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Grimm Audio UC1

Audio Interface, A‑D/D-A Converter & Monitor Controller By Hugh Robjohns
Published June 2022

Grimm Audio UC1

There are many high‑end converters that boast excellent specifications — but this one sounds good too!

Based in the Netherlands, Grimm Audio have been manufacturing high‑end professional and consumer audio products for almost 20 years. They make a very eclectic range of products, including converters, master clocks, music streamers and high‑end active loudspeakers, along with cables, replacement vintage valve mic power‑supplies, and even loudness metering plug‑ins. What they have in common is the company’s approach: Grimm are very engineering and science led, and often employ unusual technical solutions.

Over the years, Grimm Audio have produced several digital converter systems, but the only one in their current product catalogue is the UC1, which they describe as a Universal Converter. As you’d expect from the asking price, it’s very much aimed at professional mixing and mastering applications, but particularly at those using either Grimm Audio’s own LS1 active monitor speakers or PMC’s professional active monitors, which can be connected using bespoke digital audio‑plus‑control interfaces on RJ45 sockets.


At its heart, the UC1 is a 20‑channel (10x10) line‑level USB interface (there are no mic preamps) with integrated monitor control functions. Housed in a 1U rackmount chassis extending about 210mm behind the rack ears, it is an elegant‑looking device with a rich padauk (tropical hardwood) front panel. Set into this joinery is a small colour display which shows I/O signal levels, the selected monitoring source, the current volume setting, and the prevailing sample rate. The same display is also used to navigate the configuration menus, controlled through a rotary encoder and a couple of soft buttons.

Also set into the woodwork are two quarter‑inch TRS jack sockets, providing a powerful headphone output and an unbalanced stereo analogue input; both have volume controls. A second USB connection called the ‘i‑put’ is intended for stereo USB sources like a laptop or smart device. Although there are no mic preamps, the unbalanced input can accept instrument sources, as it presents a 1MΩ input impedance!

The rear panel is absolutely crammed with socketry, starting with a universal IEC mains inlet with an integrated on/off switch, accepting 90‑240 V AC. A power LED sits as the dot above the ‘i’ of ‘Grimm’ on the front panel. There’s a 3.5mm remote‑control socket, too, although this facility hadn’t yet been implemented at the time of writing. Audio connectivity is arranged with digital I/O to the left and analogue I/O across the middle, all on XLRs.

Ten physical inputs can be routed to a DAW over the primary USB port on the rear panel. These inputs comprise two rear‑panel balanced XLRs, two front‑panel unbalanced inputs on a quarter‑inch stereo TRS socket, four digital inputs on two more AES3 XLRs, plus the i‑put USB stereo input on the front‑panel.

On the output side, a DAW can send 10 separate output signals into the UC1 over the primary USB connection. General purpose outputs comprise four analogue balanced XLRs and one AES3 stereo digital XLR output, so there’s sufficient analogue and digital connectivity to connect a piece of outboard hardware (a compressor, for example) and to route signals to and from the DAW, which is what the handbook references to ‘loop’ connections are all about.

The LS1 control out port is compatible with PMC’s active monitor speaker interface.The LS1 control out port is compatible with PMC’s active monitor speaker interface.

Another pair of analogue balanced XLRs is intended for a secondary monitor speaker pair but can also be allocated as artist cue outputs. The front‑panel headphone output always duplicates the signal being sent to these outputs. The powerful headphone amp is designed to work well with higher‑impedance headphones (up to 30mW into 600Ω).

The main (bespoke digital) stereo speaker outputs appear by default on an Ethercon‑style RJ45 connector, but this connection is not Ethernet and isn’t compatible with any of the common AoIP formats. Instead, it carries a proprietary combination of AES3 audio and MIDI control data for Grimm Audio’s own LS1 digital speaker system — the data element controlling volume and muting functions directly in the monitor speaker.

However, a menu function allows the data format at this connection to be configured to PMC’s own proprietary digital speaker interface standard, extending the UC1’s application and versatility considerably. And if Grimm Audio LS1 or PMC active speaker systems aren’t available, the main monitor speaker signal can be routed internally to either the first two analogue or the AES3 digital outputs instead, with volume and muting etc performed inside the UC1.

All analogue outputs operate with a nominal level of +4dBu, which equates to a digital signal level of ‑14dBFS, corresponding to the standard EBU alignment (0dBFS = +18dBu). As shipped from the factory, the same alignment also applies to the analogue inputs, although a menu option recalibrates the A‑Ds to accept +24dBu, if necessary.

Basic traffic‑light sample‑peak metering for all inputs and outputs is normally displayed on the main volume screen, showing green for signals above ‑60dBFS, yellow at ‑14dBFS (the analogue reference level) and red at ‑1dBFS. Larger meters can be viewed by holding the top soft button for a couple of seconds, after which pressing the top button cycles between all inputs, all outputs, or everything, while the bottom button resets the peak‑hold function. The rightmost pair of meters in the output set show the main monitor output level (and name). The bar‑graph meters in this display mode have no scaling marks but the same colour scheme is used, although in this mode the ‑1dBFS red bar is almost invisible!

