What we liked
- The ability to instantly switch between transistor and tube amplification
- The smoothness of the tubes and the dynamism of the transistors
- The power output
- The magnetic mounting system
We would have liked
- A more generous battery life
- A slightly more advanced interface, as the screen is minimalist
Our expert’s opinion
With the RU9, Cayin has created a portable headphone amplifier that shakes things up and stands out as a unique proposition on the market. The ability to switch from precise, analytical transistor amplification to warmer, softer tube amplification instantly adapts the sound signature to suit the music you're listening to and your current mood, and makes it easy to pair with any type of headphones, from monitoring models to electrostatic or Planar Magnetic cans. It's an ideal solution for enhancing sound quality with a smartphone or computer, thanks to its dual AKM DAC that decodes all modern formats up to 32-bit/768kHz and DSD512. Its only limitations are a modest battery life for portable use and a format that's a little bulkier than a conventional dongle DAC. But these limitations are largely offset by the sound quality and flexibility it offers, both at home and on the go. It's the ideal companion for discerning music lovers who want to take the emotion of tube sound with them wherever they go, without sacrificing the precision of transistor technology, depending on the headphones used.
Warm and refined, the incomparable musicality of tubes is progressively migrating outside the home with the Cayin RU9 portable headphone amplifier. This latest innovation from the tube electronics specialist lets you choose between transistor or tube amplification to tailor the sound to your preferences. With its 1000mW into 32 ohms, as well as balanced and unbalanced outputs, it promises to enhance any headphones or in-ear monitors, while its latest-generation dual AKM DAC ensures music playback up to 32-bit/768kHz and DSD512 from a smartphone, tablet or computer. Wireless listening is also possible with Bluetooth aptX HD, LDAC and aptX Low Latency. But can this little all-in-one DAC compete with a classic tube headphone amp?
Presentation
Design and ergonomics
The Cayin RU9 portable headphone amplifier features a CNC-machined aluminum chassis that looks solid and durable. The finish is impeccable: polished edges, a matte surface resistant to scratches and precise assembly that recalls the care Cayin puts into its living-room electronics. Despite the presence of a tube stage, the unit retains a surprisingly compact format, with a thickness of just 15mm and a weight of around 150g. Naturally, it stands out from ultra-discreet DACs, but the Cayin RU9 is still easy to carry in a trouser or jacket pocket.
In addition, the Cayin RU9 comes with a leather pouch that not only protects the device but also allows it to be attached directly to the back of a smartphone compatible with MagSafe wireless charging thanks to its integrated magnetic system. The whole thing then takes the form of a compact block that can be easily carried with one hand. Note that magnetic attachment is also possible without the case. In both cases, it is very secure, provided you use a smartphone without a case or a case compatible with magnetic accessories.
The amplifier has a small 0.96″ screen on the front panel displaying essential information: volume level, sampling rate, amplification mode and connection type. Despite its small size and dated interface, the display remains clear and perfectly legible, even in daylight. It lets you check active settings at a glance, so you don’t need to use a third-party app. The volume is controlled using a notched thumbwheel, which is both precise and easy to use. It also serves as a control for navigating through the menus and adjusting parameters such as gain, input source selection and DAC filter. It’s not the most intuitive way to explore the numerous settings, but they aren’t intended to be changed frequently.
To complete the interface, a few physical buttons on the side panels let you control playback (play/pause, next/previous track), select the amplification mode and switch between battery and mains power.
Triple amplification mode
Cayin is a master in the design of tube electronics, with numerous benchmark hi-fi amplifiers. In 2018, the brand became the first to use a miniaturized version of this technology in a digital audio player with the Cayin N8, followed by the Cayin N8 II and Cayin N3 Ultra. The latter were a resounding success thanks to their excellent musical performance. The Chinese brand then applied this technology to a portable headphone amplifier with the Cayin C9.
The new Cayin RU9 shares the latter’s hybrid approach. Its tube stage uses fifth generation Korg Nutube 6P1 subminiature tubes. These are miniaturized double triodes that offer tube-like operation for a warm, natural sound reproduction, while heating up less than traditional tubes. These two triode devices are visible through two honeycomb grilles on the front of the DAC. They glow bright green in the dark, enhancing the device’s elegance. They also ensure heat dissipation, keeping the unit cool even after several hours of use. As a result, you can hold it in your hand or place it comfortably in your pocket.
Cayin engineers have designed and fine-tuned the tubes’ negative feedback loops to deliver distinctive timbres, and have introduced the Dual Tube Timbre function. This gives you the choice between Modern Tube mode, which attenuates distortion and maximizes fine detail, while retaining the signature tube sound, or Classic Tube mode, which offers a more harmonically rich signature. This Cayin headphone amplifier also features transistor-only amplification for enhanced dynamics. This stage is based on a four-channel architecture with two Texas Instruments OPA1662 modules and four Nexperia power transistors. This triptych makes it possible to adapt the listening experience to the associated headphones and musical genre.
