Cayin RU9 [review]

Autor:

30 czerwca 2025

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Cayin RU9 is a portable combo device based on dual AKM 4493SEQ DACs and a balanced amplifier featuring a miniature Korg Nutube 6P1 Gen5 vacuum tube. It offers advanced controls, three sound modes, and high-power output, reaching up to 1W per channel.

Cayin RU9 is quite different from its predecessors, the RU6 and RU7. Cayin still calls it a 'dongle’, but that’s an overstatement. The RU9 is more like a portable headphone amplifier/DAC combo; it’s almost as big as a smartphone, featuring wide connectivity, and includes a Bluetooth module supporting advanced audio codecs. Inside, it’s also very different: it uses dual AKM 4493SEQ DAC chips and an amplifier with a tiny Korg tube, unlike the R2R DACs in previous models.

REKLAMA

The price, just under $500, is a bit surprising since its predecessors launched around $250-300. However, the higher cost can be justified by its advanced features and versatility. Cayin RU9 has the potential to replace not just mobile DACs or portable players, but even desktop audio devices. After all, with 1 watt of power per channel, it can easily drive various full-sized headphones. The final judgment, though, will depend on its sound quality and the ease of use.

I’ve checked how Cayin RU9 performs in everyday routine and if it truly sounds like a tube amplifier.

Accessories

The set includes:

USB-C interconnect (6 cm long)
USB-C to USB-A adapter
Leather case
Magnetic ring
Documentation

The first impression is very positive, as you get a flexible interconnect with angled plugs, an adapter for full-sized USB, and a magnetic case. The last item allows you to attach the device to iPhones as well as Android smartphones, as the manufacturer includes a self-adhesive magnetic ring.

Design

Cayin RU9 is neither a typical dongle nor an ordinary combo, but rather a combination of both. The device has the shape of a thick slab, so at first glance, it looks more like a music player than a portable DAC. Only the tiny screen, typical for mobile DACs or Bluetooth adapters, reveals that the RU9 is not a fully autonomous device.

On the front, there’s a monochrome display, a knurled knob, and two 'windows’ that serve both for cooling and admiring the pleasant turquoise glow of the Korg tube. Most elements are placed in the upper part of the front, leaving the cover largely empty. The exception is the laser-engraved inscription 'TUBE/SOLID-STATE DAC AMPLIFIER’ in the bottom right corner.

The space on the edges has not been wasted. On the bottom, you’ll find the connectors: a 4.4mm output, a 3.5mm output, two USB-C ports, a power switch (BAT/DC), and a 3.5mm coaxial input/output. The power button, power LED, and MENU button are located on the right side, while the left side packs three music control buttons. The top edge is the least cluttered, with only a plastic insert that ensures stability of Bluetooth connection.

Cayin RU9 boasts superb build quality. Its perfectly fitted, machine-cut aluminium chassis, with bevelled edges and fine details, looks fantastic. The buttons are responsive with a distinct click, and the knob turns smoothly. Despite its robust appearance, it’s surprisingly light due to its thin, yet rigid construction.

Ergonomics

Cayin RU9 weighs just 155 grams, which isn’t much for an aluminium block measuring 10 x 7 x 1.5 cm. When paired with a smartphone, it creates a handy setup similar in size to a high-end music player. However, the RU9 is 3-4 times larger than the RU6 and RU7 models, so calling it a ‘dongle’ is an abuse. Because of this, the magnetic case is essential – the device needs to be attached to your smartphone. Letting it dangle from the USB-C port simply isn’t an option.

The magnetic case works much better than unfortunate elastic bands that cover the smartphone screen. Strong magnets ensure the RU9 doesn’t detach during typical use. However, the self-adhesive ring can be problematic. Despite its strong 3M tape, much will depend on where it’s stuck. Such stickers don’t hold well to silicone cases, especially those with a velvety finish. iPhone users gain an advantage in that matter thanks to MagSafe technology.

