Cayin N6ii, Unlimited Possibilities: a fully modularized smart DAP
May 13, 2021 at 7:36 AM Post #5,870 of 6,277

bluestorm1992

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What?!

N6II TI - R2R - Limited Edition. Compatible with previous motherboards.

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A screenshot of the R2R motherboard.

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May 13, 2021 at 7:38 AM Post #5,871 of 6,277
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R01 Audio Motherboard, our first R-2R implementation
N6ii-Ti (R-2R Titanium Limited Edition)


When we announced A02 back in September 2020, we conclude our announcement by saying “We do not have any Audio Motherboard in our development plan after A02, so unless a very innovative and feasible idea comes up down the road, this is it”. Apparently we have identified the innovation that warrants another Audio Motherboard.

To be honest, we are hesitated to roll out R01. This is Cayin’s first attempt on R-2R, and from Marketing point of view, we should reserve this as a selling point for new DAP down the road. The N6ii DAP is almost 2 years old (first shipment dated May 20, 2019), while its can satisfy the functional requirements of modern DAP, potential customers will probably start their search from recent models. This is inevitable for Android-based DAP as we bridged between consumer electronics and hobbyist audiophile product. While the modular approach has extended the product life of N6ii, we cannot expect the product to sustain indefinitely. For instance, R01 probably will help us to sell several hundred sets of N6ii, which is hardly feasible if N6ii is not modular based. On the other hand, a new R-2D DAP will definitely attract more attention and creates more sales. Nevertheless, we decided to bite the bullet and offer our first ever R-2R implementation to our devoted N6ii customers. The R01 is also the most resource consuming and highest production cost Audio Motherboard in N6ii platform. We want to complete the innovative mission of N6ii with our new venture, so the 6th Audio Motherboard will be the final Audio Motherboard for our 6 series N6ii DAP.

We describe the Audio Motherboard concept as "Unlimited Possibilities". With the inclusion of the R01 Audio Motherboard, we believe the N6ii platform delivers all the expectations when we announced N6ii back in 2019. To admire our R-2R breakthrough, we decided to create a R-2R Titanium Limited Edition of our N6ii DAP. We shall provide more information about this specially crafted N6ii later.

CLM00377.jpg


R-2R Implementation in R01

The digital audio section of R01 Audio Motherboard DAC circuitry can be divided into four subsystems:
  1. Digital Audio Bridge: receive different formats from source (aka N6ii FPGA), be it DSD or PCM in different sample rate, go through high precision signal enhancement, output to DSP filter.
  2. Oversampling Interpolation Filter: convert all digital audio signal to left and right channel of 24Bit/768kHz 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.

PCBA-1.jpg

PCBA-2.jpg


R2-R Ladder DAC in R01

There are loads of online materials explain the R-2R Ladder DAC technology in great detail, so we won’t bother to repeat here, for those who want to find out more about the theoretically side of R-2R Ladder DAC, you can check out HERE and HERE, or check out the tutorial video HERE.

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The basic building block R-2R ladder DAC is a matched pair of R and 2R resistors. We need 8 pieces of resistors (3 x R and 5 x 2R) to implement 4Bit R-2R Resistive Ladder Network (as illustrated in the diagram above). We can add another basic block to enhance the network to 5 bit, that will requires 10 pieces of resistors (4 x R and 6 x 2R), so on and so forth. We can work out the total number of resistors under the following formula:

Total No. of R-2R Resistor at N bit = (N-1) piece of R Resistors + (N+1) pieces of 2R Resistors​

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 choice of R resistors will affect the sound signature. After months of trial and experiment we finally find the best resistors for R01 circuit. We adopted 5.1kΩ as our R resistor, and naturally the 2R resistor is 10.2kΩ. This is not an easy decision because 5kΩ and 10kΩ high precision resistors are more available then 5.1kΩ and 10.2kΩ. We trust our ear and we are dedicated to deliver our best performance, so we stick with our finding and go for these very specific R and 2R resistors.

Once we have determined the R and 2R value, choice of resistors will be the next important step, and we need to consider the follow requirements:
  • Matching accuracy: The simple rule is that each of the R resistors is identical, and each of the 2R resistors must be exactly twice the resistance of the R. The higher the matching accuracy, the more accurate our DAC can achieve. There are two critical success factors here: the accuracy among R and 2R resistors, and the exact ratio between R and 2R in each basic building block.
  • Stability during change of temperature: The resistance value might change with temperature, therefore the accuracy of the resistors is only meaningful if we can install the resistors in a temperature-controlled environment, or the resistors can maintain its value as much as possible during temperature fluctuation. This is especially important when we are using the R-2R resistor network inside a compact audiophile DAP.
We have conducted a thorough research and negotiation and we finally selected Ultra Precision Low TCR Thin Film Resistor from Viking. These are fairly expensive resistors from production consideration. Their tolerance is as low as ±0.01% (or ±0.0001 or ±1/10,000). This is as close as we can get within N6ii Audio Motherboard form factors, and will safeguard the matching accuracy required in our R-2R ladder DAC circuit. The temperature coefficient of resistance (TCR) of the Viking thin film resistor is also very respectable. Rate at TCR10 (= ±10 ppm/℃), the resistor value will only fluctuate within 10/1,000,000 per 1 degree change in temperature.

