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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.
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:
For more detailed 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 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.
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.
Summary of RU6 Features
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.
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:
- USB Audio Bridge: receive different audio format from USB, go through high precision signal enhancement, and output I2S bit-stream for subsequent processing.
- 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.
- 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.
For more detailed 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 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.
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.
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-layer PCB, digital and analog circuits 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, 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 aluminum chassis with 1” OLED screen at 28g.
- Optional Leather cases: orange or blue
- Optional USB-C to lightning cable
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.
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. Congratulations for another wonderful product.
