I’d like to extend my thanks to Cayin’s John Huang for reaching out and sending us a sample RU9 for this review. Your support is greatly appreciated.
Introduction
The portable audio smorgasbord has diversified dramatically over the past few years, transforming from simple headphone adapters to sophisticated DAC/amplifier combinations that rival desktop equipment.
As smartphones abandoned the headphone jack, audio enthusiasts found themselves navigating an increasingly complex world of dongles, adapters, and portable solutions. Into this crowded marketplace steps the $499 Cayin RU9, a device that challenges conventional thinking about what portable audio can be.
Cayin has built a reputation for innovative audio solutions, from their acclaimed N-series digital audio players to their respected amplifier lineup. With the RU9, they’ve created something entirely different: a magnetic tube-powered DAC/amplifier that literally attaches to your smartphone. This isn’t just another portable audio device – it’s a reimagining of how we interact with high-quality sound on the go.
The RU9 represents a fascinating intersection of old and new technologies. At its heart lies a genuine vacuum tube, specifically Korg NuTube, that provides authentic tube harmonics without the power consumption and heat generation of traditional vacuum tubes. This is wrapped in a modern package featuring MagSafe compatibility, dual USB-C connectivity, and multiple sound signatures that cater to different listening preferences.
What makes the RU9 particularly intriguing is its approach to the fundamental compromises inherent in portable audio. Rather than pursuing extreme miniaturisation or maximum battery life, Cayin has prioritised user experience and sound quality, creating a device that feels more like a desktop component that happens to be portable rather than a traditional dongle with ambitions.
Cayin’s dongle evolution
Before diving into the RU9’s specifics, it’s worth contextualising this device within Cayin’s broader portable audio journey. The RU9 doesn’t exist in isolation; it represents the latest chapter in a fascinating story of technological innovation that has consistently challenged what’s possible in dongle-sized devices.
It all started with the RU6 in 2021, a device that seemed almost impossible on paper. R-2R DAC technology had long been the domain of hefty desktop units, requiring substantial circuit boards laden with precision resistors. The notion of cramming such complexity into a pocket-friendly form factor bordered on the absurd.
Yet Cayin managed exactly that, developing their own 24-bit discrete resistor ladder implementation that delivered authentic R-2R sound characteristics in a package you could actually carry around. Supporting PCM up to 384kHz whilst maintaining isolated digital and analogue sections, the RU6 proved that revolutionary audio technology needn’t be anchored to your desk.
The RU7 followed in 2023, taking a completely different technological approach with its 1-bit architecture. Drawing inspiration from Cayin’s flagship N7 digital audio player, this device featured fully balanced discrete networks built around 128 high-precision thin-film resistors.
RU7’s All-to-DSD processing philosophy converted every digital signal to DSD format before playback, bringing native DSD decoding capabilities to the portable realm. Where the RU6 had conquered R-2R miniaturisation, the RU7 demonstrated that sophisticated 1-bit processing could be equally portable.
Now the RU9 continues this pattern of technological firsts, introducing vacuum tube processing to the ‘dongle’ format. Each device in the series has tackled a different aspect of high-end audio reproduction – discrete R-2R, 1-bit DSD, and now tube harmonics – proving that Cayin’s commitment to innovation extends far beyond simple specification improvements.
This progression reveals a company unafraid to experiment with unconventional approaches to portable audio. Rather than following established dongle formulas, Cayin has consistently asked “what if we could make this portable?” The results have been devices that feel more like technological demonstrations than mere accessories, each pushing the boundaries of what’s achievable in increasingly compact packages.
I for one am, very keen to see what they come up with next. An R2R/1-bit tube DAC/amp dongle with WiFi support, anyone?
Continue to packaging and accessories…