Using the device is easy. Cayin RU9 fits comfortably in hand, never digging in or irritating the skin. I was also pleasantly surprised by the knob’s placement, which is within thumb’s reach when holding the device in one’s right hand. It’s also easy to access the buttons located on the right edge. It’s worth noting that the knob and display are recessed into the casing, preventing accidental volume changes and allowing placing the device screen-down.

Usage

The small screen is sharp and bright, clearly showing all necessary information like gain level, sound mode, battery status, active input, and the Bluetooth codec. Handling is also simple: a quick click of the side button switches sound modes, while a long press opens the menu. The scroll wheel controls navigation. There’s plenty of settings to adjust, including three gain levels, three digital inputs (USB, Bluetooth, SPDIF), and various PCM and DSD filters.

Cayin RU9 features two USB-C ports with distinct functions. The first one handles signal transmission, and can also provide power, via a selectable mode. The second port is dedicated to charging or external power, controlled by a small BAT/DC switch. When this switch is set to the second position, the device’s power output automatically increases. For example, the 4.4mm jack’s output jumps from 620mW to a full 1000mW (at 32 ohms).

It’s great that both headphone outputs can also function as line outputs, and the SPDIF function is separate. An additional 3.5mm jack handles this, acting as a bidirectional coaxial port for both sending and receiving signals. The device also includes a Bluetooth 5.1 interface supporting AAC, aptX, aptX HD, and LDAC codecs. All these features effectively transform your smartphone into a full-fledged DAP.

What’s missing, then? Analog inputs. Cayin RU9 can’t take a signal from another DAC, meaning it won’t act as an external amplifier. However, this feature is becoming less common as nowadays most mobile audio devices are designed to handle digital-to-analog conversion.

Unfortunately, I did run into a few practical issues. Despite the plastic insert, the Bluetooth interface is prone to interference – I couldn’t keep the RU9 in my pocket with my smartphone on the desk nearby, as the signal would drop. I was also a bit disappointed by the knob’s lack of precision; it has some delay and often doesn’t respond if turned too quickly.

Battery life

The device’s Achilles’ heel is its short battery life. During testing, the RU9 needed charging after just 3-4 hours, even when the screen wasn’t heavily used. This isn’t surprising given its advancement, powerful amplifier and a small 2000 mAh battery. Such figures also match the manufacturer’s predictions.

However, 3-4 hours is simply too short for comfortable use. You’ll likely need a power bank (ideally supporting QuickCharge 3.0 or Power Delivery 2.0 technologies). The good news is that Cayin RU9 recharges quickly, in just over an hour.

Specs

General

DAC: 2x AKM 4493SEQ
Amplifier: 2x Texas Instruments OPA1662 + Korg Nutube 6P1 Gen5
Receiver: XMOS XU316
Display: 0.96 inch LCD
Wireless Interface: Bluetooth 5.1 with AAC, aptX, aptX LL, aptX HD, LDAC
Connectors: USB-C (signal); USB-C (power); 3.5 mm and 4.4 mm (headphone/line out); 3.5 mm (coaxial input/output)
Features: QC 3.0/PD 2.0 fast charging, three sound modes, Bluetooth interface, desktop mode, three gain levels (-12, -6, 0 dB), additional USB-C (power)
Battery: 2000 mAh lithium-polymer
Battery life: 3-5 hours
Dimensions: 100 x 70 x 15 mm
Weight: 155 g

Audio

Support: 32 bit/768 kHz, DSD512
Frequency response: 20 Hz-32 kHz
Output impedance: 1 Ω
Power (DC): 1000 mW + 1000 mW @ 32 Ω (4.4 mm); 420 mW + 420 mW @ 32 Ω (3.5 mm)
Power (BAT): 620 mW + 620 mW @ 32 Ω (4.4 mm); 310 mW + 310 mW @ 32 Ω (3.5 mm)
Dynamic range: 105-119 dB
THD+N: 0.5-3.1%
SNR: 104-118 dB
Channel separation: 86-105 dB

Sound

Cayin RU9 sounds fantastic, which isn’t a shocker since Cayin has a long track record of excellence. Its sound signature is warm, colourful, and musical, clearly indicating the use of AKM DACs. This makes for a very enjoyable listening experience, while maintaining high technical fidelity. The extra sound modes noticeably tweak the audio, though perhaps not as dramatically as with the Cayin N3Pro player that I reviewed a few years ago. Let’s start with Solid-State Mode.

