Hi Fi/Home Theatre & Portable Audio

Cayin RU7 Discrete 1 Bit DAC/Amp Dongle

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The RU7 is Cayin’s second portable USB DAC/Amp. We learned a lot from RU6, we want to repeat the formula and do it better.

RU6 incurred the R-2R DAC technologies from R01 Audio Motherboard of N6ii DAP, and they were roughly 6 months apart. By the time we finalized the circuit design of N7 DAP, we know we should do this again, so we establish the RU7 as a high-priority project, hoping that the more affordable Dongle DAC will pass down the 1-Bit DAC technology to a larger group of users.

Irresistible Upgrade

The RU7 is equipped with three major upgrades when compared to the very well-received RU6 R-2R Dongle DAC:

Fully Balanced 1-Bit DAC architecture.

While reviewers and users appreciate 1-bit DAC technologies as natural, smooth, and realistic when compare to their analog experience in the practical world, they are inevitably not as popular as their PCM counterparts. The not-so-impressive measurements have hesitated a lot of vendors to devote their resources to 1-bit DAC. In addition, the existing solutions are far too bulky and consumed too much power for personal audio, so we didn’t have any 1-Bit DSD DAC implementation for DAP or even transportable DAC/Amp. for many years. To introduce 1-Bit DAC to portable users, Cayin offered our in-house developed micro-miniaturized 1-bit DAC circuit from fully discrete components in N7 DAP, and we passed down the DAC circuit design to the RU7 Dongle DAC almost as soon as we could.

We have provided a detailed explanation of 1-Bit DAC technologies and explained our implementation in Pure 1-Bit “DSD” DAC Explained. We can’t use the FPGA technologies from N7 here because it was oversized and drained too much power for the Dongle application. We simplified the Audio Bridge in N7 by adopting a hardware solution to take care of the All-to-DSD transcoding and upsampling. This implementation enables the RU7 to decode up to DSD256 natively, and it will handle PCM up to 384kHz flawlessly.

While the DAC of both RU6 and RU7 are resistor-network based, the 96 pieces (2×48) resistor network in RU6 is a stereo (2-ch) R-2R implementation, the 128 pieces (4×32) resistor network in RU7 is a fully balanced 4-ch DAC design. Although we need to convert the DAC output to single-ended for subsequent processing, the fully balanced DAC can render more detail and preserve more dynamics of the music. If we treat R-2R and 1-Bit as two different but equal engineering approaches to convert the digital audio bitstream to the analog waveform, the fully balanced design in RU7 will still be a step up from the singled-end design in RU6, and it can be marketed as a very pricy upgrade in audiophile marketplace.

By the way, RU6 and RU7 employ the same 0.1% 25ppm high precision thin film resistor (with different values). So if material means a lot to you, they are equivalent in quality and RU7 has an upper hand in quantity. 

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Parallel Driven Dual Phone Amplifiers

The RU6 track record suggested that our Dongle DAC is well-received by mobile phones (Android and iPhone), tablets, notebooks, and desktop computer users, and there is a niche group of users that use RU6 with their DAP. We want to provide an upgrade headphone amplification to as many users as possible with RU7, so we revised the circuit to parallel dual amplifiers, delivering 80% more power when balanced driven with similar overall power consumption.

The parallel amplification circuit of RU7 behaves differently from RU6. When driven at the same output voltage, parallel amplification will deliver more current. I paired the two Dongle DAC with Meze Liric volume matched, the RU7 offers better control and playback in a calm and at-ease manner. The difference with a 3.5mm single-ended phone out is less obvious, but when switching to a 4.4mm balanced, the edge of RU7 becomes more noticeable.

While Android phone and Tablet/notebooks/PC users will enjoy the extra power completely, iPhone users will also be benefited from the new parallel amplifier if they are using IEMs, we have tested many IEMs with iPhone+RU7 and drive them to abnormally loud levels, none of them triggered the power limiter in or test. However, if you are using demanding headphones or IEMs (especially some of the high-impedance earbugs), the power limiter will remain an issue.

