Korg NuTube portable DAC

The Cayin RU9 is a pocket-sized DAC and headphone amplifier that brings a real vacuum tube into everyday portable listening. Built around a Korg NuTube 6P1, dual AK4493SEQ DACs, and both 3.5mm and 4.4mm outputs, the RU9 is for tube character without giving up modern portability. A magnetic back lets it ride on your phone, and an internal battery means it works whether you're on the move or at a desk.

Real NuTube amplification in a portable device

The RU9 is the first portable DAC and amp to implement the Korg NuTube 6P1. Unlike digital tube emulation, the NuTube is a true dual-triode vacuum tube that adds harmonic richness through an analog circuit. Cayin pairs the tube with a three-stage amplification design using audio-specific JFETs, allowing both the single-ended and balanced outputs to deliver authentic tube character without the size or heat of traditional glass tubes.

Three selectable timbre modes

The RU9 gives you control over how much tube character you hear. Classic mode emphasizes warmth and density associated with traditional tube gear. Modern mode keeps the harmonic sweetness while reducing background noise and tightening the presentation. Solid-State mode bypasses the tube stage for a clean, neutral sound with fast transients and accurate tone, making it easy to match the RU9 to different headphones and music libraries.

Dual AKM DAC architecture

At the digital core are two AK4493SEQ DACs, each operating in mono mode for improved channel separation and lower crosstalk. This layout helps preserve spatial detail and imaging, especially with balanced output. Support for up to 32-bit/768kHz PCM and DSD512 ensures compatibility with modern high-resolution files, while measured performance keeps distortion and noise well controlled.

Balanced and single-ended outputs

The RU9 includes both 3.5mm single-ended and 4.4mm balanced outputs that can function as headphone or line outputs. Balanced operation reduces noise and improves channel separation when paired with compatible cables and headphones. This flexibility allows the RU9 to serve as a portable headphone amp or a compact DAC feeding powered speakers or a desktop amplifier.

Battery-powered portability with flexible power modes

An internal 2000mAh lithium battery provides up to 4.7 hours of playback, letting the RU9 operate independently of your phone or computer. When connected over USB, it can also draw power directly from the source or run from an external DC supply. Support for QC and PD fast charging shortens downtime and fits into modern charging habits.

Hyper mode for demanding headphones

When powered by an external DC supply, Hyper mode raises operating voltage and current for higher output capability. In this mode, the RU9 can deliver up to 1000mW into a 32Ω load, providing additional headroom for full-size headphones that benefit from stronger drive without changing devices.

Anti-vibration tube mounting

Vacuum tubes are sensitive to vibration, especially in portable use. Cayin addresses this with a custom isolation system that mounts the NuTube in a silicone housing on a flexible PCB with vertical pin support. This design reduces microphonics so the tube character remains consistent while walking or commuting.

Designed for daily use across devices

The RU9 works with smartphones, computers, tablets, digital audio players, and gaming consoles. The magnetic rear panel allows it to attach cleanly to compatible phones, keeping cables short and manageable. Support for UAC1.0 and UAC2.0 enables driverless operation on many platforms, including macOS and Windows.

Clear display and tactile controls

A 0.96-inch color display shows volume, sample rate, and format at a glance, with adjustable brightness for indoor or outdoor use. The knurled volume knob offers precise control, while side indicators provide charging and playback status without clutter.

Product highlights

• Korg NuTube 6P1 vacuum tube amplification
  
• Dual AK4493SEQ DACs in mono configuration
  
• Classic, Modern, and Solid-State timbre modes
  
• 3.5mm single-ended and 4.4mm balanced outputs
  
• Magnetic back for smartphone attachment
  
• Internal battery with USB and DC power options

Specs

• DAC: Dual AK4493SEQ
  
• Decoding: PCM up to 32-bit/768kHz, DSD512
  
• Outputs: 3.5mm single-ended, 4.4mm balanced
  
• Bluetooth: v5.1 with LDAC, aptX HD, aptX LL
  
• USB processor: XMOS XU316
  
• Battery: 2000mAh polymer lithium-ion
  
• Display: 0.96-inch color screen
  
• Material: CNC-machined unibody metal chassis
  
• Weight: 5 oz (142 g)
  
• Dimensions: 3.9 x 2.7 x 0.6 in (99 x 69 x 15 mm)

What’s included

• Cayin RU9 DAC and headphone amplifier
  
• Leather case
  
• L-shaped USB cable