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Stack Link II review

  • I first jumped aboard the Roon train back in 2015: music streaming software that levelled the playing field for manufacturers, allowing hi-fi product designers to do their job without being hamstrung by the software side, an area that was – and still is – the weakest link in many a streaming product. And being an avid Roon user limits my selection of network streamers to those that are Roon-capable. It’s a sacrifice that’s no sacrifice at all when we consider that Roon will talk to (its own) Roon Ready code sitting running inside a network streamer, as well as Apple’s AirPlay, Google’s Chromecast, Squeezelite and Sonos.

    Pre-Roon, choosing a streaming playback device involved a somewhat complex set of decisions. Beyond the obvious such as price and connectivity, there were some core functionality aspects to work out. Did I want a device to leverage existing protocols such as UPnP or Logitech Media Server? Or a more proprietary solution like an Aurender? How about a Mac Mini running Audirvana, or a Windows machine using JRiver Media Center, or an MPD-based Linux box? I’ve been through all of these and more, each bringing its own unique strengths and weaknesses to the table.

    Nowadays the biggest variable is whether I want a headless device, solely controlled via tablet/laptop/smartphone, or something with a display and perhaps its own set of front-panel controls. Once that’s sorted, I look at more standard considerations like price and output options to seal the deal.

    Under consideration here is the (Roon Ready) Stack Audio Link Streamer II. It sells for US$1355.08, the exchange rate likely responsible for the lack of a nice round number. The Link II is a dedicated streaming audio transport, designed from the ground up to deliver pure audio signals free of unwanted distortion. Proprietor Theo Stack is clearly an analog guy, with Stack Audio known for its upgrade kits to the iconic Linn LP12 turntable. He has a strong background in the principles of chassis damping and vibration control. On the streaming side, Stack enlisted the services of noted digital guru John Westlake, a man responsible for highly-regarded digital gear from Audiolab, Peachtree Audio and Cambridge Audio. Together, the duo makes a very credible attempt at a state-of-the-art streaming transport without the stratospheric price tag often accompanying such devices.

    Design and Build
    The Stack Audio Link II’s asymmetrically curved chassis design measures 174mm wide by 140mm deep and is just 25mm tall. That’s roughly the same footprint as a pair of Blu-ray cases stacked on their sides (no pun intended) for easy physical integration into most systems. The hefty-for-its-size enclosure is CNC machined from a solid block of aluminum and lined with a special EMI absorbing material to keep electrical noise at bay. A proprietary “AVDC vibration trap” is incorporated – a technique borrowed from the firm’s Soprano LP12 baseboard upgrade – featuring (what appear to be) cork-board feet underneath the unit. All this lends the unit an extremely robust and sturdy feel in hand despite its compact dimensions.

    I knew the Link II was small but didn’t truly wrap my brain around it until the review sample arrived. Pictures do not quite do it justice, both in terms of beauty nor diminutive size. Here the name “Stack” Audio seems fitting, as the Link II will comfortably fit stacked atop most of my audio gear. My one complaint concerns a form-factor mismatch with the external power supply.

    That linear power supply is called Volt and was formerly an optional extra. Now it ships with the Link II as standard. Stack also sells the Volt separately with different output voltage options, suitable for upgrading the Roon Nucleus or various Chord DACs. I find the Volt to be attractive and well-built but with a different aesthetic to the Link II itself. Seeing them together is akin to having an older flagship DAC from Japan’s Esoteric Audio – all blocky and tank-link – sat next to a modern swoopy Esoteric device.

    The Link II has what I consider a generous set of inputs/outputs for a compact headless streamer. My standard-bearer here is the venerable Sonore microRendu, which is about as simple as can be: Ethernet in and USB out. Meanwhile, Stack Audio has the same USB output but gives us a choice of Ethernet or WiFi, with no discernably audible penalty when choosing the latter. It also gives us an HDMI port and a pair of USB type A inputs (one hidden on the front panel), currently not active and described by the manual as intended for “future use”.

    More intriguing is the USB type B input which can be used to “Digitally Detox” third-party devices. This lets one use the Link II as a sort of USB decrapifier, cleaning up signals as they travel from a different transport to a DAC. It’s the same process that happens internally when using the Link II as a source, but in this scenario, we might be using a separate PC or Mac or network streaming device instead. I’ll discuss the ramifications of this in a moment.