Clocking is normally controlled from the DAW, but external word‑clock in and out are also provided on BNCs (both 75Ω terminated), and either of the AES3 digital inputs can be used as a clock source, if desired. Supporting all sample rates from 44.1 to 192 kHz, the UC1’s internal clocking system is derived from the company’s master clock products and features extremely low intrinsic jitter (less than 0.6 picoseconds).

The primary USB interface is class‑compliant and supported natively by Mac OS, Linux and, in theory, Windows 10. However, an ASIO driver is needed for Windows 7 and Grimm Audio also recommend using it for Win 10 systems — that’s what I did throughout this review. At the time of writing, firmware updates could only be applied from a Windows computer (the review unit was running v1.1.3).

Monitor Control

In addition to the 10x10 USB interface functionality, the UC1 also contains independent internal signal routing and DSP for the monitor control functions. The first two USB output channels are normally routed to the main speaker outputs for monitoring, but any of the 10 physical inputs can be selected as alternative monitoring sources when required. There are several ways to change the selected source, but the easiest is to press and hold the rotary encoder until the source label flashes in the screen. Rotating the dial then scrolls through the available options and releasing the encoder knob engages the displayed source.

The encoder knob also adjusts the monitor volume, of course, and briefly pressing the knob mutes the main speaker outputs and activates the headphones in their place. As shipped, the volume reference level is set to ‑8dB, meaning that when the volume control shows 0 the output going to the speakers is actually 8dB lower, and the control can be turned up to +8 to deliver full level. This arrangement emulates a conventional analogue volume control where the normal reference level is with the pointer at, say, 2 o’clock, typically giving about 8dB of ‘gain in hand’ if the control is turned up fully. Naturally, the UC1’s reference volume can be set to any desired level through the configuration menu, and recalled instantly if required.

At a volume setting of +8 the signal path has unity gain and is bit‑accurate. For all other settings the level‑adjusted signal is dithered at 24 bits. I was surprised to discover that the UC1’s volume control actually goes up to +18, providing 10dB of ‘positive gain’ above unity, which is handy when checking low‑level sources. However, it also means anything above ‑10dBFS will be clipped, so care is needed! For all volume settings below ‑17 the volume changes in 1dB steps, but above that level finer 0.5dB increments are used.

Not everyone is comfortable with the idea of adjusting monitoring levels in the digital domain, of course, and if an external analogue volume control is preferred then the UC1’s volume control function can be disabled for the Main output. In this mode the rotary encoder continues to serve as the volume control for the secondary speaker output (if enabled) and the headphones.

A full set of monitor control functions are included in the UC1, although with only the bottom programmable button available to access them it’s not the easiest monitor controller to use. Nevertheless, the button can be assigned to select any of the following: switch to the secondary monitor speaker output, activate a 20dB dim, enter the Compare mode (see below), recall a stored reference monitoring level, or select a ‘special function’. This last option is determined by another menu setting, which offers a choice of: mono sum (L+R), stereo difference (L‑R), or swapped channels (R<>L).

That Compare mode remembers the last volume setting employed for each individual input so, after adjusting the volume appropriately for each source, switching between them provides a level‑matched comparison. This is useful when comparing an external reference track with a DAW mix, for example. If the Compare mode isn’t enabled, the monitoring volume doesn’t change when selecting different sources.

Normally, the main speaker mute function (typically activated by pressing the encoder knob) kills both speaker outputs, but a menu option allows just the left or right channel to be muted instead, enabling the source to be heard on a single speaker. This is handy for checking the tonality of each channel or speaker individually, or to monitor mono on a single speaker (although the mono function would also need to be selected, of course).

If artist cue monitoring is required, the second monitor speaker output (and headphones) can be repurposed via a ‘Foldback’ menu option. In this Foldback mode the selected DAW monitor channels can be combined with any of the UC1’s direct inputs to provide latency‑free source monitoring — although sadly there is no provision to adjust the mix balance between the DAW replay and local source channels. For more demanding situations, a ‘Split’ menu option separates the secondary monitor signal path (and headphones) from the main monitor source selection, USB channels 9‑10 becoming the secondary source. Used alongside the Foldback latency free mode, this creates an independent external artist cue/headphone system.

Another unusual UC1 feature is that the polarity of the headphone signal can be inverted. It’s important that the polarity of the headphone signal matches the polarity of any acoustic leakage also reaching the ears, as this results in a fuller and more natural sound for the listener — something that makes a big difference when talking or singing! As different headphones are wired in different polarities, being able to invert the signal in the UC1 is really useful.

Based on both the measured technical performance and extensive listening tests, I can say confidently that the UC1 is of professional mastering quality.