The last but probably the most important reason to select Viking is because of their extraordinary support to custom make 10.2kΩ thin film resistor for our R01 project. While 5.1kΩ is a readily available off-the-shelf item from Viking’s high precision low TCR thin film resistor product line, the 10.2kΩ resistor is not, and we can’t find this anywhere else globally. There are three options to facilitate 10.2kΩ resistor in our design:
  • Connecting two 5.1kΩ resistors in series;
  • Connecting 10kΩ and 200Ω resistors in series as both value are available off-the-shelf;
  • Custom make 10.2kΩ resistor from Viking.
(1) is the “natural” choice mathematically, and we can reduce our procurement process by acquiring only one value of resistor. However this will increase the total number of resistor from 48 pieces per channel to 73. While marketing manager might enjoy the idea of using more resistors to impress consumers and asking for a higher price, Engineer will be frustrated because when you connect two resistors to achieve 2R value, the error will accumulate and increase by 100%. (2) is also theoretically sound but it will also incur additional error whenever you connect two resistors as 2R. (3) is the best engineering solution, but the R01 will involve irreplaceable custom components which will increase the BOM cost and complicated the supply chain.

At the end of the day, Cayin decided to go for route (3), and we are thankful to Viking that they are willing to accommodate our order. We have ordered 50,000 pieces of 5.1kΩ (off-the-shelf) and 10.2kΩ (custom-make) resistors respectively. Be reminded 50,000 pieces is a very small quantity for custom-make resistors and given the global shortage of semiconductor and electronic components since the COVID-19 outbreak, we are grateful to Viking for accepting and delivered the custom order. It takes 16 weeks to deliver the resistors to us, almost doubled the original schedule, which is not a big surprised if you are in this business, we have experienced worse.

Theoretically we can make 1000 units of R01 with our custom resistor on hand, the actual number should be round about 950 if we consider the average efficiency of SMT based production. We have placed another batch of Ultra Precision Low TCR Thin Film Resistor with Viking already, but there is no confirmed delivery date so far, hopefully we can make more R01 Audio Motherboard in near future.

R-2R Resistor.jpg



Oversampling or Non-oversampling

There are a lot of heated debate on oversampling and non-oversampling, we do not want to get into the detail, please Google if you are interested but I can warn you in advance that this is a never ending debate. However we can explain Cayin’s practice for your reference.

For all the Audio Motherboards (or DAP for that matters) we have offered so far, our FPGA won’t perform any oversampling processing before we feed the digital audio bit-stream to DAC chipset. However that doesn’t make us NOS DAP manufacturer because the DAC chipsets will perform oversampling internally. In fact, most, if not all, digital filters in DAC chipset are some form of interpolation oversampling filters. Cayin has tried our best to remain bit-perfect as much as possible, we prefer the natural sound signature that associated with bit-perfect processing, but we can appreciate some users might prefer oversampling approach because it will offer better SNR, optimize roll-off at high frequency range and simplify the LPF design requirements. Another frequently asked question that is related to NOS or oversampling is the pop sound when we start a track or change sampling frequency between tracks. Oversampling can resolve this pop noise graceful but NOS pre-processing will need to mute the pop noise.

Our initial R01 design was a pure NOS approach. We didn’t have the Audio Bridge and Oversampling filter in place, the digital audio signal was feed into the Serial to Parallel Shift Register directly. The prototype performs satisfactory with standardized 24bit hi-res music files. Unfortunately we run into serious problems when we conduct further test with multi-format music library and highly sensitive IEMs. We must acknowledge that the oversampling approach adds an extra processing step but it will provide a more pleasant user experience that fit the expectation of most consumers. NOS approach R-2R might works well for speaker based setup when the (mild) pop noise was emitted from speakers at 2 to 3 meters away. We finally deserted the NOS approach and included a D2D oversampling Interpolation DSP filter in our R01 design. For the record, we adopted hardware 8x oversampling solution but we only use the DSP filter upto 4x in R01.

R01 Functional Diagram FHD.jpg


The Functional Framework of R01

The phone amplification circuit design is similar to A01 and T01. It is fully optimized for fully differential amplification, and the final headphone output stage employs set of four Op-Amp in parallel amplification. This will increase the output current capability and lower the output impedance. In practical, these improvements will translate into better control, ability to sustain a heavier loading, and drive most portable headphones with lots of headroom.