Cayin RU9 – Solid-State Mode (transistor symbol)

In Solid-State Mode, the sound is musical, warm, and smooth, typical of Asahi-Kasei Microdevices (AKM) chips. But don’t worry, it’s not overly smooth or blurry; the sound remains firm, defined, and precise. Cayin RU9 in this mode still delivers a distinct and direct sound without being aggressive or tiring. This is thanks to a balanced frequency response: the bass is full, the mids are intimate, and the treble is clear without being sharp.

The RU9 creates a wide soundstage, especially through its 4.4mm output, thanks to strong channel separation. The sound has good depth and height, forming an ellipsoidal shape. The holography is impressive, with well-defined phantom images and plenty of air between them. This means headphones sound spacious and open, yet still direct, as Cayin RU9 pulls you into the music rather than pushing the sound away.

The Solid-State mode offers an intriguing blend of musicality and technical precision. It’s close to a neutral sound but avoids being analytical, providing an engaging experience with full bass, rich mids, and controlled treble. Compared to the tube modes, Solid-State sounds more 'Hi-Fi,’ with a slightly more digital or raw feel. The tube modes, especially the 'C’ setting, deliver a more natural sound.

Cayin RU9 – Classic Mode (C)

When you switch to Classic mode, the sound character noticeably shifts. The bass becomes shallower and the midrange becomes more prominent. The tone feels more natural and less sterile, almost a bit 'grainy,’ which is especially noticeable in the mids. I get the impression that in this setting, the mids really steal the show – they’re more organic and slightly 'dirty.’ However, I didn’t observe any significant impact on the soundstage, which remained wide, layered, and well-organized.

Thus, the name 'Classic mode’ truly fits its effect, and it’s great, especially if you enjoy older recordings and lighter music genres. I particularly liked it for jazz, funk, and classic rock – it’s a perfect match for that kind of music. Classic electronic music also sounded good, but modern tracks felt a bit too light on bass and too focused on the mids – that’s where mode 'M’ came in handy.

Cayin RU9 – Modern Mode (M)

The name 'Modern mode’ is also appropriate. It truly sounds more contemporary than Classic mode, yet still keeps that distinct tube character. I see 'M’ mode as a hybrid of Solid-State and Classic modes. You get fuller bass and a cleaner, smoother midrange that’s also warmer, more colorful, and more analog. Unlike Solid-State mode, there’s no digital or raw edge, and it avoids the retro-midrange focus of Classic mode.

I think Modern mode is the most versatile of the three – it offers a rich, warm sound in a contemporary style. This means you can comfortably listen to anything from jazz, blues, light rock, or funk to rap, electronic music, or heavy, guitar-driven genres – the RU9 truly handles everything in this mode.

Cayin RU9 – comparisons

Cayin RU9 vs N3Pro
N3Pro was mentioned earlier because it’s a well-known Cayin player that also features a tube amplifier and three sound modes. However, the N3Pro’s modes have different characteristics and offer more variety. The N3Pro’s Solid-State mode is more neutral compared to the RU9’s, which is warmer. The N3Pro’s tube modes are either warm and musical (Triode) or neutral and analytical (Ultra-Linear). In contrast, both tube modes on the RU9 are consistently warm, colorful, and musical. So, while they’re both Cayin devices, the RU9 doesn’t have much in common with the N3Pro’s sound profile; it truly takes the audio in a distinctly more analog direction. This can be seen as both an advantage, if you prefer a warmer, more organic sound (Cayin RU9), and a disadvantage, if you’re looking for neutrality (N3Pro).

