Grace Design m903

m903 Reference Headphone Amp, DAC and Monitor Controller

The m903 is a stereo reference headphone amp, digital-to-analog converter and stereo monitor controller housed in a 1U, half-rack enclosure. The m903 has been designed to provide audio pros in music, post and live-sound with exceptional headphone as well as stereo mains monitoring, whether from an analog or digital stereo source. It also serves the audiophile market as a reference quality DAC and system preamp. The m903 provides balanced and unbalanced stereo analog inputs via rear panel XLR and RCA jacks. Both +4dBu and –10dBV analog sources are accommodated. A pair of front panel, parallel-wired 1⁄4” TRS outputs are included to allow two headphone pairs to be used simultaneously. The m903 also adds two pair of line outputs (balanced and unbalanced) on the rear panel, allowing the system to perform as a high-end monitor controller and system preamp.

Each pair of line outputs may be level controlled separately. The m903 employs high-current, transimpedance output amplifiers optimized for driving even the lowest impedance headphones. A front panel rotary encoder provides precision level control of both headphone and line output levels and level adjustments are made in 0.5dB steps within a 95dB range. AES3, S/PDIF and optical digital sources are accepted via rear-panel XLR, RCA and TOSLINK connectors respectively. As an added convenience, a USB input is supplied for users who require a reference quality listening experience while connected to their computer. A front panel rotary switch selects between analog and digital sources, and a 4-LED array is provided to show valid digital source sample rates between 44.1 and 192kHz. A next generation, high performance 24- bit DAC, which includes Grace’s unique s-LockTM dual stage PLL, (Phase Lock Loop) is utilized for conversion and provides stunning results with extremely low intrinsic jitter.

“We’ve been fortunate to have customers who made the m902 become an industry standard during its product life cycle. As a manufacturer, you better be sure that a next generation product is something pretty special. We’re extremely excited about the introduction of our new m903 because we believe we’ve done just that”. noted Grace Design VP of Sales and Marketing Doug Wood. “The m903 is the culmination of over a year of focused development where the goal was to provide our customers with the very best performing product of its type in the pro-audio and audiophile market spaces.”

When compared to the former m902, a number of significant sonic performance related enhancements have been incorporated in the m903, which include:

- a next generation DAC and ultra low noise balanced current-to-voltage converter yields more musical performance, with a nearly 10dB improvement in dynamic range
- a new high speed, ground isolated USB interface with a completely asynchronous transfer mode that provides bit-perfect playback with zero interface induced jitter – driverless operation on Mac OS machines up to 24-bit/192kHz (Windows machines require drivers for 192kHz operation)
- a new volume control which has higher headroom and lower distortion and noise
- improved headphone output protection circuitry and a gold contact relay for eliminating ‘pops and clicks’ during power cycles   
- balanced and unbalanced line outputs with individual level control for each
- improved s-Lock PLL circuitry has lower phase noise and jitter
- improved X-feed circuit eliminates perceived low frequency content loss
- improved power supply architecture isolates headphone power amplifiers, line amplifiers, DACs and digital logic

Here is a short explanation from Gordon Rankin (Wavelength Audio, Developer of the USB chip) regarding USB Class 1 und Class 2:

Class 1 is available on almost every OS and does not require any driver and is available to 24/96 Full Speed (U1).

Class 2 is available on OSX and some Linux platforms but NOT Windows and works at speeds above 24/96 (U2) or 24/192 in this case.

Recommendation:

If you are running OSX or a Linux that is capable of Class 2 or use the custom Windows drivers then leave the setting at U2.

If you don't want to use the Windows drivers or your Linux box does not support Class 2, only then set the output to U1.