The R01 is a headphone-output only Audio Motherboard. Like E01, It does not have line out feature. We have been fighting very hard to fit the R-2R Resistive Ladder Network into the Audio Motherboards PCB. Line out from sigma-delta DAC chipset are relatively straightforward because we can take the LPF output as the foundation of line out signal. The R-2R design requires more work to deliver a dedicated line out. The background noise pattern of R-2R network also makes it inappropriate for pseudo lineout implementation. In other Audio Motherboards, the background noise basically remains the same level in all volume level, so it will affect the sensitive IEM users as they tend to listen at low volume setting (i.e, same amount of noise but lower output level). On the other hand, the background noise of the R-2R ladder DAC circuit will increase as you turn up the volume. While sensitive IEM users might welcome this pattern, it makes pseudo line out inappropriate because when you max out the volume of the amplification circuit, the background noise become audible.

Another unique feature of R01 Audio Motherboard is the inclusion of 24.576MHz Reference clock within R01. For the record, all other Audio Motherboards are relied on the two master clocks in the N6ii main units. This is mainly because the R-2R ladder DAC design required very high precision and a local reference clock will perform better than a remote Master Clock. We have also implemented two digital filters in R01: Sharp Roll-Off and Slow Roll-Off. We have worked with DAC chipsets that offers a lot of choice of digital filters, but we have also received a lot of feedback that they cannot differentiate the digital filters from each others. We learn from previous implementation and focused in two digital filters that offer distinctive sound signature.

Last but not least, the R-2R Resistive Ladder Network is also very sensitive to power supply. We have include low noise high current LDO in each channel as VREF for R-2R circuit. This will lower the non linear error rate and reduce the noise level significantly.

R01.jpg

R01-2.jpg


R01 Specification.jpg


The SRP of N6ii-Ti (R-R Titanium Limited Edition) is US$1899, SRP of R01 is US$619. Both item will be available by early June 2021.

The R01 is the final Audio Motherboard in N6ii family. However we are working on a new product to continue the Audio Motherboard concept. It is something completely new to Cayin so probably will take some time before we can deliver it to the market.

Detail of N6ii-Ti (R-2R Titanium Limited Edition) will be provided later (TBC ....)
 
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May 13, 2021 at 7:46 AM Post #5,872 of 6,277

msiekkb

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@Andykong I see this module have 3.5mm and 4.4 mm headphone output?
 
May 13, 2021 at 7:51 AM Post #5,873 of 6,277

ThanatosVI

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N6ii-R2R (Titanium Limited Edition)
I'm so glad I didn't buy one yet and neither the new A&K Se180.

R2R Board was exactly what I was waiting for.
New Titanium R2R Edition looks like the perfect opportunity to get one
Any chance on bundles with the C9?
 
May 13, 2021 at 7:57 AM Post #5,875 of 6,277

bluestorm1992

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Edit: Sorry, let me correct myself. R01 still available for a separate sale, just without the Ti chassis.
 
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May 13, 2021 at 8:02 AM Post #5,877 of 6,277

bluestorm1992

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R01 Audio Motherboard, our first R2R implementation.
N6ii-R2R (Titanium Limited Edition).


Edit 2: Correcting this post: R01 still available for a separate sale, just without the Ti chassis.
 
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May 13, 2021 at 8:06 AM Post #5,878 of 6,277

msiekkb

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One of those chips on the module is probably a SE to differential convertor. Its sad because for headphones it basically makes no difference in most cases but Cayin won't be able to sell any without a balanced output of some type (2.5, 4.4 etc).
For me best option are SE, because I have a lot of cable to 3.5mm. SE are very good as like Balanced output, Dethronay DTR1 have only SE, Lotoo have to very good SE too. For me the best module for N6 II is E01, I like sound from this module, but for my CIEM have not enough power.
 
May 13, 2021 at 8:09 AM Post #5,879 of 6,277

MarkParity

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Yeah, I asked their marketing rep, and she said only with the new machine.

Edit: I think there is another new product - a New DAP.
Oh I understand, so basically N6ii + R01 as a package and no R01 on its own, if that is the case then that would be another seriously stupid marketing decision by Cayin, but I wouldn't be surprised.
 
May 13, 2021 at 8:11 AM Post #5,880 of 6,277

bluestorm1992

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Oh I understand, so basically N6ii + R01 as a package and no R01 on its own, if that is the case then that would be another seriously stupid marketing decision by Cayin, but I wouldn't be surprised.
BTW just saw the price of the Ti machine: 11988 RMB ($1800).

Edit: R01 still available for a separate sale, just without the Ti chassis.
 
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