John Darko
Is the Grimm MU2 Music Player the most expensive product I’ve ever had in for review? At a cool €18,000, I think it might be. The internal 8TB SSD storage fitted to my unit is a €600 upcharge. “Music Player” is a euphemism for a streaming-centric preamplifier that includes a Roon Core (server), Roon endpoint, DAC, line-level analogue pre-amplifier and 6.4mm headphone output. No Spotify Connect. No Tidal Connect. No Apple AirPlay. No Google Cast. The MU2 is strictly – and proudly – a Roon-centric machine. Just add a power amplifier and passive loudspeakers; or a pair of powered speakers.
The CD-sized disc set into the MU2’s inwardly-curved aluminium top plate rotates for “no compromise” relay-based volume changes and clicks for play/pause. A longer click pulls up the menu system on the rudimentary 3.5″ front panel display. That’s useful for changing the active input. The supplied external wired infrared receiver means we must train our own IR wand. The Apple TV remote is a good choice for anyone not already using an AppleTV — I’ve bought something a little different to sidestep sending IR commands simultaneously to the MU2 and the Apple box.
Outside of Roon and click-wheel controls, a web interface allows us to change the volume, active input and to customise those inputs with gain offsets. It’s through this web interface that we can also change the brightness of the screen and front-panel LED; retina-boil-be-gone!
Metadata is only displayed on the Grimm’s screen when the internal Roon server functionality is engaged. And I’ll call it early: that metadata will be useful for desktop users but it’s too small to read when sitting more than a metre away, which most loudspeaker listeners will be. Also note: if you want to send Roon streams to the MU2 from a third-party Roon server, there is no wi-fi, only Ethernet.
On the back panel, analogue outputs and inputs show up in both single-ended RCA and balanced XLR versions. I have the XLR outputs connected to a Mytek Brooklyn+ power amplifier and the single-ended RCA outputs connected to a KEF KC62 subwoofer to work simultaneously without issue. Keeping vinyl in play, I connected a PS Audio phono stage’s XLR outputs to the Grimm’s XLR inputs, which (purists rejoice) bucks modern trends to not be digitised upon entry. Feeding the MU2 with hard-wired digital signals, I have (had) a Shanling ET3 hooked into its AES/EBU socket, a Samsung ‘The Frame’ 2022 TV connected to its TOSLINK input and a WiiM Ultra extending the unit’s streaming functionality over coaxial. The USB-A socket aside – used for streaming from USB storage devices – it’s a full house! If you want digital outputs, you want the similar-looking MU1.
Moving inside the unit, signal routing and general housekeeping are handled by “Linux Tiny Core – which is the most bare bones variant (an OS of 23 MB that runs from RAM). Apart from Linux, Roon and one or two libraries (like ffmpeg) we wrote all the software ourselves, including all drivers, display renderers, webserver, etc. We even wrote a DMA controller for our FPGA to have the fastest and most reliable connection between the FPGA and CPU. It was a lot of work but now it is starting to pay off. We are in full control of every line of code and so can always solve issues.” Those words come from company founder and lead engineer Eelco Grimm.
Now we get to the real meat. D/A conversion isn’t executed by an off-the-shelf chip from the likes or ESS Labs, AKM or Cirrus Logic. The MU2’s digital and analogue stages are, like the software, designed entirely in-house. Grimm calls its solution the Major DAC — one that offers an “optimal combination of dedicated DAC pre-processing in FPGA and our own discrete DAC hardware”. Readers of a more technical persuasion can study up on the Major DAC here. In this post, I will reproduce only that document’s bullet-pointed highlights:
- 11th-order flawless Noise Shaping
- 1.5bit architecture, 1bit cell, at 512fs rate
- 128fs Pure Nyquist oversampling principles used throughout
- Smart, extended settling technology
- Intersample overs supported without clipping
- 16-tap FIR-DAC topology
- Fully symmetric analog signal path
- Ultra-stable local power supply technology
It’s not multibit. It’s not single-bit. It’s somewhere in the middle with a 1.5-bit architecture. Stereophile measurements can be perused here.
When I speak to Eelco Grimm in an upcoming Darko.Audio podcast episode, I’ll ask him to explain the Major DAC, where possible, for dummies (like me). I’m also keen to learn from Cap’n Grimm how much the analogue output stage and power supply contribute to the MU2’s sound. Later this year, we will cover sound quality, including a side-by-side comparison to the Mola Mola Tambaqui, in a video review. Hang tight.
Further information: Grimm
