“Spring 2” Discrete R2R DAC

Spring 2 Improvements 
Spring 2 is a new generation of fully discrete resistor network (commonly known as R2R) DAC produced by Holo Audio. Compared with its predecessor product “Spring”, it has the following improvements:

– A new generation of fully discrete resistor network DAC modules with lower distortion especially for low level signals, and faster conversion speeds.
– The world’s first DAC that truly supports DSD1024 and PCM1.536M
– The USB interface uses self-developed firmware with ultra-low latency and highly reliable data transfer. The performance is 2-4 times higher than the default firmware.
– Femto clock
– Improved I2S input interface, four independent receiving circuits, which make the I2S clock signal subject to lower interference
– Improved power and analog reference power for the digital section. Superior performance, PSRR above 110db and noise below 0.2uV within 100KHz
– Remote control (L2 and above)
– Improved display

Spring 2 DAC Core Modules
The Spring 2 DAC core module is composed of a discrete resistor network. One of the resistor networks is an updated version of Spring R2R, which is responsible for the digital analog conversion of PCM. Another set of resistor networks is specially designed for DSD, which is named “Vector Stepper Resistance Network”. The two sets of resistance networks of Spring 2 DAC are fully balanced, one positive and one negative, respectively on the front and back sides of the PCB.

Delta-Sigma and R2R
As we all know, there are two types of digital audio formats, PCM and DSD. PCM consists of binary representation of each analog sample, commonly known as multi-bit. DSD contains the analog quantity by the pulse ratio of 0s and 1s, commonly known as single-bit. The original analog signal during ADC process is converted to binary, which is PCM.

Delta-sigma type DACs become a popular choice, since delta-sigma type DACs need to modulate multi-bit binary data into high-frequency single-bit data, the modulation process is a key step. This is better to perform high-quality modulation by computer software beforehand and pass the modulated signal to the DAC chip just for conversion, instead of having the DAC chip to perform real time modulation. This is why the digital signal format of DSD was born. It is to avoid the low-quality modulation that comes with the chip. 

Unfortunately, modern delta-sigma chips are designed to achieve lower distortion at lower cost and Multi-bit delta-sigma modulation has been adopted. After receiving the DSD data, these chips internally modulated into a multi-bit delta-sigma signal for digital analog conversion. The existence of DSD itself is to avoid the modulation of the chip, and such re-modulation undoubtedly makes the DSD signal lose its advantages and meaning of existence. This is why often modern chips have strong characters, rather than faithfully replay the contents in the recording. Spring 2’s “Vector Step Resistance Network” does not require additional modulation of the DSD signal, and digital analog conversion is directly performed using the DSD source signal. This can fully reflect the advantages of DSD, and faithfully reproduce the original recording, instead of adding or subtracting anything.

As mentioned above, the original analog signal is converted to binary during the ADC process, that is, the PCM format. So, PCM is a more primitive digital format, and why there is still DSD? We will need to refer to the PCM characteristics. As a binary code, the weight of the most significant bit is the Nth power of 2 of the least significant bit. In the case of 16-bit PCM, the most significant bit is 32768 times the least significant bit. The most significant bit needs to guarantee the accuracy of 1/32768, which is 0.003%, to ensure the accuracy of 16 bits. If it is 20 bits, it needs 0.0002% accuracy. Modern 0.01% precision metal film resistors are already very expensive. Therefore, DAC conversion of PCM requires extremely high accuracy, which makes the native PCM DAC, also known as the R2R DAC, too expensive. The delta-sigma DAC is digitally modulated into high-frequency pulses, which can achieve high precision at lower cost, that’s why delta sigma DAC chip became the mainstream. As mentioned above, the quality of the delta sigma chip’s own modulation function is not high, so it is necessary to pre-modulate the delta sigma data such as DSD.

