M Audio USB Duo Preamp And USB Audio Interface For Mac & PC

Published in SOS October 2002
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Reviews : Recording System

For convenience and portability, USB interfaces like M Audio's new Duo are hard to beat.

Martin Walker

Way back in July 1999 I reviewed Opcode's DATport, the very first USB-based audio interface to grace the pages of SOS. Like all USB devices, installation was simplicity itself — you just hot-plugged it into your PC for it to be recognised and have any required driver files installed — and it's perhaps not surprising that this simple approach was viewed as the Holy Grail by soundcard manufacturers. Unfortunately, however, I never did get it to play back on my PC without glitching, and when I reviewed Swissonic's USB Studio D the following March exactly the same problem occurred. Many such problems were caused by limitations at the computer end, including rogue USB Host Controller chips, but most of these issues should now be in the past. M Audio's Duo, the latest product in their range of USB devices, has finally given me a chance to see for myself how the situation has improved.

M Audio have always prided themselves on spotting gaps in the market, and the Duo is another example of this approach. It features two mic preamps using the same circuitry as their acclaimed DMP3 preamp (reviewed in SOS May 2002) and Omni 'front end' (SOS January 2001), complete with +48 Volt phantom power, can record both mic and line-level signals over USB at up to 24-bit/96kHz, and has a built-in headphone amp. It can also act as a stand-alone A-D converter when disconnected from the USB port, whereupon its S/PDIF output can be connected to any compatible digital device.


The Duo has an identically sized case to the Omni, being 21.5 centimetres wide, 14 centimetres deep, and just over four centimetres high. This makes it half-rack width, and a 5mm screw mounting hole is provided on its bottom plate to bolt it into a universal rackmount tray if required.

M Audio USB Duo £249
Good sound quality.
Two high-quality mic preamps with +48 Volt phantom power.
Can be used as a stand-alone mic preamp and A-D converter.
Zero-latency direct monitoring.
Audio application must be relaunched after any Control Panel changes.
Can't play back and record simultaneously at 88kHz and 96kHz.
No high-impedance input suitable for guitars.
DIP switch stand-alone sample rate adjustment is fiddly.
M Audio's Duo provides good audio quality in a convenient 'plug and play' package, complete with good mic preamps and a handy stand-alone A-D mode, and should appeal to a lot of musicians, subject to bandwidth limitations that mean you can have either 24-bit/96kHz recording or playback, but not both at once.

The two rear-panel XLR mic sockets provide up to 60dB of gain, and a 20dB pad can be individually switched in from the front panel for hot signals. Each input has a rotary gain control along with a green signal LED and a red LED that illuminates just below clipping. Global +48 Volt phantom power is available from a front-panel switch.

Plugging a cable into either of the two TRS-wired balanced/unbalanced line inputs disconnects the mic preamp and pad, but there's a global +4/-10 dB front-panel sensitivity switch on the front panel. The line outputs are similarly wired, and have their own +4/-10 sensitivity switch on the rear panel. Completing the rear panel are the S/PDIF output and USB port, along with a socket for the supplied 9V AC 1A wall-wart, while the front panel features a quarter-inch stereo headphone socket and associated rotary level control plus a power switch and associated red LED.

Stand-alone mode is activated from a front-panel switch which connects the mic and line input signals directly to the analogue line outputs, the S/PDIF output, and the headphone output. Since the unit then acts as an A-D converter, its sample rate needs to be set using two DIP switches on the bottom panel. Settings of 44.1, 48, 88.2 and 96 kHz are available; you can change the setting with a small screwdriver or even a biro, and although this is fiddly I doubt that these will need to be altered very often.

The internal layout is extremely tidy, and let me discover that the Duo uses AKM's AK4528 converters, which offer a significantly improved dynamic range over the more common AK4524 model. Digital interfacing is handled by the AK4103 from the same company.

