Cayin RU6 USB DAC Headphone Amplifier

The Cayin RU6 is a USB-C dongle DAC with a real discrete 24-bit R-2R resistor-ladder DAC instead of a DAC chip. It offers non-oversampling and oversampling modes, Cayin’s own resistor-array volume control, and 3.5mm and 4.4mm outputs with up to 213mW into 32Ω, in a CNC aluminium body weighing about 28g.

A discrete R-2R ladder

An R-2R ladder pairs resistors of value R and 2R, twice the value. For 24-bit decoding Cayin uses 48 resistors per channel (23 × R and 25 × 2R), 96 in all. Cayin searched for resistors with extreme precision that stay stable as the temperature changes, and chose ultra-precise 0.1% thin-film parts with a low temperature coefficient of 25ppm/°C. R-2R is known for a natural, realistic sound, a different experience from chip DACs.

NOS or OS

• Non-oversampling (NOS): keeps the original sample rate with no digital filter or post-processing, optimised in the time domain for low phase distortion and jitter and no ringing: musical, natural, organic and coherent.
• Oversampling (OS): upsamples by interpolation before conversion, improving resolution, noise and anti-aliasing in the frequency domain: impressive detail and extension, a clean, sharp presentation and a darker background.

Volume done properly

An R-2R DAC needs a perfect incoming data stream, and a phone’s digital volume would badly harm the sound. Off-the-shelf volume controls couldn’t meet the RU6’s needs for precision, low noise and low power, so Cayin developed its own discrete resistor-array volume control: 100 steps (0–99) through nine resistor segments switched by relays, transparent to the natural R-2R sound. The relays add a very short delay when you change the volume. Hardware buttons set the volume and double as menu keys in Mode settings, and a 1-inch OLED screen shows playback information.

Two main boards and the signal path

To minimise interference, the circuit is split across two six-layer boards, with the digital and analogue sections separate so noise from the reference clock and DSP can’t bleed into the amplifier. Cayin’s diagram shows the path: a USB audio bridge and a digital audio bridge, clocked at 24.576MHz, convert everything to PCM (NOS, or oversampled to 384kHz) for the 24-bit R-2R DAC, followed by a low-pass filter, the resistor-array volume, a line driver and a headphone amplifier with gain control. For the 4.4mm balanced output, a unity-gain inverting amplifier creates the negative phase.

Low power, wide compatibility

A discrete R-2R circuit uses relatively little power compared with highly integrated DAC chips, so it is kind to your phone’s battery. It works with iPhone and iPad, Mac, Android phones and tablets, and players with USB audio output; Windows 7 to 10 need Cayin’s free USB Audio 2.0 driver. It plays PCM up to 384kHz and DSD64/128/256, with high and low gain for different earphones and headphones, and is small enough for the smallest pocket, for music, video or gaming.

Good to know

• Use a shielded USB cable, like the one supplied.
• In the box: shielded USB-C to USB-C cable and a USB-A to USB-C adapter. A leather case (blue or orange) and a USB-C to Lightning cable are sold separately.
• For more power, Bluetooth and an ESS DAC see the Cayin RU7.