V line on the USB connection in order to reduce noise. As far as I’m aware Stack is the only company offering this feature but that’s because there aren’t many DACs that run happily without the signal that this voltage provides, which they use to confirm the connection rather than to power the USB input board. There are two buttons on the front, one for power and the other to run detox mode, which cleans up noise on the USB input.

The 18 volts that this bridge runs on is provided by a switched mode plug top supply but those after greater results can upgrade with the Link Linear PSU which comes in a similarly high quality bit of casework to the Link II. This is a rather more chunky device but it’s still a one hand lift and comes with a good quality connector cable, if you buy one with a Link II there is currently a £50 saving.

The first DAC I partnered with the Link II was AURALiC’s rather good VEGA G2.1 reviewed last month (issue 191), it took the place of an AURALiC ARIA G1 and sounded notably leaner than that significantly more spendy streamer (more than twice the price). However it didn’t take long to get used to the Link II’s balance, in fact the second piece I played seemed to have plenty of body, but there were occasional tracks where a bit more warmth in what is essentially the lower midrange would have been nice. But soon the coherence and ease of this streamer started to make itself felt, it’s surprisingly revealing of detail and very good at presenting it in a musically engaging manner. Herbie Hancock’s take on ‘It Ain’t Necessarily So’ [Gershwin’s World, Verve] had good weight albeit the treble was open and sparkling if a little exposed. I switched the Vega G2.1 to its smooth filter setting (from the starker ‘balanced’) and things relaxed rather nicely, letting the great pace of ‘Jackie Wilson Said’ [Van Morrison, Saint Dominic’s Preview, Warner Bros] come through, the brass section giving it energy and presence. Other recordings revealed strong image depth and really muscular bass, the lack of warmth giving the low bass more impact because it’s delivered with such precision.

It’s interesting that the bass is on the one hand seemingly tighter and apparently lighter with the Link II yet that it has serious power when genuinely low notes come along. It certainly seems well-defined and articulate and this makes for strong timing. Switching to the Linear power supply refines the presentation, dropping noise and removing a slight eagerness in the treble. It meant that I could go back to the more precise ‘balance’ filter on the VEGA G2.1 without revealing anything untoward, and this made the music that much more snappy and exciting with it. Patricia Barber’s ‘Bye Bye Blackbird’ [Nightclub, Premonition] sounded really remarkable, the vocals in particular are so well recorded that it makes for a very real musical experience. But switching sample rates proved to be the undoing of the Stack/Auralic partnership, the change causing the DAC to deliver noise instead of music and only rebooting would overcome this.

Which meant I could try the Link II with a DAC more appropriate to its price point, this took the form of an iFi Pro iDSD. This was also interesting because it seemed to work with the 5V on the USB line switched off, something that did remarkable things for the sound. I was playing London Grammar’s ‘Hey Now’ [If You Wait, Metal & Dust] and finding it a bit humdrum, cutting the 5V gave it so much emotional impact that I almost let me lip quiver. Sadly a few tracks later the DAC had had enough and wouldn’t play ball so the power had to go back on, this was hardly disastrous however and I continued to use this combination for several weeks, enjoying every moment of it, especially when I upgraded the power amp to a Moor Amps Angel 6 and discovered that it was delivering even more than had previously seemed the case.