Cayin RU9 vs RU6
I haven’t tried the RU7, so I can’t compare it directly, but I really liked the older RU6. The RU6 had a balanced, natural sound, typical of R2R DACs. In my opinion, the RU9 sounds warmer, smoother, more colourful, and more spacious than the RU6. The RU9 also gives you three sound modes, while the RU6 only had an oversampling option (NOS and OS), which made the sound more technical. I think the RU9 is a significant step forward in both sound quality and features. However, keep in mind the RU9 is much larger and more expensive than the RU6 or RU7.

Cayin RU9 vs Questyle M18i
I also compared the RU9 to the Questyle M18i, a smaller, simpler device I tested previously that also features a display, battery, and Bluetooth mode. The RU9 sounded warmer, more colorful, and more natural in every mode. The M18i, on the other hand, was more neutral and balanced, clearer and more technical sounding. I also found the RU9’s soundstage slightly wider and airier. It turns out that the RU9 acts much better as a Bluetooth adapter – the Questyle M18i struggled in Bluetooth Mode, downgrading the sound in comparison to USB Mode. Overall, I think the Cayin RU9 performs better than the M18i. If the size difference isn’t an issue, the extra cost for the RU9 is worth considering.

Cayin RU9 – synergy

The Cayin RU9 generally pairs well with many headphones, but it truly shines with balanced or brighter-sounding ones. It acts as a 'musicalizer,’ adding warmth and richness. If you pair it with headphones that are already warm or dark, you might find the sound becomes too warm, as the RU9 won’t make them brighter or cooler. However, it can make bright or harsh headphones sound smoother and more listenable.

For instance, I loved how it performed with hybrid or tribrid IEMs like FiiO FH19 or Craft Ears Aurum. They sounded excellent in all modes, with no fatiguing treble and improved richness. However, my over-ear Sennheiser HD 6XX headphones, which naturally got a gentle treble, sounded a bit too dark paired with Cayin RU9 in tube modes. The Solid-State mode worked best for them, providing the clearest sound. This shows that the RU9 benefits from good synergy, and its three sound modes are still a useful tool for fine-tuning.

There’s no need to worry about signal purity – with sensitive IEMs, a slight hiss can be detected, but only in pauses between tracks and in complete silence. This also applies to Bluetooth mode, which satisfies with its low noise level. Shocks will also not be a problem – tapping or shaking the device did not cause the characteristic tubes’ microphonics.

I don’t think anyone will complain about the amp power either. Demanding headphones were driven without any problems, especially in desktop mode (with an external power supply). I successfully used it with quite demanding tribrids, desktop planar headphones, and high-impedance dynamic headphones. The bass was full, the soundstage was large, and the dynamic range was wide.

Summary

Cayin RU9 isn’t perfect. The knob has some lag, the Bluetooth can be prone to interference, and its design is bulky. The 3-4 hours battery life is also a drawback.

However, its advantages outweigh the issues. The RU9 is a superbly crafted, good-looking, and user-friendly device with powerful features. Its two USB-C ports, bidirectional SPDIF, high power, and convenient display make it a joy to use. The three sound modes – Solid-State, Classic, and Modern – are very impressive, with the latter two genuinely giving the sound a true tube character. I was also pleased with the Bluetooth interface featuring Qualcomm and LDAC codecs, as it doesn’t significantly worsen the sound compared to a wired connection.

Cayin RU9 isn’t cheap (499 USD), but you get a lot for your money. In my opinion, it can easily replace a dedicated music player and work well as a desktop device, making its overall value surprisingly good. Just know that the RU9 is a strictly musical device—in every mode, it sounds colourful, quite warm, and engaging, and the Classic and Modern modes truly sound like tubes. If you prefer an analytical, highly technical sound, you might not be satisfied. But if you’re tired of sharp, lifeless 'transistor’ sound, you’ll likely be delighted, just as I was.

Recommendation mark for Cayin RU9

Pros:
+ Magnetic case
+ Solid build quality
+ Comfortable to use
+ High functionality
+ Two USB-C ports
+ Powerful amplifier
+ Three sound modes (Solid-State, Classic, Modern)
+ Bluetooth interface with aptX and LDAC codecs
+ Colourful, warm, and musical sound with a tube character
+ Ellipsoidal and airy soundstage