Sound Impressions
I’ve been testing the RU9 for the past week – following a mandatory 100-hour burn-in period – with a range of test tracks, numerous IEMs, and a couple of newly-purchased headphones. The testing also involved brief comparisons with other dongles I have on hand, but also some higher-end DAPs, the latter of which I’ll only mention briefly because, in truth, that’s not a fair fight.
The RU9’s sonic character varies significantly depending on the selected mode, making it adaptable to different musical preferences and headphone pairings. In solid-state mode, the device delivers clean, accurate reproduction with minimal colouration. The 4493 DAC implementation provides competent digital-to-analog conversion, though listeners accustomed to higher-end DAC chips may notice some hardness in the upper midrange and treble frequencies, belying the DACs’ digital delta-sigma nature.
Modern tube mode emerges as the most versatile option for many listeners, myself included. The Korg NuTube processing adds subtle harmonic enrichment that helps soften the DAC’s inherent character while maintaining good technical performance. Soundstage presentation remains spacious, and detail retrieval stays strong while gaining musical engagement. This mode successfully bridges the gap between analytical accuracy and musical enjoyment.
Classic mode provides the most pronounced tube colouration, delivering warm, intimate sound that emphasises musicality over technical precision. The increased harmonic distortion creates a more enveloping presentation, though it comes at the cost of some soundstage width due to increased channel crosstalk. This mode works particularly well with genres like jazz, classical, and acoustic music where the warm presentation enhances the listening experience.
For a relatively compact device, the RU9 demonstrates impressive headphone driving capability. That said, it’s still better-suited to IEMs, in my opinion, providing clean, quiet operation with excellent noise performance even with highly sensitive models. Its transparency is such that it allows different models to express their individual characteristics clearly, rather than colourimng the sound to a uniform.
Full-size headphone performance varies with the selected power mode. In battery operation, the RU9 handles moderately efficient headphones competently, though demanding sets like Sennheiser’s classic flagship HD800S benefit from the additional power available in DC mode. The three-decibel increase in output level when using external power brings noticeable improvements in dynamics and bass control in this instance.
Balanced output via the 4.4mm connection provides the best overall performance, with improved channel separation and lower noise floor compared to the single-ended output. This optimisation toward balanced operation aligns with modern headphone trends and provides the cleanest possible signal path.
The digital filter options offer subtle but meaningful adjustments to the sound character. Different filter implementations affect phase response and transient behavior, allowing fine-tuning of the presentation to match personal preferences or specific recordings. While these changes are subtle, they demonstrate Cayin’s attention to providing user customisation options, with the Super Slow option my personal taste for the type of music I listen to.
It’s worth noting that while the RU9 delivers excellent sound quality for its price point and form factor, it doesn’t quite reach the sonic heights of dedicated digital audio players or more expensive, less portable DAC/amplifier combinations. The constraints of size, power consumption, and cost inevitably mean some compromises in the DAC implementation and analog circuitry.
Users stepping up from basic dongles will be thrilled with the improvement, but those accustomed to high-end desktop gear or premium DAPs might notice the limitations.
As I mentioned earlier, the entry-level DAC, while competent, lacks the refinement of more expensive chips found in reference-grade equipment. This isn’t a failing so much as a reality of the portable audio world – physics and economics don’t disappear just because we want them to.
Wired vs wireless
A quick note on the ever-divisive debate between wired and wireless sound quality. As expected, despite the solid LDAC connection to my LG V60 smartphone, sound quality for critical listening is simply better with a wire. Bass has more punch and texture, the soundstage is notably expanded, and finer details are easier to discern. This is especially true when using hi-res digital files or DSDs, where the deficit – and subsequent compression – of Bluetooth audio is most apparent.
That said, sound quality on the go is still excellent with LDAC (and any of the higher resolution AptX codecs). If you’re using serious IEMs or headphones that cost orders of magnitude more than the RU9 itself, I strongly suggest sticking to sources that support hi-res Bluetooth, else you may as well stream Spotify over SBC or AAC and stick to relatively cheap IEMs where the loss in quality will be far less noticeable.
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