M21 Digital to Analog Converter

How does the M21 improve upon the M1? 

The M21 builds upon the M1's discreet output buffer and expands further. It incorporates the Native DSD converter initially developed for the M12 Source Controller, essentially upgrading the M1 design. With a focus on DAC functionality rather than being a pre/DAC like the M12, it's akin to the M12 minus analog inputs. To accommodate these enhancements, it adopts the larger chassis of the M12, along with superior analog power supplies, making it more potent than the M1. Additionally, the inclusion of the ladder DAC alongside the existing sigma-delta DAC offers three DACs in total within the M21.

What are the differences between the three DACs?

The M21 introduces a true DSD converter, originally developed for the M12 Source Controller, offering seamless integration as a standalone DAC. This converter operates without downsampling to PCM, a common practice in other DACs, thereby avoiding issues like snaps or pops during signal processing transitions. The M21's analog domain approach, complemented by an analog attenuator, ensures smooth transitions between DSD and PCM streams.

For PCM processing, the M21 features two converters. One utilizes a sigma-delta type based on the ADI 1955, maintaining consistency with Bricasti's standard DAC design found in previous models. The second is a Ladder DAC based on a 20-bit ADI part. Both converters benefit from advanced anti-aliasing filters and analog filtering, mitigating transition issues through DSP or the analog stage.

A critical aspect of the M21's design is its clocking and clock recovery mechanism. Employing a DDS (direct digital synthesis) technique with a femto clock, it reclocks the incoming data stream, eliminating jitter and ensuring precise reconstruction with a new clock.

Are all the conversion processes performed inside a custom chipset?

Typical audio chips are relatively inexpensive, with the pricier ones costing $15-20, a high amount for a stereo audio DAC chip. These chips, although not specifically designed for DSD, can accept DSD input and process it for conversion. However, true DSD chips are rare, which prompted us to design one for the M21. The Native DSD conversion in the M21 is achieved through discrete analog circuits entirely within the analog domain.

While true 1-bit converters are challenging to implement in analog, many DAC chips use digital methods to convert the data stream, filter it into a multi-bit PCM stream, and then apply DSP to handle DSD issues. Some DACs resample DSD to sample rate converters (SRCs) and convert everything to a fixed PCM rate before converting it to analog audio.

What are the main highlights of the DSD converter on the M21?

Discussing the challenges of DSD, the rapid speed of DSD one-bit signals, such as DSD 64 at 2.8 MHz, limits the possibility of any signal processing due to its binary nature. Consequently, chips and ladder DACs must convert it into PCM. The concept of the Native DSD circuit initiated in the M12 and continued in the M21, aims to create a genuine DSD converter entirely within the analog domain. This involves using a 10GHz flip-flop to modulate the data stream, followed by an analog noise filter and a low-order low-pass filter to manage noise and signal distortion. A Burr Brown level control is then employed to handle fades, mutes, and signal levels, seamlessly integrating with PCM DACs. Despite its effectiveness, this approach is not cost-effective, as it relies on discrete components rather than inexpensive DAC chips. Therefore, while DAC chips claim to accept any DSD rate, the actual internal processes remain obscure.

What is the design of the M21's analog volume control system?

The M21 features an analog level control to manage NDSD, enabling analog mutes and fades. Utilizing a Burr Brown volume control chip implemented as a current-to-voltage device ensures exceptional performance. Unlike most DACs, where such control is digital, the M21's pure 1-bit converter demands analog processing, including filtering. This setup allows for true DSD playback with seamless transitions and PCM playback. The analog attenuator offers the additional benefit of directly driving amps with a preamp function, unlike the digital attenuator in the M1. When set to 0dB, the M21's level control initiates a hard bypass of the attenuator during data playback, with transitions activating it. This versatility enables the M21 to serve as a standalone DAC or interface directly with amps.

When connected directly to amps, the M21 provides an analog attenuator and balanced outputs of up to 4.5V and unbalanced outputs of about 3V. With its analog attenuator, users can easily adjust levels without additional adjustments. We recommend trying the M21 connected directly to the M28 at shows.