Frequency Response –
Testing Methodology: RMAA via Startech External Sound Card
The RU6 has a linear frequency response suggesting that it represents audio with great fidelity. Due to the quality of my sound card, I am unable to reliably test other measures such as distortion and crosstalk so they will be used as a personal reference only. Qualities here can also impact the sound as I will detail via subjective listening.
Output Impedance & Hiss –
Testing Methodology: SPL volume matched comparison through an inline splitter to THX789 + SMS
The RU6 was designed to provide a low noise floor despite the new DAC design and its amp section was also designed with IEMs in mind. The single-ended output offers a low 0.5-Ohm output impedance that is doubled to a still respectable 1-Ohm through the balanced out. Listening to the Campfire Audio Ara with its mechanical crossover and ability to discern between 1-Ohm and sub-1-Ohm sources revealed an expected result – the RU6’s single-ended output provided a slightly fuller and smoother sound. Even the most source sensitive IEMs will pair well with the RU6 as a result and, as the balanced output is still only 1-Ohm, you can happily pair this with IEMs as low as 8 Ohms in impedance and as low as 4-Ohms through the single-ended output.
As far as noise is concerned, the RU6 does produce a noticeable background hiss above many modern portable sources, even in low gain. I also noticed that OS mode produced a slightly more prominent noise than NOS mode for those that are especially particular. Do note that these comments are relative to most competitors that offer a jet black noise floor in all scenarios, so the RU6 is still performing acceptably here with hiss only being noticeable on the most sensitive IEMs such as the Andromeda 2018 and Ara. However, no other IEMs in my collection presented issues, even the 6.4 Ohm Fir M4 produced no perceptible background hiss. You also won’t have any issues with dynamic driver IEMs either. Unless you have an especially sensitive IEM, this is unlikely to be an issue even at low listening volumes but it is something to be noted for the purpose of critical evaluation.
Testing Methodology: SPL volume matched comparison through an inline splitter to THX789 + Topping D70S to Soft Ears RS10 (flat impedance). Powered by Shanling M2X.
Those expecting the RU6 to deliver sound in a typical warm, analogue fashion that one may associate with a tube amplifier may be disappointed. From my empirical experience, any linearly measuring DAC should deliver a more consistent sound than that between amplifiers. Of course, there are differences between them making DACs a valuable avenue for upgrading and fine-tuning an audio setup. The RU6 does just this, balancing ample transparency with a well-bodied, coherent and natural note delivery with no offensive qualities. It does indeed differ slightly from your typical sigma-delta based dongle in terms of note presentation without sounding overtly tonally coloured. It also has a highly impressive amp section for such a compact source in addition to delivering an uncharacteristically wide soundstage. Cayin has been in the portable source game for a long time and it has enabled them to execute this unorthodox design with aplomb.
NOS vs OS
I can only speak from a subjective standpoint and I found my impressions to align well with those already online. I should note as well that the two modes don’t differ hugely, delivering noticeable but hardly transformative changes. I found NOS mode delivered a slightly warmer, fuller and wider sound with a very linear high-frequency delivery. By comparison, OS mode had a bit more energy, etch and presence in the highs but also sounded a bit more brittle. Lows were cleaner and more linear. I didn’t find the soundstage in OS mode to be quite as wide but offered a sharper centre image. I could see listeners preferring either mode depending on pairings, but personally preferred the wider, smoother sound provided by NOS mode. As such, I will use this mode for testing below.
The RU6 really isn’t overly coloured in the bass but has a pleasant, punchy character alongside a slightly elongated decay – a character that is consistent throughout. It boasts impressive dynamics and extension, with a well-weighted sub-bass that occupies a balanced position. Above is where you’ll hear hints of colouration in the form of a lightly enhanced mid-bass that instigates its punchy presentation. Notes are a touch warm and full but not exceedingly so, and the note structure isn’t rounded or bloated in turn. The RU6 offers a surprisingly assertive attack.
Though not on the level of a desktop source or larger DAP, it certainly offers greater authority than most competing dongles. Interestingly, decay is on the slower side which contributes towards a smooth yet well-textured presentation. Combined with the uptick of fullness, timing and separation are lesser qualities of this source if not something that stands out as an immediate weakness. Where the RU6 excels is with regards to its ear-pleasing tonality that works in tandems with textured and slightly drawn-out notes that enhances the perception of body without upsetting the tonal balance.
