Media Server Reviews

In 1979, I visited Philips Electronics' renowned Research Center in Eindhoven, Holland, to examine a prototype of what would eventually be called a compact disc player. In 1989, I returned to the Eindhoven lab to witness the birth of the first sigma-delta DACs, which eliminated the problem of large linearity errors at low recorded levels in resistor-ladder DACs.

Perhaps the century-long presence of the Philips research center in the Netherlands led to the country being a hotbed of science-based audio engineering. Kees Schouhamer Immink, who was responsible for the optical-disc reading mechanism used by compact disc players, is Dutch, as are class-D amplifier authority Bruno Putzeys and loudspeaker designer Martijn Mensink, whose Dutch & Dutch 8C set new standards for carefully controlled dispersion. Respected Dutch engineer Peter van Willenswaard was for many years one of Stereophile's European correspondents. Finally, I have been aware of Eelco (pronounced ale-co) Grimm's work since I first encountered him at an Audio Engineering Society convention in the early 2000s.

Following an early career as an audio reviewer and magazine editor, Eelco founded Grimm Audio with Guido Tent, Bruno Putzeys, and Peter van Willenswaard in 2004 (footnote 1). (He is also on the faculty at HKU University of the Arts Utrecht, Music and Technology, and a contributing member to EBU and AES committees examining loudness in broadcasts and streaming.) Starting with the AD1 DSD64-output A/D converter, the company's products—hardware and software digital format converters, microphone power supplies, and plug-in loudness apps, as well as the AD1—have been aimed at professional audio. But following the introduction of its LS1 digital active loudspeaker in 2010, Grimm Audio turned its attention to high-end consumer audio. Grimm calls the company's latest product, the MU1 ($10,500 without internal storage), "the ultimate digital music source."

The MU1
This unobtrusive-looking music streamer is finished in matte black with a small, rectangular color display in the center of its front panel. A white LED to the left of the display illuminates when the unit is powered up. The inset back panel offers two transformer-coupled AES/EBU digital outputs; a proprietary digital output for Grimm's LS1 loudspeaker on an RJ45 jack; AES/EBU and optical and coaxial S/PDIF digital inputs; a gigabit Ethernet port; two USB 3.0 ports; a BNC jack for an antenna for an FM receiver (still to be implemented); a 3.5mm jack for a remote-control receiver; and the IEC AC input, above which is a small pushbutton to turn the MU1 on.

The MU1's elegance is inside, where an Intel twin-core i3 processor running at 2.4GHz is supported by a solid-state drive and 4GB of dynamic RAM for the Linux-based operating system, powered by a custom, low-noise switch-mode power supply. The MU1 includes a Roon Core and is fully integrated with the Roon Server app. "We partnered with Roon Labs," says Grimm Audio on the company's website, "who in our opinion are in the lead when it comes to a rich and engaging music experience." Eelco added in an email that "when we met with [Roon] in 2016 at the High End Munich, it was soon clear that they had all their metadata ducks in a row. ... We started to work seriously on the MU1 (with two people full time) [at the] end of 2016."

Audio data can be output from the network and USB ports when a Roon-Ready DAC is connected to these ports and selected as an Audio Zone with the Roon server app. However, these outputs bypass an important feature of the MU1: a custom-programmed Xilinx FPGA (field-programmable gate array) that can upsample 44.1kHz and 48kHz data either to 88.2kHz/96kHz (2Fs) or to 176.4kHz/192kHz (4Fs), in each case with an increase in bit depth to 24 bits. The upsampling feature only works with the AES/EBU and LS1 outputs. If you set DSD playback to "Native" in Roon's Audio Settings menu, the FPGA downsamples DSD64, DSD128, and DSD256 data to 24/176.4k PCM for streaming from the AES/EBU outputs; DXD data (PCM with a sample rate >192kHz) are downsampled, too. The upsampling is "smart" in that if you select 4× upsampling with the menu for playback of CD sample-rate files, when you then play files sampled at 88.2kHz or 96kHz, the MU1 changes to 2× upsampling in order not to exceed the capability of the AES/EBU format, which cannot transmit data with sample rates greater than 4Fs.

Upsampling and downsampling are achieved with what Grimm refers to as a "Pure Nyquist" decimation filter. Frequencies that are above half the lowest sample rate—the "Nyquist frequency"—of the incoming data with upsampling or of the outgoing data with downsampling must be removed with a low-pass digital filter. The quality of this filter is of prime importance with digital audio reproduction, and you can see from recent reviews of digital processors in Stereophile that there is much debate whether this filter needs to have a slow rolloff or a rolloff that is very fast. With either kind of filter, the mathematical operation needs to be as accurate as possible to avoid losing baseband resolution.

"Filtering is done in a single stage in the FPGA," Grimm says, "without compromises for the extremes that are required for the signal data path, filter coefficient resolution, and filter length....[T]his technology helps to reduce the errors in oversampling filters of downstream D/A converters by replacing their first, most calculation-intense internal oversampling step with an extremely high-precision one in the MU1."

I was asked a question about my review of the Chord Hugo M Scaler in March 2020 concerning the fact that when a D/A processor is sent upsampled data, its own reconstruction filter is still operating on those data. However, as this filter's low-pass corner frequency is now one or two octaves above the upsampler filter's corner frequency, it is effectively removed from the process. The only time there might be two reconstruction filters in series with the MU1 is when it sends 4Fs data (data sampled at 176.4kHz or 192kHz) to the DAC over an AES/EBU link. Eelco Grimm explained that "4Fs is transferred transparently unless you engage the volume control option. We don't touch anything when there is no need to. If you hear any change in sound of 4Fs sources, it is not caused by filtering in the FPGA."

Filtered PCM data sent to the MU1's AES/EBU and LS1 outputs are clocked with very low jitter; Grimm says that improves the timing accuracy of downstream converters.

The MU1's specifications say that it supports "all sample rates and formats." I asked Eelco about how the MU1 handles MQA, as the instructions say to set the MU1 to "No MQA Support" in Roon. "By telling Roon that the MU1 does not offer MQA support, Roon will do the first unfold," he replied. "If you tell Roon that the MU1 supports MQA, it will just forward the stream unaltered to the MU1. If you then turn off volume control and upsampling in the MU1, the MQA stream will transfer transparently to its AES output. If you have a DAC connected that recognizes MQA on its AES stream, it will be able to do the full MQA unfold and filter stuff. The disadvantage is that you lose the volume control and the upsampling on non-MQA streams."

Setup and use
Operation of the MU1 is implemented with a large, bronze-colored wheel inset on its top panel, using combinations of short and long pushes and rotation. For example, a long push (>2s) then turning the wheel allows you to browse the menu displayed on the MU1's front panel, selecting 2× or 4× or no upsampling and activating or deactivating the volume control for the AES/EBU digital outputs. A short push (<2s) pauses playback of the music playing in Roon, while turning the wheel operates the digital-output volume control, if activated. Pushing and holding the wheel, then turning it, allows you to choose streaming from Roon or select one of the digital inputs.

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Footnote 1: Putzeys left Grimm Audio in 2015 to develop class-D amplification, Mola Mola amplifiers, and the Kii Three loudspeaker, while van Willenswaard is now retired.