Grimm Audio MU1 music streamer

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.

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.
Grimm Audio BV
The Netherlands
(+31) 40 213 1562

dumbordumber's picture

A lot appears to have been spent on the casing of this item, particularly the top. However, the interior is a very different kettle of fish. It looks lashed together. Standard length cables, folded and cable-tied. The DC, digital and signal cables are all unprotected from RF and EMI. The casing interior has no damping of any sort, no special layout, or suspension of circuit boards, just old-school steel pillars. In fact the interior of this device largely looks like a computer that has been assembled from fairly standard off-the-shelf components by your local computer shop. I have no doubt that it sounds good if those components are well matched and well chosen, and if the power supplies are decent. But so do many home built computers costing far less. This really is a lamb in wolf's clothing.
If I had bought this device and not known what was inside, I would have been expecting, for the price, something akin to Naim's layout and military wiring. If I had opened it up and found what lay inside, my face would have looked...well, Grim.

Jack L's picture


I concur with yr candid comment: "grim" - uninviting !

It should sound superb as reflected from its lofty price. Yet its so-so internal fabrication looks like some "of-the-shelf" stuff built by a neighbourhood computer repair shop. Hopefully it's actually "a lamb in wolf's clothing" !!

I said so from my recent experience. Just 2 weeks back I got a very basic DAC of an audio unknown brand-name in New York for an unbelievable very dirt crappie price from Amazon.

I acquired it sorta for 'fun' as digital is always my back burner music source vs vinyl classical music I addicted to.

It got basic Toslink & coaxial input & unbalanced RCA audio O/P & earphone jack featuring no-joke 24bit 192KHz sampling rate with 3 year free warranty.

Beyond my expectation for such crappie price, this New Yorker sounds pretty decent with its coaxial input & audio O/P hooked up to my design/built phono-linestage with straight-line bypass mode ON.

This Little David from New York makes ALL my CDs & DVDs sound so much better than before ! WoW. So pleasantly surprised !!

Play digital smart, dear consumers !

Listening is believing

Jack L

Jack L's picture


Despite its very dirt cheapie price vs it's decent sound, this litte no-name New Yorker DAC is built in a small all-aluminum box so rigid & strong with 'seamless' corner joints looked like 100% watertight !!!!

So gratified !

Jack L

mememe2's picture

"$10,500 without internal storage". Two hoary homilies come to mind - 1. More money than brains and 2. You can put lipstick on a pig but it's still just a pig.

cgh's picture

Knowing nothing about the manufacturing process I was surprised, many years ago, to learn how expensive it is to make the box. I happened to be at a manufacturer's plant in CO after a RMAF when these parts came back from being dipped in acid and anodized.

Anyway, that's an attractive box IMO. The top reminds me of those popular pictures attempting to explain general relativity (curvature related to stress energy). Guessing it isn't cheap to make.

Jack L's picture


So comparing the "off-the-shelf" like fabrication quality of its inart, how much as a consumer like you, would guess to pay for the for the box alone ? Say 60% ??

Are we paying our hard-erned money for the music or its "attractive" box ?

By the same token, the dirt dirt cheap price I paid for my basic 24bit192KHz DAC of New York which sounds so decent, is built in a small "water-tight" aluminum box strong & rigid like a miniature tank. Where the manufacturer got the money to cover the cost of the box alone ??

Jack L

Jack L's picture


So comparing the "off-the-shelf" like fabrication quality of its inart, how much as a consumer like you, would guess to pay for the for the box alone ? Say 60% ??

Are we paying our hard-erned money for the music or its "attractive" box ?

By the same token, the dirt dirt cheap price I paid for my basic 24bit192KHz DAC of New York which sounds so decent, is built in a small "water-tight" aluminum box strong & rigid like a miniature tank. Where the manufacturer got the money to cover the cost of the box alone ??

Jack L

partain's picture

Unlike in the political universe , where never do promises of benefits "trickling down" come to fruition , Trickling down is inevitable in electronics.
Good thing.
I'd be willing to pay $700.00 , or so .

AaronGarrett's picture

I have one and my experience exactly mirrors JA. On low resolution material like streaming radio, which I listen to a lot, the difference is enormous upsampled via AES/EBU. Stuff I couldn't listen to I now really enjoy. For me this alone is worth it, I can enjoy a lot more music than I could without the MU-1. In addition for Native DSD it is great. And although JA didn't mention it (because it is not presently a feature) it will be able to do FM radio as well and be a very full-functioning digital preamp.

But as JA says also there are a lot of other very serviceable and cheaper solutions if this isn't an issue for you.

davemill's picture

I am looking forward to the release of the Boulder 812 which will additional provide balanced analog inputs and will be at a LOWER price point. I refuse to spent that amount for this type of device unless it is engineered and manufactured in the US which the 812will be and will no doubt perform extraordinary well.

Dear Stereophile Administrator, the CAPCHA authentication process at least for MacOS and iOS devices using Safari needs to be fixed. For at least six months, I have always had to use the forgotten password process to log in even though I am entering the correct password. This is a security issue.

Foottapper's picture

Hi John, thanks for the review. Since a couple of months I’m thrilled to have the MU1 at home, just love the ease of use ánd jaw dropping sound. Any chance that you’ll review the LS1be’s?