Recording - January 2013 - 34

B Y M I K E M E T L AY

Grace Design m903 Reference Headphone Amplifier
As we’ve said many times in these pages, recording musicians can choose unfortunate times to gloss over important stuff; if we don’t understand what’s going on under the hood of a particular device, we may tend to shrug and assume it’s going to give us what we need and it can’t be improved upon. For example, beginning recordists tend to ignore the innards of computer interfaces and converters as “black boxes” whose functions can’t be fathomed, only accepted. First we learn to worry about getting really good preamps and A/D converters on our inputs, but we accept the D/A conversion and output circuitry of our interfaces as whatever the manufacturer sees fit to give us. Shouldn’t we be as concerned, or more, about how we hear as how we record? Grace Design thinks so, and has been backing up that belief with exceptional listening amplification systems for many years. The latest from The Brothers Grace is the m903; it’s not only an extension and refinement (read, complete redesign) of the m902, but it offers added features that now make it not only a brilliant headphone amp, monitor controller, and D/A, but an unparalleled audio listening interface for modern computers. Front and back and under the hood The m903 is a tabletop box of square footprint, a bit over 8 inches on a side. Its hefty 5-pound chassis is Grace’s usual elegantly milled and finished stainless steel, solid as a rock and gorgeous to behold. A quick tour of the front and rear panels makes its audio path very clear; it accepts audio from balanced or unbalanced analog, three types of digital, or computer USB inputs, and delivers it to the headphones and/or to two separate line-level outputs. The front panel has a dual-digit LED information display, two 1 /4" headphone jacks that share a single output signal, and two knobs, one an endless encoder for volume control, output selection, and function settings, the other a stepped input source selector. LEDs indicate input clock rate (up to 192 kHz), digital lock, the Crossfeed function (see below), and output source; a large lit Power button completes the layout. The rear panel sports a standard IEC 3-prong power socket, a mini-USB input, coaxial (RCA) and optical (Toslink) S/PDIF inputs, AES3 digital input on XLR, balanced and unbalanced inputs on XLR and RCA respectively, and two sets of line outputs, unbalanced RCA and balanced 1/4" TRS. A wireless remote is optional. Start simple, go deep Using the m903 couldn’t be simpler: hook it up to your various sources, plug in monitors and/or headphones, select the source of the moment, select your output, and crank it up. But if you want more control, there’s an entire menu of added functions hidden inside the m903. It’s a bit fiddly to use at first, with only one encoder for control and only the 2-digit LED for feedback, but the functions there are (mostly) items you’ll set and forget. The m903 lets you set the L/R balance between channels individually for the phones and the two Line outputs, in 0.5 dB increments to ±6 dB; you can also preset output levels for powerup and offset levels between the sources by up to 9.5 dB, disable front-panel output
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RECORDING January 2013

It’s time to get serious about studio listening
destination switching, gang both Line outputs to be controlled at the same time (good for satellite/subwoofer control), toggle Mono mode, and mute outputs not currently selected for volume control. The menu is also where you activate Crossfeed, which simulates the crosstalk between the ears that one gets when listening to music on speakers. It’s not an outrageously effect-y thing, but definitely has an impact on how we perceive stereo. I’d leave it off for working with my own tracks and mixes, and it does weird things to binaural recordings (which depend upon strict isolation between the ears), but if you’re listening to material that benefits from being heard over speakers, it really adds to your listening enjoyment. I loved hearing the stereo remasters of the Beatles’ back catalog with Crossfeed! The menu functions can also be tweaked via the wireless remote control, which speeds things up a bit. I consider the remote a handy addition, but the m903’s functional design is clean enough so that it’s not a necessity. From source to signal Analog sources are treated very kindly by the m903, assuming they’re of good quality to begin with. I ran the unbalanced and balanced inputs from various outputs of my analog and digital mixing consoles and from my rackmount audio devices, and was pleased by what I heard (or unsurprised at the faithful reproduction of lowlevel hum from one cheap mixer with a bad chassis ground).

I didn’t do a lot of digital-source testing, primarily working with coaxial S/PDIF at 16-bit/44.1 kHz, but I liked what I did test. The m903 has its own low-jitter clock regeneration system called sLock; when you feed a digital signal into the m903, it grabs the clock signal and then regenerates it to run its D/A converter without jitter for better audio quality. This is independent of how the m903 handles USB, which is worth a discussion in its own right... USB done right USB is the most common interface standard for computer audio these days—another good example of “what you don’t understand can hurt you”. USB was never designed to provide jitter-free audio (arguably it wasn’t designed for audio at all!), and the clocking of audio down the bus can be interfered with by everything from competing audio sources to your mouse and keyboard. To avoid the clocking/jitter problems this can cause, the m903 uses a system developed by Gordon Rankin of Wavelength Audio where the m903 becomes the master clock for the USB connection and forces the computer to follow its clocking when delivering its audio. There are two modes, one for audio up to 96 kHz and a special one for 176.4 and 192 kHz; the latter requires a free driver for Windows PCs, but both are “plug and play” on Macs. I found the

Recording - January 2013

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