The RU6 has a slightly smooth articulation and a laid-back upper midrange but, overall, upholds a highly natural and well-balanced image. It carries over a light warmth from the bass but not too much fuzz or bloom so as to uphold respectable definition and separation. In addition, though slightly full-bodied, it isn’t defined by either this or warmth, simply carrying over a hint of both. The net result is a coherent and well-structured presentation with enlarged yet wholly resolved notes. Vocals are highly natural, and the presentation is coloured to a very tasteful extent that upholds genre versatility. Clarity is just sound though wouldn’t appeal to lovers of a clean, reference tonality.
The highlight of the presentation is surely its staging capabilities, being impressively layered and wide. However, though the RU6 has decent extension up top, this is not its outstanding feature. Chiefly, it is a combination of its laid-back tonality and smoothly presented notes. Though somewhat esoteric, the best description I can provide is the manner in which notes fade in and out of the stage in a progressive manner. This was clearly noticeable to me on the vocal harmonisations during the chorus of Daft Punk’s “Get Lucky” around the 50-second mark. These qualities grant the RU6 a sweet, musical and forgiving nature whilst maintaining strong but not market-leading resolving power and clarity.
Analogue descriptors shouldn’t suggest that the RU6 is blunted or ill-defined in the high-end though it does carry some of these qualities. Chiefly, the source provides an even, just slightly laid-back lower-treble that retains good detail presence and crispness followed by a darker middle treble that paves the way for a clean and immaculate black background. I do personally find this style of tuning very appealing as you get a pleasantly focused and detail-dense foreground with zero intensity or glare. Instruments are well-bodied and carry a very convincing texture and timbre. Compared to my benchmark desktop stack and even some competitors like the Hidizs S9 Pro, the initial leading edge of notes is a smidge smoother. This gives it a slightly more organic nature as notes are well-resolved but lack sharpness or grain entirely.
Though a touch smoother, this is not for lack of presence, clarity or separation, all of which perform at a good level even if they aren’t exactly highlighted to the listener. Above there may be more contention as the RU6 isn’t the airiest source and highs can sound slightly over-damped at times with a slightly truncated decay due to the darkness of its background. The OS mode does provide a little more air and shimmer for those desiring it but I wouldn’t consider it to be a perfectly linear performer in either setting. The RU6 does, however, extend impressively which permits respectable headroom with minimal glare or brightness. This is a clean, and well-contrasted performer with plenty of character and no lack of detail even if this isn’t its defining trait.
The RU6’s smooth note delivery works wonders for its soundstage in addition to its raw sense of space being far greater than the class average. Its dark background and smoothly tapered periphery almost appear to exacerbate its sense of space. Furthermore, imaging appears more multi-dimensional if a little floatier than competing sources. I also found the RU6 to push slightly laterally compared to my D-S sources, yet whilst maintaining a solid centre-image. Separation is surprisingly good as notes aren’t overly warm or enlarged combined with the larger stage dimensions. The ether surrounding each element is palpable and notes have a nice sense of pop atop the dark background. Though note definition and clarity aren’t huge, this means the RU6 showcases what small details it does resolve to the listener very well.
Driving Power –
Though the star of the show is the R-2R DAC, the RU6 provides admiral driving power from its amp section that is surely playing into its overall musical performance. As you’d expect, IEMs are driven with aplomb and anything from sensitive multi-BA monitors to more power-hungry hybrid and dynamic driver models are no issue here. Compared to my desktop stack, I was hearing almost as much extension and sub-bass power, very convincing dynamics overall.
Stepping up to headphones, and the RU6 does well with portable or easier to drive models such as the Grado SR325X and the Oppo PM3. For full-sized headphones, I did hear it reaching its limitations. Though volume was sufficient on high-gain, I did find dynamics took a step back compared to my larger portable sources and desktop stack. While this was never the intention of the device, I was impressed by the soundstage and tonal balance it was able to achieve when paired with headphones even if they weren’t always ideal. The RU6 has one of the best Amp sections amongst dongles but remains suitable for efficient headphones only.