Whether operating in tube or transistor mode, the Cayin RU9 delivers a high-level performance, with power ratings of up to 1000mW into 32 ohms with the balanced output and 420mW with the unbalanced output. This energy reserve enables it to effortlessly power virtually all hi-fi headphones and IEMs, even the most demanding models. To optimize the match, three levels of gain are available. It should be pointed out, however, that this maximum power can only be achieved with an external USB power supply. That said, the power reserve is still very comfortable in battery mode, with 620mW balanced and 310 mW unbalanced, which is more than enough to cover the vast majority of headphones and in-ears.
Battery or USB power supply
Unlike the majority of portable DAC/amps, which are content to draw on the battery of the smartphone to which they are connected, the Cayin RU9 has its own internal 2,000 mAh battery. This technical choice has two advantages: it preserves the phone’s autonomy and guarantees a more stable power supply for the analog stages. In everyday use, the RU9 runs on its internal battery, with average autonomy of between 3 and 5 hours, depending on the listening mode chosen (tube or transistor, unbalanced or balanced output). This is an acceptable compromise given the power output and on-board technology, but it does mean regular recharging is required for long sessions when traveling.
Cayin has also designed this DAC for sedentary use, notably on a desktop. In dongle mode, the device deactivates its battery to draw power directly from the USB device to which it is connected. This saves power when connected to a computer, for example. In DC mode, the RU9 connects to an external USB power supply via a dedicated USB-C port. It’s in this context that it unleashes its full potential, automatically activating Hyper mode, which authorizes a maximum power of 1000mW into 32 ohms on the balanced output.
Comprehensive connectivity
The Cayin RU9 features both a 3.5mm unbalanced and a 4.4mm balanced output, allowing you to connect all types of headphones and in-ear monitors. Added to this is a 3.5mm S/PDIF mini-jack port, capable of operating as both coaxial input and output. This makes it possible to directly connect a digital source such as a CD player, or to connect the RU9 to an external DAC. Playback of music stored on a smartphone, tablet or computer is possible via the dedicated USB-C port. A second USB-C power port enables the device to be charged or continuously powered from the mains, while continuing to listen to music. The RU9 can therefore operate without interruption, even during charging.
Bluetooth aptX HD and LDAC
In addition to wired connections, the Cayin RU9 headphone amplifier is equipped with a Qualcomm QCC 5125 Bluetooth 5.1 controller, enabling wireless streaming from any smartphone, tablet or computer. As well as the basic SBC and AAC codecs, it supports the high-resolution LDAC and aptX HD formats, as well as aptX Low Latency to minimize latency with video content. Consequently, you can enjoy Cayin’s signature sound even when listening to music wirelessly, with a sound quality far superior to that of conventional Bluetooth connections.
32-bit/768kHz and DSD512 DAC
To make the most of its hybrid amplification, the Cayin RU9 DAC incorporates a powerful conversion stage with the integration of two AKM AK4493SEQ chips, renowned for their warm, smooth signature. These DACs are mounted in a symmetrical configuration to decode each channel separately. This technical choice improves channel separation, reduces crosstalk and optimizes the signal-to-noise ratio. Signal processing is entrusted to an XMOS XU316 controller, one of the most modern and powerful USB chips on the market. With its 16 processing cores, it ensures smooth transmission, full compatibility with Windows, macOS, Android and iOS, and optimal management of PCM and DSD streams. The Cayin RU9 DAC supports the majority of audio formats up to a very high resolution of 32-bit/768kHz and DSD512.
Listening conditions
As soon as you open the box, the Cayin RU9 is ready for use. The included USB-C cable makes it easy to connect to an Android smartphone, laptop or tablet. For users of an older-generation iPhone or iPad, a Lightning to USB-C adapter (not included) will be required to enjoy all features. The front panel OLED display and notched thumbwheel make it easy to get to grips with the device. Changing the amplification mode, adjusting the gain or selecting the input take just a few clicks. For this review, we paired the amplifier with a wide range of headphones, from the Meze 105 Silva, Sennheiser HD800S, Audeze LCD-2, Focal Hadenys and FiiO FT5 (Planar-Magnetic) open-back models, to the Focal Azurys, Meze 99 Classics and Denon AH-D7200 closed-back headphones.
