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.
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 24bit 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 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 earphone 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 R01 Audio Motherboard DAC circuitry can be divided into four subsystems:
- USB Audio Bridge: receive different audio format from USB, go through high precision signal enhancement, output I2S bit-stream for subsequent processing.
- Digital Audio Bridge: convert DSD to PCM; convert all audio signals to left and right channel of 24 Bit/384kHz serial audio data signal.
- Serial to Parallel Shift Register: convert serial data signal to parallel data signal and transmit to DAC circuit
- 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 in view of the space and power supply limitation, the oversampling feature will absorbed into the Audio Bridge and fulfilled by software based DSP. We added an USB Audio Bridge to handle the USB Audio input since this is the only digital input to RU6. The details R-2R design is also incurred from R01 Audio Motherboard include 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 Dongle DAC project. They will offer satisfactory matching accuracy required in R-2R ladder DAC circuit. The temperature coefficient of resistance (TCR) of these resistors is also respectable. Rate at TCR25 (±25 ppm/℃), the resistor value will only fluctuate within 25/1,000,000 per 1 degree change in temperature.
For more detail explanation of our R-2R implementation, please refer to our
R01 Audio Motherboard announcement.
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 anti-aliasing of the digital signal. The Oversampling DA mode will offer noticeable improvement on details and frequency extension. The playback is clean and sharp with darker background.
On the other hand, NOS DA mode maintains the sampling rate of the original bit-stream. 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 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 to Cayin RU6. R-2R DAC circuit is very demanding on incoming data integrity, digital volume of mobile phone 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, Music player app. supports bit perfect USB is recommended. If we use HiBy Music App as 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 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 different plug-in device. 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, you 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 in 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 are better volume chipset in the market, but they are either too big physically, or drain a lot of power, making it inappropriate for Dongle DAC application. Eventually Cayin bite the bullet and developed a high precision resistor array volume control circuit that provides 99 steps volume control through 9 segments of resistors and switching relays.
Resistor Array volume control is not a new technologies, it have been around for a very long time. They are very high quality volume control design, extremely transparent when implemented correctly and you can find them in a lot of high-end preamp and integrated amplifies. Each segment of resistor array can only provide ~10 steps of volume control, this is obviously not enough in real life application, so you need to "hop" to another set of resistor array for another 10 steps, and so on, and so on.
We don't recall anyone used resistor array volume control in a Dongle DAC, probably not even in portable DAP. There is no way we expected this final outcome when we started the RU6 project back in early February 2021 (when we completed R01 R&D). We basically completed the RU6 circuit design in 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 basically an out of proportion implementation for Dongle DAC and that us another month to finalise the furrent 9 segment resistor array design.
Our main hesitation with resistor array volume control is not about technical difficulties or budget consideration. The Resistor Array volume control involves switching relays when you hop from one segment to another. The relay will produce 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 set back from user experience point of view. For dedicated audiophiles who put audio performance, this is a small price to pay in order to implement 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 up front.
Amplification, Headphone Output and Other Issues
First of all, another confession. The RU6 is NOT a full 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 unity gain amplifier, all it does is to convert the original stereo signal to 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 full balanced design.
Design and implement full 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 are 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 impression 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 top priority, we can only advise you up-front that RU6 is not the best choice for you.
Summary of RU6 Features
- 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
- 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
- 99 steps Multi-segments High Precision Resistor Array Volume Control
- Hardware Volume +/- button
- TWO 6-layers PCB, digital and analog circuit on separate boards
- 3.5mm single-ended phone output delivers 138mW per channel at 32Ω loading
- 4.4mm balanced phone output delivers 213mW per channel at 32Ω loading
- Compatible with Android, iOS, iPadOS, macOS, Windows 7/8/8,1/10 and DAP with USB Audio output
- Type-C USB Audio, Shield USB-C to USB-C cable bundled.
- High/Low gain control
- Compact and seamless CNC aluminium chassis with 1” OLED screen at 28g.
- Optional Leather cases: orange or blue
- Optional USB-C to lightning cable
The suggested retail price of RU6 is
$249.99
The USB-C to Lightning cable is an optional accessories 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 local availability date. The global logistic is not at their best, so please be accommodative when it take a bit longer to arrive.
