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Steinberg AXR4

Thunderbolt Audio Interface By Sam Inglis
Published July 2019

Steinberg AXR4

Steinberg have made interfaces before — but none so ambitious as the AXR4. Can it hold its own against the competition?

Steinberg's Cubase music production program is a household name, at least in houses where music production happens; and the Hamburg-based developers are also responsible for numerous other well-established software titles. Over the years, they have introduced a fair quantity of hardware products, too, including fader controllers, MIDI interfaces and audio interfaces, all targeted squarely at the home- and project-studio end of the market.

With the AXR4, however, Steinberg have struck out in a different direction. This is very much a flagship product, occupying a premium price bracket and targeted at users who are willing to invest in pristine sound quality, flexibility and expandability. It's a 1U, rackmounting, multi-channel Thunderbolt audio interface that incorporates technologies developed by Steinberg's parent company Yamaha for their advanced digital consoles. The aim, clearly, is to stake a claim to territory currently occupied by manufacturers such as Apogee, Lynx, Focusrite and Antelope Audio. It's a crowded market with plenty of very strong competition: is the AXR4 good enough to take a piece of it?

The One & The Many

Multi-channel audio interfaces typically take one of two design philosophies. Some provide lots of one type of I/O, such as analogue line-level ins and outs, to cater for users who just want to connect an analogue console, or lots of outboard mic preamps. Others try to offer as many different types of input and output as possible, for those who never know what might cross their path from one day to the next. The AXR4 undoubtedly falls into the latter category. It features 12 analogue inputs in total, of which the first four have mic preamps; the first two also provide optional high-impedance modes for DI'ing electric guitars. These are joined by a wide array of digital inputs, of which up to 16 are available for recording at any one time: two optical ports provide either stereo S/PDIF or up to 16 channels of ADAT-format I/O depending on sample rate, and there is also a DB25 connector, which delivers eight AES3 inputs and outputs. There's also MIDI and word–clock I/O.

On the output side, there are eight line-level outputs on quarter-inch jacks, plus two independent front-panel headphone sockets, and a matching selection of digital options. Again, up to 16 digital channels are available for playback. Digital I/O format is switched in blocks, so for example if you use the AES3 channels, the second pair of optical connectors becomes unavailable.

The inclusion of multiple AES3 inputs and outputs is unusual, and is a clear statement of the AXR4's professional intent. Nor is it the only novel feature on offer. The AXR4 supports sample rates up to 384kHz, at word lengths of up to 32 bits, and employs a technology called Super Suppression Phase Locked Loop to deliver supernaturally low levels of jitter. Its mic preamps are also not your common-or-garden designs: they use a topology called AXR that is employed in Yamaha's premium digital consoles, and offer a licensed recreation of the Rupert Neve Designs Silk circuitry for situations where intentional colouring of the sound is required.

If there's an obvious limit to the professional intent behind the AXR4, it's that there are no RJ45 connectors. Audio-over-IP is making steady inroads into this sector of the market, with an increasing number of products able to act both as 'native' interfaces and as Dante or AVB hubs. Steinberg haven't gone down that route, and nor have they included any other massively multi-channel format such as MADI. If you require more than the 28 inputs and 24 outputs available at base sample rates on a single AXR4, you'll need to stack multiple units: a single computer can host up to three of the devices, for a potential 84 ins and 72 outs.

Cut Me Some Rack

I have never understood why audio interface manufacturers are so reluctant to allow complex interfaces to occupy more than one unit of rack space. With so much functionality on board, both the front and back panels of the AXR4 are inevitably crowded. On balance, however, I think Steinberg's engineers have done a pretty good job of working within their self-imposed limitations. All of the I/O (other than the AES3 channels) is on full-size connectors, though there is minimal clearance in between the quarter-inch jacks on the rear; and although the front-panel display is small, it's bright, colourful and clear, and capable of metering all 28 inputs and 24 outputs (at base sample rates) at once.

