Specs and tech
The technical foundation of the RU9 centers around the AKM AK4493 32-bit DAC chip, a capable delta-sigma converter that supports PCM up to 32-bit/384kHz and DSD256. While not the flagship AK4499EX found in premium equipment, the 4493 delivers solid performance with measured dynamic range exceeding 120dB and total harmonic distortion below 0.0003%.
The star of the technical show is undoubtedly the Korg NuTube 6P1, a teeny tiny vacuum tube that operates at just 1.5 volts while generating authentic triode characteristics. Unlike traditional tubes requiring hundreds of volts and significant heat dissipation, NuTube consumes mere milliwatts while providing genuine even-order harmonic distortion. This technological breakthrough makes legitimate tube sound possible in battery-powered portable devices.
But RU9 is not a tube-only device; rather, it offers three distinct operational modes accessible via the timbre button. Solid-state mode bypasses the tube circuitry entirely, providing clean, reference-style sound. Modern mode engages the tube processing while maintaining technical accuracy, offering a middle ground between analytical and musical presentations. Classic mode delivers full tube saturation with warm, engaging sound characteristics.
Digital signal processing offers multiple reconstruction filter options, allowing users to tailor the frequency response and transient behavior to their preferences. The selectable filters affect pre-ringing, post-ringing, and ultrasonic response, providing subtle but meaningful sonic adjustments.
Power delivery varies significantly between battery and external power operation. In battery mode, the balanced output delivers approximately 550mW into 32 ohms, while external power operation unlocks hyper gain mode with output exceeding 1000mW. Single-ended output provides roughly half the power of balanced operation, making the 4.4mm connection the preferred choice.
The internal battery capacity of 2000mAh provides roughly 4-5 hours of operation depending on volume levels and connected loads. The dual USB-C implementation allows simultaneous audio input and power delivery, enabling desktop-style operation without battery limitations.
Aside from wired USB and S/PDIF inouts, RU9 also features full-blown Bluetooth connectivity, supporting high-resolution wireless codecs such as LDAC, aptX HD, and aptX Lossless. This transforms the RU9 into a Bluetooth receiver capable of driving wired headphones and IEMs from wireless sources.
However, the implementation isn’t without limitations – occasional electromagnetic interference can affect signal stability, and the Bluetooth range falls short of the best-in-class wireless audio devices. Despite these compromises, the Bluetooth capability dramatically increases the device’s versatility, allowing wireless streaming from phones, tablets, and computers to traditional wired audio gear.
Compatibility considerations extend to legacy devices through selectable USB Audio Class support. Users can choose between UAC 2.0 for modern equipment or driverless UAC 1.0 for older devices including gaming consoles like the Nintendo Switch, ensuring broad compatibility across different source types.
Device summary
- Digital conversion: Dual AK4493SEQ DACs supporting native DSD512 and 32bit/768kHz PCM
- Amplification (tube): KORG Nature 6P1 dual triode tube
- Amplification (solid state): Ti OPA1662 OPAMPs, 4x Nexperia audio power tube, 6x Toshiba JFETs
- I/O interface: XMOS XU316 USB, S/PDIF
- Analogue outputs: 3.5mm single-ended and 4.4mm balanced dual headphone/line out ports
- Wireless: Qualcomm QCC5125 supporting Bluetooth v5.1 (LDAC, aptX HD and aptX LL)
- Battery: 2000mAh polymer lithium-ion with PD 2.0 support
- Price: $499
- Website: en.cayin.com
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