Review sample provided by Grimm Audio
Retail price including VAT:
No internal storage: €17.999
Available SSD storage: 2 TB (€240) / 8 TB (€640)
Other storage sizes on special request
When I reviewed the original MU1 in 2021, I instantly became a fan, and the server has been a central part of my system ever since. A few years later, I was informed that another server was being developed and would be a groundbreaking product. With the MU1 being so great, what would set the new product apart? Of course, that new product is the MU2 Streaming DAC and Media Player, and as it turned out, they are similar, yet different. And that is both comforting and exciting.
Principally, the MU2 does not deviate all that much from the MU1, which is a good thing! But instead of relying on active digital speakers or an external DAC, the MU2 includes the company’s proprietary ‘Major’ DAC and a relay-based analog preamp section. This allows the designers total control over the front end and provides customers with a simple, single-unit, easy-to-use central audio system hub.
MU2 Streaming DAC and Media Player
The MU2 combines the functions of several components, including a Roon Server and Roon Endpoint, with internal SSD storage capability, a DAC and Master Clock, a Preamplifier with both digital and analog inputs, and analog volume control.
The MU2’s main music source is a Roon Core, Server, and End Point that offers playback from local files network files as well as streaming music via Tidal and Qobuz. You can stream from the MU2 to any other Roon endpoint from any brand, even multiple endpoints simultaneously.
Functionally, so far, the MU2 works just like the MU1. What differentiates it from the MU1 is that it combines its excellent server and streamer sections with an all-new Grimm-proprietary ‘Major’ DAC, a dedicated FPGA processor board with the highest possible quality in analog – and digital signal processing and dejittering, and an analog preamp with relay-based, analog volume control. Besides Roon, the MU2 offers two analog inputs and three digital inputs. There’s one balanced XLR input and one analog RCA input for analog, and TOSlink, RCA/Coax, and AES/EBU for digital. Analog outputs are available on RCA and XLR, and there’s even a 6,3 mm headphone output. Thus, the MU2 can function as a compact and beautiful yet very complete central hub in an audio system.
Key Technical Aspects
The MU2 is powered by in-house-designed switch mode power supplies that are clean and powerful and in-house designed voltage regulators (series and shunt type) to offer extremely high electrical isolation of sensitive circuit parts. The audio file and streaming functions are offered by Roon Labs software. The operating system is Tiny Core Linux. Grimm Audio has developed most of the other software that runs in the MU2.
On board is an exceptionally low jitter clock oscillator that is guaranteed accurate since each is measured with a proprietary jitter analyzer before it leaves the factory. Further, the MU2 employs an extremely powerful FPGA processor with highly sophisticated upsampling and noise-shaping algorithms.
The preamp section has an analog audio signal path that is Class A throughout, focusing on ultra-low harmonic distortion and ultra-low phase modulation. The relay-based analog volume control lets the user control external analog sources equally well as digital and internal sources.
The MU2 is intended to be used by the entire family. Its compact single-box design can easily be placed in any environment. The push/rotary dial on top offers a lot of functionality, while remote control is possible via Roon, a web interface, and any IR remote handset. An IR receiver unit is included with a cable that plugs into the back of the MU2.
Driven to keep improving, Grimm Audio works actively to provide ongoing updates. Whenever they improve the sound quality or enhance the feature set of the MU2, these improvements are offered to all existing customers via software or hardware updates (when possible). Software updates are free and can be installed automatically via the update server.
Finally, in case of a malfunction, the Grimm Audio Support Team is dedicated to getting you to enjoy the music again within the shortest timeframe possible. Grimm Audio offer a 5 year limited warranty to customers who fill in their warranty card.
Above: main web screen of the Grimm Web Interface, reachable by simply typing the unit’s IP address in a browser window.
If you need to know the unit’s IP Address, here are two tips: Roon shows the IP address in its settings general section, and the MU2 also shows its IP address on its front panel display on the second screen of the menu.
Below: various Web Interface menus and settings.
Grimm Audio’s proprietary Major DAC is a discrete 1.5-bit PWM converter that runs on the FPGA processor. It has a discrete design that combines the company’s Pure Nyquist upsampling filters with a “fundamentally flawless” 11th-order noise shaper of 1.5 bits. Grimm Audio have published interesting white papers, including one covering their “Pure Nyquist” filters. For this, please head to the Grimm MU2 page and see the Downloads tab.
For this review, I’d like to focus on the DAC itself. The following is a simplification of the information found in the Grimm Major DAC document, which can also be found in the aforementioned downloads section.
Creating a high-quality analog signal from a digital signal has always been a challenge for any manufacturer. Companies have tried various approaches, including using “discrete DAC” methods to achieve better results. After three years of work, Grimm Audio developed a unique digital-to-analog converter using a combination of FPGA pre-processing and their own discrete DAC hardware.
Existing DAC techniques use multi-bit, bitstream, or PWM conversion. The conventional multi-bit conversion demands extreme precision, making achieving more than 18-bit precision difficult. These types of DACs show distortion and cross-talk, causing graininess or harshness in sound character.
To get around these problems, Philips introduced the ‘bitstream’ single-bit technology in the 80s, later used for SACD as the ‘DSD’ format. Single-bit DAC is inherently linear, but it can only represent two signal levels, causing a high noise level. The technique works by quickly switching the single-bit value through oversampling and then pushing the high noise away from the audio band into the inaudible region above 20 kHz using a noise shaper. This process brings challenges to a DA converter, such as more stringent requirements on clock jitter performance and significant high-frequency energy on the DAC output. Additionally, digital noise shaper systems operate with limited stability, compromising transparency and audible coherence.
A PWM DAC overcomes the disadvantages of single-bit architecture by using multiple bits and varying the width of the single-bit stream. However, a noise shaper is still required and demands extreme processing power, resulting in a compromised implementation and sound quality.
The MU2 ‘Major DAC’ threads the theoretical optimal middle ground between all these options. It uses a 1.5-bit architecture, meaning amplitude linearity is inherently guaranteed as the 1.5-bit value is represented with a single-bit D/A cell in ‘PWM style.’ Just as with PWM DACs, the noise shaper runs with an effectively constant efficiency, realizing a linear operation over the entire dynamic range.
The input of the noise shaper is fed from the extreme precision “Pure Nyquist” digital FPGA filter, running at 128 times the base rate, which results in a zero error operation of the noise shaper. Moreover, the 1.5-bit DAC choice offers such a stable noise-shaping operation that it allows for a highly optimized, unique 11th-order noise shaper.
Finally, the Major DAC uses a 16-cell FIR DAC topology per channel to filter high-frequency noise before entering the analog signal path, which is implemented fully symmetrically using high-quality components and a relay-based volume control section.
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