Unsurprisingly, the AXR4 is a pretty deep and heavy unit, and appears to be constructed to a very high standard indeed. The entire top panel is made of a metal mesh, which reveals an interior absolutely stuffed with components. All those holes presumably help with cooling, allowing the AXR4 to operate without an internal fan but making it a bad idea to use it as an ashtray. The heat situation is also no doubt improved by the use of an external power supply, which attaches using a locking four-pin XLR.

The back panel of the AXR4 is a  crowded affair, boasting twin Thunderbolt ports, MIDI I/O, oprtical I/O and eight audio ins and outs on quarter-inch jacks.The back panel of the AXR4 is a crowded affair, boasting twin Thunderbolt ports, MIDI I/O, oprtical I/O and eight audio ins and outs on quarter-inch jacks.

There are two Thunderbolt sockets, both of which use the older Mini DisplayPort connectors rather than the current Type C ports specified for Thunderbolt 3. In practice this makes little difference, as the standard is fully backwards-compatible and many users still have older Macs with TB2 ports, but it's an odd decision to build a state-of-the-art interface with a deprecated connector type. In any case, you'll need to fork out for cables and/or adaptors no matter what ports your computer has, as it seems my campaign to shame manufacturers of Thunderbolt interfaces into actually supplying Thunderbolt cables has yet to bear fruit.

There are plenty of controls on the AXR4's front panel, but the only ones that operate in the analogue domain are the potentiometers governing the headphone output levels. They are outnumbered by digital controls comprising 12 tiny buttons and two rotaries. One of the latter is used to adjust the Silk emulation for the selected mic preamp, while the other is a general-purpose assignable jobbie. In its default role, this offers basic monitor control, but in conjunction with various buttons, it allows you to surf menus in order to configure a wide variety of AXR4 setup parameters.

Panel Beating

I won't go through the AXR4's front-panel setup options in detail, partly because life is too short and partly because they are all duplicated in its control-panel utility. This rejoices in the snappy title of dspMixFx AXR, and at the time of this review was available only for Mac OS, although the Windows version should be available by the time you read this.

Some manufacturers try to make their control-panel programs fun to use by copping the look and feel of an expensive analogue console. Others embrace the latest fads in user-interface design, which usually involves making everything as dark and as touch-friendly as possible. Steinberg have done neither, preferring to create something that is very plain and functional. The dspMixFx utility thus could not be described as beautiful, but the more I used it, the more I warmed to it; although it gives you control over a huge number of AXR4 parameters, it's never intimidating, always clear, and usually defaults to doing the thing you'd want it to do anyway.

The AXR4's unglamorous but eminently useful dspMixFx software.The AXR4's unglamorous but eminently useful dspMixFx software.

Its role is to configure the AXR's internal digital mixer, which is a powerful beast. In essence, what you get is a separate fader page for each of 12 output pairs; these, in turn, have their own output faders, one of which can be assigned to the front-panel multi-function rotary to give basic monitor level and mute control. If you prefer, mixing and routing can alternatively be configured using a matrix window, as found in RME's TotalMix. Each input channel in the mixer also has two insert slots, into which you can load compressor and EQ plug-ins, and a send to a global reverb engine. The insert effects can optionally be applied to the recorded signal, if you like. DAW return channels also appear in the mixer (though they can't be processed), as does the stereo stream from the AXR4's two-channel driver (see box).

The AXR4 driver installation also includes something called the AXR Extension. If you have a recent version of Cubase, this should replicate all of the dspMixFx AXR functionality within the DAW, thus eliminating the need to constantly Cmd+Tab between applications. For instance, if you create a mono or stereo audio track in Cubase and assign an AXR4 input to it, you'll see all of its input parameters in a new Inspector window associated with that track. I was not supplied with a compatible version of Cubase for this review, so wasn't able to test this functionality, but similar ideas have been implemented by PreSonus with Studio One and their own hardware, and by Apogee with Apple's Logic Pro, and are unquestionably a Good Thing.