Cayin RU6: R-2R USB Dongle DAC with Head-Amp
Oct 11, 2021 at 9:07 AM Thread Starter Post #1 of 3,546
Nov 16, 2013
While we are busy working on our new flagship products, we have also devoted a lot of resources to passing down our new technologies to more affordable applications. Our R-2R based R01 Audio Motherboards for N6ii were well-received by the Personal Audio community. While a lot of users urged us to develop a high-end R-2R DAP with the latest CPU and software technologies, our priority is to make the R-2R technologies we developed for N6ii more widely available before we move to another high-end R-2R DAP.

RU6 Sales Poster.jpg

Background Story

The RU6 is the first portable USB DAC/Amp from Cayin. We have noticed the enormous demand in the Dongle DAC market, but we are not satisfied with the highly integrated off-the-shelf solutions, so we take our time to look for the right technologies. By the time we completed R01 Audio Motherboard R&D for N6ii DAP, we noticed the R-2R resistor ladder network can be implemented as low power consumption DAC circuit that offers outstanding audio performance. The bigger difficulties are space and operation limitations but we are confident that R-2R Dongle DAC is technically feasible.

The RU6 Dongle DAC features an in-house developed 24-bit Discrete R-2R resistor ladder circuit that can decode up to 384kHz. The hardware volume control buttons are backed by a sophisticated resistor ladder volume circuit, and the user can select between Non-Oversampling or Oversampling mode through a simple menu setting. Two headphone outputs, 3.5mm and 4.4mm, are in place so you can connect RU6 to a wide selection of earphones in the market.

24Bit Discrete R-2R Reistor Ladder DAC

The basic idea of R-2R ladder is a matched pair of two resistors, the first is “R” and the other is “2R”which has twice the value of R. To achieve 24bit decoding, we need 48 pieces of resistors (23 x R and 25 x 2R), and that’s good for one channel. So for a stereo 24Bit R-2R decoder, we need exactly 96 pieces of resistors. The digital audio section of the R01 Audio Motherboard DAC circuitry can be divided into four subsystems:
  1. USB Audio Bridge: receive different audio format from USB, go through high precision signal enhancement, and output I2S bit-stream for subsequent processing.
  2. Digital Audio Bridge: convert DSD to PCM; convert all audio signals to the left and right channels of 24 Bit/384kHz serial audio data signal.
  3. Serial to Parallel Shift Register: convert serial data signal to parallel data signal and transmit to DAC circuit
  4. 24-Bit Discrete R-2R Precision DAC: convert digital audio signal to analog audio signal through a high precision R-2R Resistive Ladder Network.
Basically (2) to (4) is incurred from R01 Audio Motherboard, we can't accommodate the hardware interpolation filter in RU6 because of the space and power supply limitation, the oversampling feature will be absorbed into the Audio Bridge and fulfilled by software-based DSP. We added a USB Audio Bridge to handle the USB Audio input since this is the only digital input to RU6. The details R-2R design also incurred from R01 Audio Motherboard including the R and 2R value (5.1kΩ and 10.2kΩ respectively). Unfortunately, we can’t afford to use the same high-precision resistors, so we go down one step and adopted high precision low TCR Thin Film Resistors rated at ±0.1% (or ±0.001 or ±1/1,000). This is as close as we can get with the Dongle DAC project. They will offer the satisfactory matching accuracy required in the R-2R ladder DAC circuit. The temperature coefficient of resistance (TCR) of these resistors is also respectable. At the rate at TCR25 (±25 ppm/℃), the resistor value will only fluctuate within 25/1,000,000 per 1-degree change in temperature.

For more detailed explanation of our R-2R implementation, please refer to our R01 Audio Motherboard announcement.

RU6 PCB.jpg
RU6 01.jpg
RU6 02.jpg

Non-Oversampling/Oversampling DA mode

Cayin offers both Oversampling and Non-oversampling DA modes in RU6. For Oversampling mode, the Digital Audio Bridge will upsample the digital audio data to 384kHz through digital filters. This will enhance the resolution, reduce noise and improve the anti-aliasing of the digital signal. The Oversampling DA mode will offer a noticeable improvement in details and frequency extension. The playback is clean and sharp with a darker background.

On the other hand, NOS DA mode maintains the sampling rate of the original bitstream. This will get rid of the digital filters in OS mode, and maintain the signal in perfect time sequence. The phase distortion and jitter will remain at a very low level without ringing artifact. Playback will become very musical with a natural, organic, and coherent presentation.

