Cayin RU7 USB DAC Headphone Amplifier

The Cayin RU7 is a USB-C dongle DAC with a difference: instead of a DAC chip it uses a discrete, fully balanced 1-bit resistor-network DAC, closely related to the one in Cayin’s N7 player. It plays native DSD and can convert everything to DSD, drives headphones with up to 400mW balanced through parallel amplifiers, has Cayin’s own discrete volume control, and comes with a handmade leather case.

A discrete 1-bit DAC

Cayin designed a micro-miniaturised 1-bit DAC from discrete components: a fully differential four-channel resistor network of 128 high-precision thin-film resistors (four groups of 32), closely resembling the discrete DAC of the N7. For accurate 1-bit DSD decoding the resistors must be precise and stay stable as the temperature changes, so Cayin chose parts with a 0.1% tolerance and a low temperature coefficient of 25ppm/°C. The network’s output passes through a three-level active low-pass filter and is converted to a single-ended signal for volume adjustment.

All to DSD

The RU7 plays PCM from 44.1kHz to 384kHz and native DSD64 to DSD256. Its audio bridge receives every format and puts it through high-precision signal enhancement; in the user-selectable All-to-DSD mode taken from the N7, everything is converted to DSD64, DSD128 or DSD256 before it reaches the 1-bit DAC. Cayin’s functional diagram shows the whole path, from the USB-C input to the two outputs.

Parallel amplifiers, real power

Two amplifiers work in parallel, the second a unity-gain inverting stage, giving four-channel balanced drive: 400mW per channel into 32Ω from the 4.4mm Pentaconn output and 160mW from the 3.5mm output, with an output impedance of 1.0Ω and 0.5Ω.

Shared line out

Either output can be set to a fixed line level that bypasses the volume control, 1.2Vrms from 3.5mm or 2.4Vrms from 4.4mm, chosen to keep as much of the 1-bit DAC’s detail and dynamics as possible with minimal noise and distortion. Connect it to a portable or desktop amplifier or a home system and use the RU7 as a miniature 1-bit DAC.

Discrete volume control

A discrete resistor DAC is very demanding on the integrity of the incoming data, so Cayin recommends leaving your phone’s digital volume at maximum. As off-the-shelf analogue volume chips couldn’t meet the RU7’s needs for precision, low noise and low power, Cayin developed its own resistor-network volume control: 100 steps through three segments of resistors and switching relays (steps 1–49, 50–79 and 80–100), precise over a wide range and gentle enough for the most sensitive in-ear monitors. The relays add a short delay when you change the volume, which is normal.

Two boards, low power

The circuit is split across two six-layer circuit boards, with digital and analogue circuits on separate boards so noise from the reference clock and DSP doesn’t bleed into the amplifier. A discrete resistor network also uses less power than a highly integrated DAC chip, so although Cayin redesigned the amplifier for much more output, the whole RU7 draws only about 100–110mA from your phone.

Design and compatibility

The 66 × 24 × 12mm CNC aluminium body weighs about 25g, with chamfered edges and smoothly curved surfaces for better handling. Its 0.96-inch 128 × 64 OLED screen shows the data rate and volume, and the hardware volume buttons double as menu controls in Mode settings. It works with Android phones and tablets, iPhone and iPad, Mac, Windows 7 to 11 (with Cayin’s USB audio driver) and music players with USB audio output.

A handmade leather case

The bundled green leather case is soft, handmade and custom-made for the RU7, with a precise cut-out for the screen, giving a tight fit and an elegant look. It is magnetic: two round metal plates with adhesive pads are included, so you can fix the RU7 to the back of your phone case without leaving residue.

Good to know

• Keep your phone’s volume at maximum and set the level on the RU7.
• The RU7 has no power switch: unplug it when you finish listening, or it keeps drawing on your phone’s battery.
• Windows computers need Cayin’s USB audio driver. iPhones and iPads with a Lightning socket need a USB-C to Lightning cable such as Cayin’s CS-L2C, sold separately.
• In the box: RU7, USB-C cable, USB-A to USB-C adapter, green leather case, two magnetic plates with adhesive pads and user manual.