Driver Installation

The Duo is bundled with drivers suitable for Windows 98SE, ME, 2000 and XP, along with Mac OS 9.1 or higher (including OS X). Also included are the latest version of Emagic's Logic Delta, Bitheadz's Unity Piano and Sonic Foundry's Acid XPress. It's difficult for any manufacturer to keep the supplied driver CD-ROM in sync with the latest releases available on the manufacturer's web site, but M Audio managed it, and I was able to install the latest 1.26 drivers for my Windows 98SE-equipped PC from it. Most USB audio peripherals seem to appear to Windows as several devices, and mine detected three in all, as well as requesting the original Windows CD-ROM.

After installation, a new USB Audio Configuration Manager icon appears on the Taskbar, allowing you to choose between ASIO/EASI and MME drivers. However, unlike a standard PCI soundcard, you don't get these options simultaneously, but must instead choose the most appropriate for your application before you launch it. Confusingly, both options appear to the application, but you'll get 'hardware missing' error messages if you attempt to use the inactive option.

The default is ASIO/EASI operation, which is probably the most appropriate option for musicians. However, I found that powering down the Duo and then restarting it sometimes resets it to MME operation, so it's as well to check the current mode by hovering your mouse over the Taskbar icon before launching your application.

Control Panel

Also on the USB Audio Configuration Manager page is a button that launches the Control Panel, although this can also normally be launched directly from within a suitable ASIO/EASI host application. It contains user settings for Bitwidth, Available Channel Sets and Latency, as well as providing a readout of firmware and driver versions, although the latter didn't appear in my setup. When you select a Bitwidth of 16 bits/sample in the Control Panel there is just one option in the Available Channel Sets box — 'Channels 1-2 In, 1-2 Out' — and in the Available Sampling Rates box your applications can choose from 11.025, 22.05, 44.1 and 48 kHz.

Given the finite bandwidth of USB, audio devices capable of 24-bit/96kHz operation have to make some compromises, and when you shift the Bitwidth to 24 bits per sample you get three options for Available Channel Sets. If you stick with 'Channels 1-2 In, 1-2 Out' you get the same four sample rates. However, if you want to move to the higher sample rates of 88.2kHz or 96kHz you must choose either 'Channels 1 & 2 In' or 'Channels 1 & 2 Out', effectively sacrificing full-duplex operation to use the full USB bandwidth in one or other direction.

There are five Latency options ranging from the default Medium down to Very Low and up to Very High. An additional Advanced button opens another small dialogue where you can select Thread Priority. In most cases the Normal setting should work, but if you're using other USB devices simultaneously, raising this to High may avoid clicks and pops. You'll need to relaunch your application after changing this or the Bitwidth setting.

  M Audio USB Duo: Brief Specifications  
  Mic inputs: balanced/unbalanced XLR sockets, up to 60dB gain, with switched 20dB pad and global +48 Volt phantom power.
Line inputs: balanced/unbalanced quarter-inch TRS jack sockets, hardware-switchable +4dBu/-10dBV sensitivity.
Line outputs: balanced/unbalanced quarter-inch TRS jack sockets, hardware-switchable +4dBu/-10dBV sensitivity.
Peak input and output signal: +16dBV (mic input), +19dBu (+4dBu line setting), +2.1dBV (-10dBV line setting).
A-D converters: 24-bit 64x oversampling (part of AK4528 Codec chip).
D-A converters: 24-bit 128x oversampling (part of AK4528 Codec chip).
Dynamic range: 105dBA (line in to line out).
Total harmonic distortion: <0.002% (line in to line out).
Frequency response: 22Hz to 22kHz, +0.3/-0.2dB.
Supported bit depths: 16 and 24.
S/PDIF: out, up to 24-bit 96kHz operation supported.
Internal sample clock: 44.1, 48, 88.2, and 96kHz.