High Precision Resistor Array Volume Control

Most USB dongle DAC will rely on the volume control of your mobile phone to control the volume of the headphone output. Unfortunately, this is not a viable solution for Cayin RU6. R-2R DAC circuit is very demanding on incoming data integrity, the digital volume of mobile phones will damage sound quality badly, so the more effective solution with R-2R design is to keep the incoming bit-stream at full volume and then implement a high-quality analog volume control after the R-2R DAC circuit. For these reasons, the Music player app. supports bit-perfect USB is recommended. If we use HiBy Music App as an example, you need to turn on Exclusive HQ USB Audio Access and USB Output setting > Lock USB Audio Vol in your App setting (as illustrated below), this will safeguard the quality of USB Audio output. For applications that don't provide a similar feature, you'll need to turn the volume to 100 manually. This is inconvenient to users for sure, but fortunately, most, if not all, current mobile phones will "remember" the volume setting of the different plug-in devices. So as soon as you unplug the RU6 from your mobile phone, the volume will fall back to the setting BEFORE you plug in RU6, and when you plug in RU6 again, your phone will raise the volume to 100 automatically.


Cayin tested numerous off-the-shelf volume options, unfortunately, they cannot meet the high precision, low noise, and low power consumption requirements of the dongle DAC application. For instance, we have used PGA2311A in several DAPs and R01 Audio Motherboards, we tried to repeat the trick but the RU6 developed unacceptable background noise. There is a better volume chipset in the market, but they are either too big physically or drain a lot of power, making them inappropriate for Dongle DAC application. Eventually, Cayin bite the bullet and developed a high-precision resistor array volume control circuit that provides 99 steps of volume control through 9 segments of resistors and switching relays.

RU6 06.jpg
RU6 07.jpg

Resistor Array volume control is not a new technology, it has been around for a very long time. They are very high-quality volume control designs, extremely transparent when implemented correctly and you can find them in a lot of high-end preamps and integrated amplifiers. Each segment of the resistor array can only provide ~10 steps of volume control, this is not enough in real-life applications, so you need to "hop" to another set of resistor arrays for another 10 steps, and so on, and so on.

We don't recall anyone using resistor array volume control in a Dongle DAC, probably not even in portable DAP. There is no way we expected this outcome when we started the RU6 project back in early February 2021 (when we completed R01 R&D). We completed the RU6 circuit design in a very fast track but we stuck at the volume control issue for 3 months trying different solutions and going nowhere. Using Resistor Array Volume Control is our final trump card. It is an out-of-proportion implementation for Dongle DAC and that is another month to finalize the current 9-segment resistor array design.

Our main hesitation with resistor array volume control is not about technical difficulties or budget considerations. The Resistor Array volume control involves switching relays when you hop from one segment to another. The relay will produce a very mild pop sound with speakers and is inaudible when you are 2 or 3 meters away from the speaker. Unfortunately, the pop noise will become very annoying to sensitive IEM users. For this reason, we need to mute the output for a short moment when a relay kicks in. This will introduce a small delay (around 40ms) in volume adjustment, definitely a setback from a user experience point of view. For dedicated audiophiles who put audio performance, this is a small price to pay to implement an R-2R resistor ladder network in a dongle DAC. but we can understand some users might consider this as a deal breaker, that's why we explained this in detail upfront.

Amplification, Headphone Output and Other Issues

First of all, another confession. The RU6 is NOT a fully balanced designed Dongle DAC. The 4.4mm phone output is balanced-driven, but the signal path is primarily a single-ended design. We added an extra op-amp (identical to the primary headphone amplification Op-Amp) in the final output stage as a unity gain amplifier, all it does is convert the original stereo signal to a negative phase. This will provide extra power and current from the regular 3.5mm phone out, but the channel separation is not as good as a fully balanced design.

Designing and implementing a fully balanced amplification circuit for R-2R is not difficult, we have done that with R01. The primary limitation is space, and power consumption is also something we need to consider. If you hesitated because the 4.4 phone out is not fully balanced design, please wait till you have the chance to audition the RU6 in person, or when there are reviews or user impressions that provide all the information you need about RU6.

To be honest. if you study the specification of RU6, they are not particularly impressive. They are not bad for sure because we have done our engineering work properly. Measurement is not the strong suit of R-2R technologies, especially with NOS design. For those who put measurement as a top priority, we can only advise you up-front that RU6 is not the best choice for you.