Listening impressions
Transistor mode
With its transistor amplification, the Cayin RU9 headphone amplifier delivered a clear, perfectly controlled reproduction. Attacks were crisp, transients clear and the soundstage unfolded with excellent clarity. With demanding headphones like the Audeze LCD-2, dynamics were expressed without restraint, with firm, deep bass that was perfectly sustained. Vocals were natural and without excessive coloration, while the high frequencies soared without ever becoming harsh. This mode offered a transparent, analytical listening experience, while retaining enough substance to avoid the dryness of overly clinical neutrality.
Modern tube mode
Switching to modern tube mode altered the signature slightly. Without offering the coloration of a true EL34 or 300B tube amp, the atmosphere was adorned with a certain softness and lightness. The midrange gain was more dense, timbres became more fleshed-out and acoustic instruments revealed a subtly warmer texture. With Miles Davis’s Ascenseur pour l’échafaud, the double bass became fuller and the piano sounded rounder, with an extra weight that made th soundstage more realistic. The effect remained subtle enough for the reproduction to retain its initial precision, with clean contours, a spacious soundstage and powerful bass. This is probably the most versatile setting, combining the richness of tubes with transistor-like control.
Classic tube mode
Finally, the classic tube mode more closely embodied the essence of tube sound. The sound was mellower, with enhanced smoothness at the top end of the spectrum. Cymbals weren’t as bright, blending more into the musical fabric. Vocals were magnified and emphasized slightly, with a warmth that created a strong sense of closeness. With headphones known for their analytical sound, such as the Sennheiser HD800S, this mode transformed the listening experience, smoothing out the sometimes overly sharp high frequencies and replacing them with a very pleasant fluidity. This is the most appealing setting for long, relaxing sessions, or for bringing musicality back to headphones that are a little too technical. On the downside, you lose some fine detail in the high frequencies and the bass becomes softer.
Cayin RU9 vs Chord Mojo 2
Compared to the Chord Mojo 2, a benchmark for portable DACs and amplifiers, the Cayin RU9 plays a different tune. Where the Mojo 2 focuses primarily on extremely accurate and transparent sound reproduction, with exceptionally sophisticated digital management thanks to its internal DSP, the RU9 impresses with its sonic versatility, offering three amplification modes, two of which harness the warmth of Nutube technology. The Mojo 2 boasts longer battery life and a resolutely analytical approach, while the RU9 offers a more flexible experience, capable of switching from the neutrality of transistor mode to the mellow musicality of tube mode. Two philosophies that complement each other rather than oppose each other: the Mojo 2 embodies British rigor, while the RU9 embodies Cayin’s generosity and flexibility.
Who is the Cayin RU9 designed for?
The Cayin RU9 is aimed at headphone enthusiasts who like to keep control of their system’s sound and who want to be able to switch between the precision of transistors and the warmth of Nutube tubes at any moment, depending on the music genre or their current mood. This versatile device is suitable for use on the move and at home. Its compact size and USB power supply make it an excellent desktop headphone amplifier, capable of fitting neatly onto a desk while offering performance worthy of larger devices. When on the go, it is best suited to short sessions such as daily commutes due to its maximum battery life of 3 to 5 hours. For longer trips, you will need to rely on the battery of your smartphone, use an external battery, or plan ahead to recharge it in order to extend your listening time.
FAQ – Frequently asked questions
Yes, thanks to its power of up to 1000mW into 32 ohms with the balanced output (with external power supply), the RU9 is capable of driving demanding headphones without difficulty. Even on battery power, its 620mW are sufficient for the vast majority of audiophile headphones and IEMs.
The battery life varies between 3 and 5 hours, depending on the mode used (tube or transistor, balanced or unbalanced output). This makes it ideal for everyday use, but for longer trips, we recommend using an external battery or connecting it to the mains.
Yes, the RU9 offers three distinct signatures: a neutral, precise transistor mode, a modern, balanced and musical tube mode, and a warmer, softer classic tube mode. This makes it possible to adapt the listening experience to the headphones used, as well as to the music genre.
It gives off a little heat, especially in tube mode and on the balanced output at high power. However, heat dissipation is well controlled: the device remains comfortable to hold and is safe to use.
The Cayin RU9 features Bluetooth 5.1 with support for the LDAC and aptX HD high-resolution codecs, as well as aptX Low Latency for movie viewing and AAC for Apple devices. Quality remains inferior to wired mode, but the experience is highly satisfactory for practical everyday use. Note that Bluetooth can only be used for reception, not transmission.
Yes, it works perfectly with iOS devices equipped with a USB-C port. For older-generation models, a Lightning to USB-C adapter (not included) will be required.
Yes, you can use one USB-C port to power or charge the device, while using the second port for DAC/amp mode.
Under macOS and Linux, no installation is necessary. For Windows computers, an ASIO Cayin driver lets you take full advantage of hi-res streams up to 32-bit/768kHz and DSD512.