In Use

Configuring the UC1 through the menu system is straightforward. The interface is divided into six tabs, each offering four parameters. The first two tabs cover core functions like source selection, special monitoring modes, foldback, muting, clocking, second speaker mode, and the Compare mode. The next three set user preferences such as which outputs carry the main monitor speaker signal, the maximum analogue input level, volume control status, Split mode, headphone polarity, volume reference level, display brightness, and whether the mini‑meters are shown on the default volume screen. The final menu tab provides system hardware and firmware information.

The UC1 is well constructed with an in‑house SMPS power unit on a small PCB mounted alongside the main circuit board, which covers most of the chassis floor. Small vertical cards lie behind the front panel controls. The analogue outputs are derived from AKM AD4396 D‑As, while the analogue inputs are handled by Burr Brown PCM4202 A‑Ds. These are both well‑known, high‑performance converters, but the A‑Ds were selected specifically as being amongst the few that don’t employ gain-ranging. As a result, they have a slightly higher noise floor than more modern designs, but suffer no noise modulation: a trade‑off that Grimm believe important.

Inside: most of the circuitry is on a single PCB, with another adjacent board for the internal switch‑mode power supply, and another for the controls mounted vertically behind the front panel.Inside: most of the circuitry is on a single PCB, with another adjacent board for the internal switch‑mode power supply, and another for the controls mounted vertically behind the front panel.

In my usual bench tests, I measured the AES17 dynamic range of the A‑D converters at 116.9dB and the D‑A at 117.3dB (A‑weighted). Both are very respectable figures, albeit slightly trailing the current state of the art. (In my league table of measured devices the UC1 sits above Antelope Audio’s Orion but below RME’s AIO Pro, and similar to Prism’s Lyra and the original RME AIO card.) I measured excellent THD+N figures of 0.0007% and 0.0009% for the A‑D and D‑A, respectively, and both converter sets delivered absolutely ruler‑flat frequency responses from 5Hz to 22kHz. All of my other standard measurements exceeded the published specifications.

Unusually, the A‑D converter chips have been configured with the internal high‑pass filter disabled. This is a deliberate design decision, of course — Grimm Audio felt that the filter adversely affected the sound quality — and this obviously extends the LF bandwidth slightly and removes the phase shift associated with an HPF. The downside is that inherent converter drift can potentially create a DC offset in the captured signal. This can be removed in the DAW after recording, although I should make clear that during the review period I rarely saw DC offsets above a negligible 0.06%. I have been informed subsequently that Grimm have developed a firmware fix that allows the factory to ‘dial in’ a corrective offset, and earlier models already in the field can be calibrated in the same way by dealers.

UC1 Verdict 

Based on both the measured technical performance and extensive listening tests, I can say confidently that the UC1 is of professional mastering quality. There are a few high‑end products that achieve slightly better technical measurements in some areas, but the overall sound quality of the UC1 as a USB interface is genuinely excellent. In undemanding monitoring setups it also serves as a convenient and effective monitor controller. However, I found only being able to access one signal‑conditioning function through the soft button frustrating, and for those who like to switch between monitor speakers or regularly check the mono sum and stereo difference signals, a more conventional monitor controller with a button per function would be more practical.

Nevertheless, the UC1 is very capable and offers a wide range of customisable facilities through its menu system. Its technical performance is very good, and it sounds great too — that’s an important point, since in my experience those two things do not always go hand in hand! I enjoyed using the UC1, and in straightforward interface/monitoring setups it works extremely well, with a good range of connectivity. The ability to plug a guitar or bass into the front‑panel jack socket and track an overdub is handy, too. Hopefully, the planned remote control feature will involve an external panel with one‑button‑per‑function controls to access all those hidden signal‑conditioning facilities more readily. For anyone who prefers to use external mic preamps, then, or who has a PMC active monitoring setup, the UC1 has a lot going for it, and deserves a closer inspection.  

Standalone Mode

Although the UC1 is intended to be used with a DAW over the primary USB connection, it can also be employed in a standalone converter mode. When engaging standalone mode in the menu, a fixed internal A‑D and D‑A routing scheme is adopted, in which analogue inputs 1‑2 feed digital outputs 1‑2, and digital inputs 3‑4 feed analogue outputs 3‑4. The main speaker output and monitoring path signal selection continues to work as normal with the physical inputs.


  • Combines a 10x10 USB interface with monitor control facilities.
  • Great sound quality and technical performance.
  • Powerful headphone amp.
  • Can accept instrument inputs.
  • Compatible with PMC active monitor speaker interface.


  • Only one assignable button to access special monitoring functions.


An elegant and high‑quality multichannel USB interface with monitor controller facilities and some unusual features and options.


€4780 including VAT.

Grimm Audio +31 40 213 1562

€3,950.41 (about $4400).

Grimm Audio +31 40 213 1562