RU6 05.jpg
RU6 Open Box Optional.jpg

Summary of RU6 Features
  1. 24-Bit Discrete R-2R Resistor Ladder DAC
    • 1/1000 ultra-high precision TCR25 low-temperature coefficient thin film resistors
    • Support up to PCM 384kHz and DSD 64/128/256
    • Low power consumption
  2. User-selectable NOS/OS DA Mode
    • NOS mode: digital filter-less, low phase distortion, low jitter, no ringing artifact
    • OS mode: increased sampling rate, enhance resolution, reduce noise, improve anti-aliasing
  3. 99 steps Multi-segments 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 138mW per channel at 32Ω loading
  7. 4.4mm balanced phone output delivers 213mW 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” OLED screen at 28g.
  12. Optional Leather cases: orange or blue
  13. Optional USB-C to lightning cable
The suggested retail price of RU6 is $249.99
The USB-C to Lightning cable is an optional accessory at $19.99
Two protective cases (blue and orange) are also available at $19.99

We have commenced mass production of RU6 already, they should be available by 1 December 2021, please consult your Cayin dealer for the local availability date. The global logistic is not at their best, so please be accommodating when it takes a bit longer to arrive.

RU6 Open Box.jpg

RU6 Specification.jpg

RU6 Functional Diagram V1.0 20211027.jpg

RU6 PCB explode.jpg
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Oct 11, 2021 at 9:33 AM Post #2 of 3,546
So why these USB dongles can't also work via BT to compete with BTR5 or other BT receivers too?:thinking:
Would it considerably increase the build size or something ?
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Oct 11, 2021 at 9:35 AM Post #4 of 3,546
I have not used a dongle before but I am interested in this one
Oct 11, 2021 at 10:22 AM Post #5 of 3,546
So why these USB dongles can't also work via BT to compete with BTR5 or other BT receivers too?:thinking:
Would it considerably increase the build size or something ?

Perhaps expense and more importantly, noise potential.
Oct 11, 2021 at 10:29 AM Post #6 of 3,546
Perhaps expense and more importantly, noise potential.
Well, I'm no expert but I think BT can be turned off when not in use perhaps, like on phones?
Anyway it would have been much more appealing for my use case.
Oct 11, 2021 at 12:38 PM Post #10 of 3,546
Since the cat is out of the bag, and there were discussion of our forthcoming USB DAC/Amp Dongle in multiple threads/forums/groups, we decides to start a new thread ahead of our regular schedule.

RU6 Design SQ.jpg

While we are busy working on our new flagship products, we have also devoted a lot of resource to pass down our new technologies to more affordable applications. Our R-2R based R01 Audio Motherboards for N6ii was well-received by the Personal Audio community. While a lot of users urged us to develop a high-end R-2R DAP with latest CPU and software technologies, our priority is to make the R-2R technologies we developed for N6ii more widely available before we move to another high-end R-2R DAP.

The RU6 is a scaled-down implementation of the R-2R resistor ladder network DAC from R01 Audio Motherboards. I don't have a lot of information at this stage but I'll update the thread and the first post regularly until we have the complete product announcement clarify all the details to our users. This is what I can share at this moment:
  1. First ever 24Bit discrete R-2R DAC in USB DAC dongle, with 48 high precision resistors per channel.
  2. Choice of NOS mode and Oversampling mode
  3. Build-in high precision Volume control
  4. 3.5mm single-ended and 4.4mm balanced phone out
  5. Availability: Q4 2021
  6. Price: $2xx, to be confirmed
To be honest, R-2R is not a user friend technologies for USB Dongle application. To start with, the size of the RU6 is fairly big when compare to other USB dongle in the market: that the minimal price when we replace an off-the-shelf DAC chipset with 96 resistors. We have also run into a lot of compatible and operational problems during the later part of the project. At the end of the day, we have decided to go for best possible sound performance with some compromise in user operation. This is a tough call from business perspective because we know convenient and user satisfaction are indispensable factors for mass-market consumer electronic products. Putting sound performance over operation convenient will limit our users to a very niche group of customer based, but our passion to high quality audio performance drove us to do crazy things again.

I can only show you some rough photos on the engineering sample at this moment, I'll upload more photo when we completed our trail production run later this month.

RU6 Engineering Sample 1920.jpgRU6 Engineering Sample 02 1920.jpg
Thanks for the update, Andy. I'll buy one. If it sounds better than the L&P W2, I don't really care as much about usability features. Well, I do care. But I care about great sound the most. So I am probably in your target customer base. Too bad it doesn't have a true line out. Would have been nice to pair it with the C9.
Oct 11, 2021 at 2:59 PM Post #14 of 3,546
give it 2 - 3 years and dongles will run the portable game.
Quite possible-you can even argue the W2 already knocks out most $1-1.5k DAPs from a pure sound quality aspect. It's really just the use case of battery burn/not having a separate LO'd music player that can be an inconvenience to most.
Oct 11, 2021 at 2:59 PM Post #15 of 3,546
give it 2 - 3 years and dongles will run the portable game.
Only if they get their power use under control. I am actualy here on Head-fi because I was destroying my phone batteries prematurely. Time for a dedicated device, I thought. Happy I did, but 2-3 charge cycles for a new phone in a day using a dongle vs. 1 charge every 3 days when not using is a huge difference.

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