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Shard 3.5mm and 4.4mm Line Out

This is the most wanted feature in the RU6 thread, period.

We disagree from the engineering perspective because the resistor network DAC behaves differently from highly integrated DAC chipsets. In short, thermal noise is the more dominating noise pattern in RU6 and RU7 and the noise level will increase when you turn up the volume, i.e., warming up the PCBs. This pattern is not causing a lot of problems because in regular applications. When you turn the volume of your Dongle DAC to a very high level, most likely you are using a hard-to-drive low-sensitivity IEM or headphones, and these headphones are, in most cases, immune to background noise. On the other hand, if you are using the Dongle DAC as a line out, you need to drive the dongles fairly hard. Some users set the dongle volume to 100 when they use it as Line out. No doubt this will introduce thermal noise into the system. and then the noise will feed into your desktop or home audio system and be amplified the second time. That explained why we are reluctant to introduce shared line out in discrete resistor network Dongle DAC.

Since we don’t have the space to implement a dedicated “unamplified” line out in the Dongle, a shared line out will drive the headphone amp to work at a fairly high load, hence warming up the circuit board and the space inside the Dongle. As a result, this will increase the thermal noise in the DAC circuit, hence not a desirable implementation.
Given the demand, we have implemented the shared line-out feature in RU7 because the 1Bit DAC is relatively more stable than R-2R DAC on thermal interference. Since this is a fixed voltage output from the phone amplifier, so they
are not significantly different from manually setting the phone out to a specific gain/volume setting, but this is more complicated than setting the volume to maximum and calling it a day. We measured the output carefully and determined an optimized setting that will retain the resolution and dynamic of the RU7 circuitry while controlling the noise and distortion at an acceptable level, and we ended up with 1.2V for singled-end and 2.4V for balanced. For the record, this is significantly lower than the standard line-level output of our DAPs, but it should work conveniently when you switch between phone out and line out in the RU7 menu.

We must emphasize once again that this is a shared line out only, it is inferior to an unamplified dedicated line out for sure. In addition, since RU7 does not have an MCU built in, we can’t implement a protection mechanism to reset the line out back to phone out whenever you unplug your 3.5mm/4.4mm connection, so please be reminded SERIOUSLY that you must switch the output back to a phone out immediately when you disconnect the line out cable from RU7. While the 1.2V/2.4V output level is not terribly high, it can still be alarming if you are using very high-sensitive IEM.

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Summary of RU7 Features

  1. Fully Balanced 1-Bit Discrete Resistor Network DAC
    • 1/1000 ultra-high precision TCR25 low-temperature coefficient thin film resistors
    • Native decode DESD64/128/256, support PCM up to 384kHz.
  2. User-selectable All-to-DSD
    • Users can specify DSD64/128/256 DSD output formats through menu setting
    • Different DSD output settings might sound slightly different and drain more power when a higher-resolution output format is selected.
  3. 100 steps 3-segment High Precision Resistor Array Volume Control
  4. Hardware Volume +/- button
  5. TWO 6-layer PCB, digital and analog circuits on separate boards
  6. 3.5mm single-ended phone output delivers 160mW per channel at 32Ω loading
  7. 4.4mm balanced phone output delivers 400mW per channel at 32Ω loading
  8. Compatible with Android, iOS, macOS, Windows 7/8/8,1/10, and DAP with USB Audio output
  9. Type-C USB Audio, Shield USB-C to USB-C cable bundled
  10. High/Low gain control
  11. Compact and seamless CNC aluminum chassis with 1” 128×64 OLED screen
  12. 66mm x 24mm x 12mm, ~25g net weight.
  13. Green Leather cases bundled
  14. Optional USB-C to lightning cable CS-L2C (purchased separately)

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