Packaging and accessories
The RU9’s presentation is a spartan affair. Inside a simple box featuring understated artwork on the outer cover you’ll find a black presentation case, with the RU9 seated in a precisely-cut protective foam shelf.
Under the shelf, Cayin includes a collection of accessories alongside documentation that, while covering most of the essential operational details, is printed really, really small for my ageing eyes.
The included protective leather case is really nice, and goes beyond simple scratch protection. For example, it has embedded magnets on the rear surface and a metallic plate on the front that strengthens the MagSafe connection when paired with an iPhone. This also allows small MagSafe power banks to attach directly to the case for convenient charging via short USB-C cables. Additionally, Cayin provides a magnetic attachment ring for smartphones lacking built-in wireless charging or MagSafe, or for use with DAPs.
In use, the case doesn’t block any important functions, with precise cutouts providing access to all connection ports at the bottom, side function buttons, the display screen, and tube and antenna windows. Additional accessories include practical connectivity solutions: a USB-C to USB-A adapter and a well-designed (but flimsy-feeling) right-angle USB-C to USB-C cable.
Design and features
The RU9’s physical presence immediately sets it apart from conventional portable DACs – and even dongles. In fact, it’s a bit of a stretch calling this a dongle, in the traditional sense of the word.
Measuring approximately 10cm in length, 7cm in width, and 1.5cm in thickness, it’s rather outsized compared to typical dongle-style devices. However, this larger footprint serves a purpose, housing the vacuum tubes, amplification circuitry, and battery while maintaining relatively low weight.
Build quality on the whole is very good, despite the lightweight (150g) construction and barebones styling. The rectangular form factor channels classic portable device aesthetics while incorporating modern design elements.
When paired with standard-sized smartphones, the device’s width closely matches typical phone cases while extending approximately two-thirds of the phone’s length, allowing discreet mounting that preserves camera functionality without obstruction.
The front panel features the Korg NuTubes behind a transparent window, creating an appealing visual element that serves as both functional component and design accent. The display, while admittedly small, provides essential information about volume levels, input sources, and operational modes.
Actually, calling the RU9’s aesthetic modern might be too generous. The device looks like it escaped from the bridge of the USS Enterprise circa 1975 – all angular lines, utilitarian button placement, and that conspicuous tube window that screams ‘I am a serious audio device’ rather than whispering it elegantly.
While this retro-futuristic design certainly makes a statement, it’s more science fiction prop than sleek contemporary gadget. The overall appearance suggests that Cayin’s design team was more concerned with functionality than fashion, which isn’t necessarily wrong, but it does make the RU9 stand out in ways that might not always be flattering.
The display deserves particular mention for being both useful and frustratingly inadequate. While it provides essential operational information, the tiny screen seems designed for users with the visual acuity of a hawk.
Reading the display requires either excellent eyesight or the willingness to bring the device uncomfortably close to your face – a design choice that feels particularly puzzling given the device’s otherwise user-focused approach. It’s functional enough to get the job done, but you might find yourself squinting more than you’d like.
Practical design elements enhance daily usability significantly. The top panel features a tinted wireless antenna window that prevents signal interference while maintaining structural integrity. Control placement follows logical patterns, with three identically sized transport buttons positioned on the left side offering tactile feedback for skip-forward, skip-backward, and play/pause functions.
The right side houses power management and mode selection controls. The power button provides dual functionality through long-press activation/deactivation and short-press display control. Below sits a multicolor status indicator displaying charging status in red and various sampling rates through different color codes. The menu/timbre button cycles through operational modes with single presses or accesses configuration settings through extended activation.
The bottom panel maximises connectivity options in the available space. Balanced 4.4mm and single-ended 3.5mm outputs support both headphone and line output modes with selectable gain settings. Separate USB-C ports handle audio input and power delivery respectively, with color-coded identification simplifying connection management.
A combined S/PDIF input/output port expands digital connectivity options, while a battery/DC mode switch enables external power operation for enhanced performance capability.
Lastly, the volume control, implemented as a rotary encoder, proves less successful. Its operation can be inconsistent, particularly during rapid adjustments, and the lack of display activation during volume changes creates usability challenges.
One intended but understated feature is the RU9’s ability to act as a digital-to-digital converter (DDC). Desktop aficionados will know that this type of device is used to ‘clean’ a ‘dirty’ USB signal, from a PC for instance, and ‘filter it’ through a second and less noise-prone interface like S/PDIF. As it happens, RU9’s coaxial S/PDIF output (and input) port makes it possible to take the incoming USB signal and output it (via S/PDIF) to an external DAC.
Another cool ‘desktop’ feature is battery bypass, made possible by connecting the RU9 to an external power source (including a battery bank) to completely bypass the battery and power the amplification and digital circuits directly. This has the ancillary function of pushing balanced power output beyond the 1kW mark for harder-to-drive IEMs and less sensitive headphones.
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