STEREO GUIDE verdict
The Cayin RU9 is a mobile USB DAC and headphone amplifier with genuine Nutube tube technology. It offers dual AK4493 DACs, HiRes support up to DSD512, Bluetooth 5.1 and 3.5 and 4.4 mm outputs. In particular, the option to switch between tube and transistor sound ensures customized listening pleasure at audiophile level.
Pros
- Very spacious sound image - especially with tube circuitry
- Powerful exit step
- Leather case with magent holder included in the scope of delivery
- 3.5 and 4.4 mm outputs
Cons
- You're guaranteed to want to listen for longer than the battery lasts
-
Sound9.8
-
Usability / Connectivity9.3
-
Price/Performance9.5
The hi-fi cliché says that tube amplifiers are reserved for the most traditional group of music lovers: Big speakers, vintage turntables, 50 years of hi-fi history and a certain disdain for all things digital, that’s how you imagine the typical tube listener.
Not true, of course. And this new device finally turns the cliché on its head: Cayin, the specialist for fairly traditional home tube amplifiers, has brought the sound of the vacuum tube into the mobile world with the RU9. The compact USB D/A converter and headphone amplifier in the size of a pack of cigarettes fits all modern smartphones, tablets and laptops.
During operation, a blue glow behind two barred viewing windows on the top reveals the traditional tube technology. The Nutube double triode – one element per channel – works together with state-of-the-art digital processing and hi-res conversion and is primarily aimed at users of wired headphones and high-quality IEMs.
Nutube 6P1 – real tube sound signature in minimal space
At the heart of the RU9 is the Nutube 6P1 vacuum tube, designed in a collaboration between Korg and Noritake Itron Corp. The fact that this double triode was developed at all in the digital age is due to demand from the music industry. Electric guitarists in particular love the warm, lively sound signature when such a tube is gently driven into the distortion range.
The special feature in contrast to traditional tubes: it requires neither a power-hungry filament heater nor hundreds of volts of voltage between the cathode and anode. The energy requirement is only one fiftieth of that of conventional tubes. This also makes it suitable for mobile applications, such as a dongle DAC.
Despite the limited space in the housing, Cayin has managed to implement the tube in this 5th generation of its mobile tube circuit in a shock-proof manner via a suspension and at the same time keep the thermal management under control. The free electrons in the vacuum, which are essential for tube amplification, are generated by a small surface heater in semiconductor technology, which is not unlike a small OLED display. The circuit does not require an over-all negative feedback and should therefore, according to the manufacturer, particularly support the natural sound aspects of the tube.
Features and technology: Dual DAC, HiRes and flexible outputs
On the digital side, Cayin relies on two AKM AK4493s DAC chips in a dual-mono configuration. These converters are known for their high resolution and fine dynamics. In combination with the tube stage, the result is a hybrid concept: maximum precision in conversion, combined with the organic, slightly saturated sound character of a tube.
The RU9 goes all out with hi-res files: PCM data streams up to 768 kHz and 32 bits can be played via USB-C, DSD up to 512, which corresponds to 8 times the resolution of SACD.
The RU9 is versatile: in addition to an unbalanced 3.5 mm output, a balanced 4.4 mm Pentaconn connection is also available. This means that even more demanding headphones with high power requirements can be operated. One highlight is the ability to switch between
Also a very flexible mobile tube DAC on the input side
UAC1.0 and UAC2.0 are supported via USB-C, so the RU9 works with the vast majority of devices with or without drivers. There is also an additional S/PDIF input via the somewhat hidden 3.5 mm input, which you have to select in the menu if you want to listen to a CD transport or similar via the RU9. The manual selection also applies to the wireless Bluetooth 5.1 with LDAC, aptX HD and AAC. This means that the small DAC can be used not only wired, but also wirelessly with various devices in the highest possible quality – from smartphones to laptops and games consoles.
There are also three sound profiles (“Classic”, “Modern”, “Solid State”), which offer finer-grained options for different listening habits. The main difference between the two tube modes is that in “Classic” the negative feedback is reduced to a local minimum per amplifier stage, which results in slightly higher distortion values but, in theory, more natural amplification, while the “Modern” tuning is optimized for the lowest possible tube distortion. In the latter mode, the internal tube is bypassed and only transistorized operational amplifiers are used.
Practice: Operation, menu navigation and suitability for everyday use
The display on the front provides clear information about volume, input mode, sampling rate and battery status. The RU9 is operated via a combination of side buttons and a smooth-running volume roller, which allows very fine tuning.
At 10 × 7 × 1.5 cm and weighing just 150 grams, the RU9 remains easy to transport. You can attach it magnetically to your smartphone or its case, but we found this almost a little too thick for a jacket pocket.
The housing is made of CNC-milled metal and gives a high-quality impression. The integrated 2000 mAh battery allows up to five hours of listening pleasure, regardless of the source device, and can also be recharged during operation via the second USB-C port if a second cable is connected
Sound test: Cayin RU9 on IEMs and over-ear headphones
In the first round of testing, we used the Cayin primarily with IEM, i.e. wired in-ear monitors: Sennheiser IE 900, Beyerdynamic Xelento 2 and Final B3 are our references in this segment.
Special attention was paid to operation with the rather high-efficiency Sennheiser. And it was here that the RU9 proved to be the ideal choice. Dynamic shading in the piano range in particular was a real high-end pleasure via the combination in tube mode, where the IE 900 showed its detail resolution and the RU9 its sound warmth to the full. The louder we listened, the more we heard the Sennheiser as powerful, dynamic and focused on the ends of the spectrum as we were used to. The only exception was that the Cayin took the edge off overly sharp recordings and, while not softening them, made them sound a little silkier than usual.
Tube mode vs. transistor – two characters, one device
On the Beyerdynamic T5p and Final high-end over-ears, the Cayin was really in its element. It impressed with an enormously spacious listening experience, which should be welcomed by all those who want to escape the narrowness of the usual headphone sound worlds. This was especially true in combination with the tube mode, which gave the sound a little more smoothness and a greater sense of space.
Surprise combo: RU9 with Fosi Audio i5 in the test
One wonders: Can this be improved? And surprisingly, my curiosity led me to the answer: the Fosi Audio i5 – an over-ear with planar-magnetic transducers – proved to be a real dream partner for the Cayin RU9. This open headphone in the class up to 600 euros was actually not even scheduled for a review on STEREO GUIDE. But I simply couldn’t resist and was thrilled.
What I particularly liked about this mobile hi-fi system was the extremely spacious listening experience by headphone standards, the fine resolution in the mid-high range without sharpness and the high level of differentiation. With the combination of RU9 and i5, I was able to hear the different acoustics of the concert halls better than usual with songs of which I have various live versions – such as “Thunder Road” by Bruce Springsteen or “Country Feedback” by R.E.M.. I had the feeling that despite surprisingly subtle differences between transistor and tube timbre, the latter provided a denser, more authentic atmosphere.
Conclusion and alternatives
The Cayin RU9 has a lot of competition in the mobile dongle DAC segment, but it has an unbeatable unique selling point: its tube sound. The RU9 is therefore clearly positioned in the upper segment, but is aimed at all those who don’t want to miss out on that tube feeling on the go.
Cayin RU9: Technical specifications
- Manufacturer’s recommended retail price: 598 euros
- Design: Mobile headphone DAC with tube circuitry
- Dimensions (W x H x D): 10 x 15 x 1.5 cm
- Weight: 150 g
- Special features: Transistor and tube amplification USB input, S/PDIF input, Bluetooth with LDAC, AAC and aptX HD
- More at: cayin.com
