In this article, I’m looking at the Cayin RU6 dongle DAC. The RU6 is unique in that it’s the first known device of this type to utilize a discreet R-2R DAC architecture. It’s priced at $249.

Rather than the current standard delta-sigma DAC implementation, the RU6 blazes its own path by utilizing a modern version of the retro R-2R architecture. Known for its natural and realistic audio quality, R-2R promises an analogue but precise experience. But I’m all about the sound and end-user experience, so if you want to nerd out on R-2R technology you can learn more about it at Electronics Tutorials.

Disclaimer: This sample was provided by Cayin for the purpose of an honest review. All observations and opinions here are my own based on my experience with the product.

Cayin RU6

Specifications

• 24-bit discrete R-2R resistor ladder DAC
• User selectable NOS/OS DA modes
• High-precision resistor array volume control
• Supports up to PCM 384kHz, DSD256
• 3.5mm and 4.4mm headphone outputs
• CNC chassis
• Dimensions: 65×25.4×13.7(mm)
• Weight: 28g
• Low power consumption
• TWO 6-layers PCB, digital and analog circuit on separate boards
• Compatible with iPhone, macOS, Windows and Android devices
• Low and High gain modes
• Frequency response: 20Hz-20kHz
• Output impedance: 0.5Ω (3.5mm) / 1.0Ω (4.4mm)
• Output power: 138mW@32Ω (3.5mm) / 213mW@32Ω (4.4mm)
• Price: $249

Package and Accessories

The Cayin RU6 comes in a handsome dark grey box with a colour image of the DAC on the front. Inside the box is the obligatory black foam insert holding the RU6 securely in place. Under the foam are the rest of the accessories which include a USB-C to USB-C cable, a USB-C to USB-A adapter, a user manual and a pair of Hi-Res Audio stickers.

Design

RU6 has a sand-blasted aluminium chassis. While the shape of the chassis is fairly standard for a dongle, the RU6 oozes a sense of prestige and premium. There’s a glossy glass faceplate on the top of the unit along with a 1-inch OLED display. The display shows information such as the volume level, sampling rate, NOS/OS DA mode. Here is also where you can navigate the menu options which include Low/High gain, screen brightness, screen timeout length and NOS/OS DA modes.

Both sides of the chassis have a chrome strip running down the centre. On one of these sides are the 3 buttons that control the volume and menu navigation. The buttons are small and unobtrusive so they don’t snag on things like pockets but they still have a tangible tactile click.

As far as outputs go, there’s a single-ended 3.5mm jack and a balanced 4.4mm jack on one end of the device. Output impedance is low on both so the RU6 is well-suited for extra-sensitive IEMs. In terms of output power, RU6 pushes 138mW@32Ω from the 3.5mm jack and 213mW@32Ω from the 4.4mm out. If those numbers don’t indicate anything to you – it just means that RU6 has ample power to drive most IEMs and many headphones with ease.

At the other end of the device is the USB Type-C port for data transfer. RU6 comes with a USB-C to USB-C cable but iPhone users will need to purchase a separate cable. One other really neat feature of RU6 is its built-in magnet layer. This is great for securing the unit to the back of your phone or a computer case to prevent it from swinging about.

If RU6 wasn’t unique enough already, Cayin also managed to implement a 99-step resistor-based volume control. This helps to maintain signal integrity by fixing the incoming bit-stream at full volume rather than amplifying a weak signal with variable input volume. In addition, the fine volume control is especially useful for dialling in precise levels when using sensitive IEMs.

Furthermore, a built-in mute function prevents pops during resistor activation (different sets control specific volume ranges). Although this sometimes causes a brief delay when changing volume I don’t find it distracting.

NOS/OS DA Modes

When in non-oversampling mode (NOS), the RU6 maintains the original sample rate of the source bitstream, so there’s no post-processing. According to Cayin, this reduces jitter, low phase distortion and ringing artefacts. This is RU6 in its most musical and analogue form, much akin to traditional tube amplifiers.

In the oversampling mode (OS) the sample rate is upsampled to 384kHz, enhancing the resolution and anti-aliasing while also reducing background noise. In this mode, the background is blacker and frequency extension is improved. As a result, the OS mode increases detail retrieval and sounds slightly cleaner overall.

Sound

I tested the Cayin RU6 with a variety of IEMs such as the itsfit Fusion, DUNU Studio SA6 and FiR Audio 5×5. I also added some headphones to the mix including the HIFIMAN Sundara and HarmonicDyne Zeus. The RU6 drove everything easily with power to spare.

So with all the technology and extreme engineering that went into creating the RU6, how does it actually sound? Well, that’s the best part; RU6 has an organic, natural and analogue sound that other dongles in its class simply cannot match.

There’s nothing ‘digital’ about the sound – it does indeed sound analogue. But what’s so outstanding about it is you’re not giving up any fidelity for the natural tone. It doesn’t compromise on technicalities to give you that silky smooth presentation.

Detail retrieval is exceptional; pair RU6 with a highly resolving IEM or headphone and you’ll find every micro-detail present and accounted for. This device miraculously blends transparency with smoothness, unlike some sigma-delta DACs.

To my ears, the RU6 has an uncoloured sound but that’s not to say it doesn’t exhibit any character. I find the bass textured and full-bodied with excellent extension. Midrange notes are spacious and articulate. In fact, it’s the sense of spacing and layering that is one of RU6’s greatest strengths.

In the (my preferred) NOS mode, the treble sounds more natural and doesn’t contain any hint of artefacts. It has great precision along with a lovely, open feeling, lending to great staging and accurate imaging. As a result, the aforementioned layering is superb and helps to create a holographic soundstage.

Comparisons

Earmen Sparrow ($199)

The Earmen Sparrow is a delta-sigma dongle equipped with an ES9281Pro DAC chip. It has 125 mW of output power from its 2.5mm jack compared to the 213 mW of the RU6’s 4.4mm jack. That’s a pretty significant difference and makes RU6 more suitable for demanding IEMs and headphones. Having said that, I haven’t come across an IEM that needs more power than the Sparrow can deliver.

Physically, the Earmen Sparrow is much smaller than the Cayin which is another factor that might be relevant to you. However, the RU6 has dual gain modes, NOS/OS modes plus more precise and discreet volume control.

The most notable difference I hear between these two devices is in the treble. The Sparrow’s high frequencies occasionally exhibit some ringing (more akin to RU6’s OS mode). Other than that, Sparrow has a lower noise floor and a cleaner overall presentation.

The Sparrow might be a hair more resolving but the RU6 has more character and an organic tone. One other thing I notice is the RU6 has a wider soundstage and more tangible layering than the Sparrow. At the end of the day, the differences are minute and choosing between the two will come down to sonic character, size and features.

Verdict

The Cayin RU6 differentiates itself from other DACs not only with its R2R technology but also with its organic, analogue sound. If you’re someone who values character over absolute neutrality, this is the dongle for you.