    Lastly, a switch allows users to deactivate power from the USB output. Many DACs demand power to be carried via USB, either for operation or at least for the initial handshake. But some do not. In those cases, there may be a theoretical benefit to cauterising an unnecessary power feed. That said, mandatory inclusion of the Volt PSU makes this less useful, as the power situation is already well-sorted compared to the original stock wall-wart.

    System hardware
    The Link II was paired with several DACs, each coming from different price categories: an original Schiit Bifrost (US$449 nearly a decade ago), a Keces S3 (US$1549), a Yulong Audio DA1 (US$4100), and a Resonessence Labs Invicta Mirus Pro Signature (US$8000). Headphones amplification was handled mainly by a Pass Labs HPA-1 (US$3,500) with occasional detours into tube goodness via the Cayin HA-1A mk2 (US$899). Headphones included the Meze Elite and Empyrean (US$4000 and $3000 respectively), Audeze LCD-2 (US$999), Kennerton Audio Thekk (US$3k), Campfire Audio Cascade (US$799), and the 64 Audio A18t custom in-ear monitors ($3000). As usual, balanced power came from an Equi=core 1800, whilst all cabling came from Audio Art.

    Note that while the Link II can be set to function in a variety of modes including Squeezelite, HQPlayer NAA, UPnP/DLNA, Librespot (for Spotify streaming), and Shairport Sync (for Airplay), I used it solely as a Roon Ready endpoint in my system. These other functions can be easily accessed by logging into the Stack Audio web interface through your browser of choice. I can’t comment on how well the other streaming inputs perform as I am fully immersed in the Roon ecosystem, but the sonic performance I experienced should apply similarly across the board.

    Evaluating the sonic performance of streaming audio ‘transports’ is a tricky business. They don’t have a sound on their own. Rather, they impact the sound spilling from the downstream DAC. It’s why I had the Link II take turns with four different DACs of varying cost, vintage, and performance. As we’ll soon see, using a single DAC can lead to incomplete conclusions.

    My baseline for comparison was a Dell Micro PC with an Intel i5 processor, running Windows 10 and configured as a Roon bridge. I augmented this little computer with an iFi DC iPurifier2 to clean up incoming power, plus an iFi iSilencer for filtering on the USB output. Many folks I know use Intel’s NUC as a source, often running some sort of upgraded power supply, and this mildly-upgraded Dell Micro sits roughly on the same level.

    When using the 1st-gen Schiit Bifrost, the differences in performance were rather stark. The Dell transport made it sound washed out, thin, and gave an overall “grayish” tone that made music not as engaging as the Link II. Note: this was Schiit’s original USB implementation. One that even they admitted was inferior to S/PDIF. Switching to the Link II had instruments snapping into focus, delivering a welcome dose of richness.

    Switching to the Keces S3, which features a modern XMOS USB solution and an ESS Sabre chip, I wasn’t expecting as large of a performance delta between the Dell and Stack Audio’s streamer. Isn’t the patented ESS Hyperstream processing supposed to suppress jitter anyway, and thus minimize/eliminate transport differences? Then again, jitter isn’t the only unwelcome aspect in a signal, and the Link II is surely more focused in this regard than my modestly-upgraded Dell. Whatever the mechanism, I was surprised to hear another substantial difference between the Dell and the Link II.

    The Keces S3 is an articulate DAC that digs deeper than the Bifrost’s warm-ish signature, and the Link II teased out a significantly more nuanced, delicate performance. Layering and separation were clearly superior as heard through any of my headphones but most obvious through the Meze Elite. The Link II’s quieter background exposed more explosive dynamics, whilst the low-end went deeper and with more authority. A night and day difference? Not exactly, but anyone familiar enough with the Keces unit would pretty easily spot the improvement.

    From Schiit to Keces, we’ve already tripled our DAC budget, and increased our sonic dividends along the way. What happens if we 3x that DAC spend? That leads us to Yulong’s superb DA1, complete with proprietary FPGA processing, latest-gen AKM D/A chip, and massive outboard power supply. The Yulong is an entirely higher class of device, in both price and performance, so it should be even more capable of identifying differences between the Link II and the lowly Dell transport. Right?