Audio Performance

The Duo's playback quality sounded very good, and extremely similar to my Echo Mia, which is hardly surprising consider they both use exactly the same converters. I'm also pleased to report that for the first time ever with a USB audio device, I experienced no playback glitches with the Duo, even at the lowest latency setting, as confirmed by listening to a 10kHz sine wave, which will give audible clicks if even a single sample goes missing. Some glitches did appear when I resized the playback window, and whenever I attempted to type this review in the background, but even these were cured by increasing the Thread Priority to High. I also had no problems running Pro 52 at the lowest ASIO latency settings at both 44.1kHz and 96kHz.

As with all USB audio peripherals, bandwidth limitations mean that switching to higher sample rates like 88.2kHz and 96kHz restricts you to recording or playback, but not both.

Although the latency values aren't stated in the Control Panel, they are reported back to the host application as 1323 samples (Medium), which equates to 30ms at a sample rate of 44.1kHz, while the Low setting is 882 samples (20ms at 44.1kHz) and Very Low is 441 samples (10ms at 44.1kHz, or just 3ms at 96kHz). If you rarely adjust your bit depth, driver type or latency, using the Duo will be fairly straightforward, but I did find it frustrating to have to relaunch my music applications each time I changed one of these, and you'll get plenty of error messages if you forget your current settings and choose the other driver type, an invalid sample rate, playback in recording-only mode, and so on. It would also help if the Taskbar icon indicated whether or not the Duo was on-line.

Like the DMP3 and Omni before it, the Duo's IC-based mic preamps sounded excellent, providing low noise and colouration, and I experienced no recording problems at any of the available bit depths or sample rates. It's frustrating to lose playback ability while you're recording at 24-bit/96kHz, but you can use the Duo's hardware 'zero latency' monitoring instead.

To measure RMS background noise I plugged in a dummy line-level cable to bypass the mic preamps, and measured -88dB when recording at 16-bit/44.1kHz, -95dB at 24-bit/44.1kHz and -93dB at 24-bit/96kHz. These figures are higher than many soundcards, but perfectly acceptable given the convenience of USB.

Finally, with the front-panel button pushed in, I found the Duo very easy to use in its stand-alone mode — you just select the appropriate sample rate using the DIP switches, and connect the Duo's S/PDIF out to the S/PDIF input of your other digital device. This is an ideal way to improve the recording quality of an elderly DAT machine for instance.


If you want a couple of mic preamps, 24-bit/96kHz conversion, and USB connectability, there are several products that can provide it. If money's no object the best of the bunch is likely to be Apogee's Mini-me, with built-in soft limiting, UV22HR dithering, and AES-EBU as well as S/PDIF outputs, but its high price tag will probably put this out of reach of many musicians.

A much closer competitor in both price and performance is Edirol's UA5 (reviewed in SOS March 2002). This offers a more versatile arrangement of co-axial and optical digital in and out, as well as a front-panel high-impedance guitar input, and if you want a portable setup its 9V DC supply would be easier to arrange from batteries than the 9V AC of the Duo. Where the Duo scores is in its ability to run as a stand-alone A-D converter box (there's no direct connection between the UA5's analogue inputs and rear-panel digital outputs except via USB), and in providing significantly more gain (60dB as opposed to 50dB) on its mic preamps to cope with low-level sources.

As long as you don't mind the inconvenience and possible confusion of relaunching your applications every time you change a setting, M Audio's Duo is certainly a lot easier to install than a PCI soundcard, and judging by my tests, it offers excellent glitch-free 24-bit/96kHz recording or playback. Its stand-alone mode also makes it significantly more versatile than most, and for those who have a laptop, or don't want the hassle of opening up their desktop computer, its high-quality mic preamps are the icing on the cake.

  Test Spec  
  M Audio Duo Windows 98SE driver version 1.26.
Intel Pentium III Coppermine 1GHz PC, Asus TUSL2-C motherboard with Intel 815EP chipset, 512Mb PC133 RAM, running Windows 98SE.

£249 including VAT.
M Audio +44 (0)871 717 7100.
+44 (0)871 717 7101.
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