Front and rear panel views of the m902B.
Michael Grace of Grace Design comes from a background of high-end consumer audio, which has traditionally paid more attention to build quality than the pro audio manufacturers do. Such is the case here with his m902B Reference Headphone Amplifier ($1,850 list); at first glance, it says "quality."
The m902 is a real "fit the need" kind of product, one which functions as a headphone amp, a preamp, and a D/A converter in one neat, compact package; it's a perfect match for today's workstations. Both balanced and unbalanced analog inputs are supported as well as AES, TOSLINK, S/PDIF, and USB digital. Balanced or unbalanced line output versions are available for feeding power amplifiers or powered speakers. [The "B" in Jung's own m902B stands for "balanced," as the unit retains all features of the standard Grace Design m902, yet is augmented with fully balanced line outputs in place of the standard m902's unbalanced outputs. — Ed.]
A simple push of the large rotary encoder volume knob switches control between line and dual headphone outputs in half-dB steps displayed on a nice, big, blue LED readout. Press and hold the same knob, and you enter the submenu where additional modes can be accessed such as Balance, Crossfeed, Exclusive Output, Output Toggle Lock Out, Gain Set, Power Up Level, and IR Remote Control Enable.
All of m902's digital inputs are re-clocked with s-Lock, a crystal- based PLL (phase-locked loop) used for regenerating the incoming digital clock and providing an extremely stable, ultralow jitter clock to run the DACs. The s-Lock system can lock to input sample rates of 44.1 kHz or 48 kHz +/- 10 Hz, 88.2 or 96 kHz +/- 20 Hz, and 176.4 kHz or 192 kHz +/- 40 Hz. If the incoming signal clock frequency is outside of these tolerances, the s- Lock circuit will not lock and the s-Lock indicator will shut off. Even if the s-Lock does not achieve lock, the digital audio receiver circuits still achieve decent jitter performance.
I set up the m902B at my workstation with its balanced line outputs into my Bel Canto stereo amplifier driving a pair of SLS ribbon monitors. I listened to several digital sources as well as balanced and unbalanced analog inputs, and all sounded just the way I would expect: clean and quiet with solid imaging and easy on the ears, meaning no harshness. The low end was nice and solid with good bass extension.
Headphone listening was with my reference Grado open-air HP1, as well as the Audio-Technica closed-back studio monitor, ATH-M50. The m902's 1-ohm output impedance has excellent control of the lower frequencies, which provided nice and tight yet full bass on both sets of phones.
The X-Feed Mode in the m902B contains circuitry that electronically simulates the acoustic experience that occurs when listening to speakers. What is really amazing about this feature is that the ear/brain mechanism seems to accept this as sort of a binaural experience, thus reducing listening fatigue. The best part is that it does not mess with the tonal balance like many such circuits do.
I love my Grado headphones, and the Grace m902B makes them sound better than any headphone amp I have tried to date — so much so I had to buy it. I'm not nearly as familiar with the Audio-Technica ATH-M50, but they also sound good with the Grace amp; the A-Ts definitely have the isolation advantage over my Grado phones, making them perfect for studio use or location recording.
Equally comfortable when controlling speakers or driving headphones with excellent DA conversion and build quality, the Grace m902B is the perfect professional workstation monitor solution at around $1,750 street.
Contact: Grace Design | www.gracedesign.com
Tom Jung is the founder of Digital Music Products (DMP), a Grammy winning engineer, and a technical advisor to Pro Audio Review.