    Not so fast. This is a prime example of my earlier statement that one DAC/streamer pairing isn’t enough to flesh out the complete picture. Yulong’s 4th-gen Jitter and Interface Control (or JIC for short) is among the most comprehensive data marshalling solutions out there. To quote my own Darko.Audio review of Yulong’s DA Art Aquila II, which uses a version of the same JIC system: “This means USB and SPDIF signal selection, demodulation, FIFO buffering, femtosecond clock PLL synchronization, and outgoing signal buffering are all done β€œin-house” before the signal is handed off…” with the end result being a sort of cooling effect in terms of transport quality demands.

    In short, I notice zero difference between the Dell and the Link II with the Yulong in play. Even removing the Dell’s iFi augmentation brings no appreciable change. If I, as a DA1 user, had just spent four figures adding the Link II streamer to my rig, I would be sorely disappointed. This is no fault of Stack Audio but rather an inherent property of Yulong’s JIC design – a great reminder that one size does not fit all when it comes to audio equipment upgrades.

    Lastly, I went big with the Resonessence Labs Mirus Pro Signature Edition. Though Resonessence has now sadly folded, this DAC is still among the absolute best available. Will a high-end DAC expose more upstream differences (like the Schiit and Keces) or will it nullify them (like the Yulong)?

    I’ve paired the Mirus Pro Signature with very expensive disc-based transports from the likes of Mark Levinson, EMM Labs, McIntosh, Esoteric, Metronome, and Accuphase, plus file-based players from Innuos, SOtM, Aurender, Linn, Auralic, Naim, and many more.

    Despite being significantly more affordable than most on that list, I’d place the Stack Audio Link II close to the very top of the heap. It unleashes the clarity and transparency I’ve come to expect from my reference DAC, without the somewhat thin, wispy feel I hear from the Dell. Most tellingly, this is one of the few times an external transport has come close to matching the Resonessence Labs’ built-in SD card playback function.
    Tonal firmness is definitely well-represented, as are bass weight and dynamic punch. How about midrange fluidity? Holographic imaging? Micro-detail retrieval? Yes, yes, yes, yes, yes and yes.

    Digital Detox
    This brings us to the Digital Detox function of the Link II. At face value, I wasn’t really sure why such a thing would be useful. Don’t we already have excellent network audio playback capabilities baked-in? What would an external device bring to the table? Aren’t we just adding unnecessary complexity to the signal path?

    Out came an old MacBook Air. Leashed to the Link II via USB, and playing Tidal or Idagio or Apple Music through their native apps, the sonic prowess of DACs connected to this general-purpose laptop was elevated to near parity with the Link II. Schiit’s Bifrost could no longer split the difference between the Link II and the Digitally-Detoxed MacBook Air, and, as before, the Yulong DA1 didn’t care one bit. Using the Keces S3, I heard just a hint of improved resolution sans MacBook, mostly during very careful listening. Only the uber articulate Resonessence Labs machine split the difference more clearly: that the detoxed MacBook Air was impressive but the Link II proved a shade better.

    The Stack Audio Link II network bridge does not attempt to be all things to all people. It seems strongest when leveraging the killer user interface of Roon, though alternate modes exist for those who haven’t converted to Roon (yet). It isn’t a music server with massive onboard storage. It won’t rip CDs. It lacks any sort of display, and it is limited to USB output. Although even that could be rectified with a decent USB-S/PDIF converter.

    In exchange for those limitations, it offers subjectively stunning good looks and objectively robust build quality. More importantly, it gives some of the most satisfying sound quality I’ve heard in my system. And it reminds us that some high-end DACs – like the Resonessence Labs (but not the Yulong) – can benefit from better streaming source quality. Factor in the impressive Digital Detox functionality which permits integration of less capable streaming hardware and we have streaming Future-Fi writ large.

    Further information: Stack Audio

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    Written by John Grandberg

    John Grandberg is a US-based audio journalist who has been immersed in the scene for over a decade. A recovering percussionist, he has a particular affinity for headphones and associated gear.

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