Behind Holo R2R Design
Having said that, to achieve real direct conversion of PCM source data, an R2R DAC is required. The mainstream delta sigma chips modulates the PCM data at high frequencies and its no longer PCM data. However, in order to achieve good results with R2R decoding, resistance accuracy problem must be resolved. Even the best 0.01% resistors available on the market does not have enough accuracy. One solution is to trim the resistor with a laser to accurately tailor the resistor to higher accuracy. This method can only be used in the process of semiconductor chips, and the equipment required is also very expensive. And we adopted dynamic compensation technology to make an additional resistor network to compensate for the accuracy error of the main resistor network. For example, an ideal value is 32768, and the actual value is 32700 due to the resistance error. The auxiliary resistor network produces a 68 that is added to the main resistor network to form an ideal value of 32768. With this technology, we can achieve an incredible 0.00005% accuracy of the compensated equivalent resistance. This is also the main reason why the distortion of the Spring can be as low as -114db. Of course, the extra auxiliary resistor network adds a lot of cost and requires very complicated digital processing. But its worth all the efforts. To date, Spring is the world’s lowest distortion R2R DAC. No other R2R DAC on the market, despite the price, has lower distortion than the Spring.

Insights about Spring 2 improvements
Spring 2 supports DSD1024 and PCM1.536M via USB. This is the first DAC in the world that supports such a high bit rate. All of this comes from the XMOS firmware developed by Holo Audio. The XMOS original firmware architecture cannot meet the requirements of the DSD1024. This is because the original firmware is not optimized for the “dual issue” command of the XU200 series chip, which is required to achieve the claimed 1000mips performance. In addition, the original firmware architecture does not provide the low latency required by the DSD1024. So, we abandoned the original firmware architecture and used a new low-latency, high-reliability architecture. The “dual issue” feature is also used to process critical data, fully utilizing 1000mips performance. This made Spring 2 the first DAC in the world to truly support DSD1024 and PCM1.536M. Of course, not everyone needs such a high rate. The improved performance of the new architecture’s USB firmware allows for lower latency and more performance margins when dealing with low-rate signals. So, even if you play 44.1K, you can hear better sounds.

In the last generation of Spring, one thing that was criticized by users was that the Spring did not use a femto clock. This is because the development time of the Spring took nearly two years and at the beginning of the development, there was no suitable femto clock available on the market. Now we included the femto clock to improve the previous shortfall in Spring 2.

Different Sampling Modes
Spring 2 has a variety of sampling modes and flexible conversion modes

– NOS mode, without digital oversampling, directly converts the original data. Since digital oversampling can cause time domain distortion such as ringing effects, NOS can avoid these problems. Generally speaking, NOS will have a significant impact on other performance indicators, but the design of the Spring 2 can also maintain good performance indicators in NOS mode.
– In OS mode, the PCM is oversampled to a higher frequency PCM, the DSD is oversampled to a higher frequency DSD, and then perform digital analog conversion.
– OS PCM mode, whether it is PCM input or DSD input, oversample to PCM for digital analog conversion.
– OS DSD mode, whether it is PCM input or DSD input, oversample to DSD for digital analog conversion.

Spring 2 configurations
Spring 2 has 3 configurations. The L1 is not equipped with a remote control, and the film capacitor of the main power supply is standard. L2 is equipped with a remote control, and the film capacitor of the main power supply is two Mundorf gold and silver oil-immersed film capacitors. L3 is based upon L2 and additionally equipped with Silver winded power transformer, silver internal power cable and Audio Magic fuse.

Spring 2’s digital input interface includes USB (ground isolation), RCA, BNC, AES, fiber, and HDMI interface I2S. All digital input interfaces support DSD (DOP mode). The “spring” analog output interface is single-ended and balanced. The sampling rates of the supported PCM and DSD are listed in the table below.

Dimensions: 430mm (width) * 300 (length) * 55 (high, feet not included), the height of the feet is 12mm
Weight: 8.5 kg