Grace M903 Headphone Amp, DAC & Preamp

Stereophile Review by Wes Philips and John Atkinson

The m903 keeps everything great about its predecessors, adds some useful new features, and ups the ante with even more breathtaking audio performance. A new volume control provides higher headroom and lower distortion and noise. New generation DAC's and ultra-low noise balanced current to voltage converter yields even more musical performance compared to the m902. Improved s-lock PLL circuitry delivers even lower phase noise and jitter. An improved power supply isolates headphone amplifiers, line amplifiers, DAC's and digital logic, resulting in lower noise and distortion.

Perhaps the most exciting improvement is the all new high speed USB interface. This cutting edge technology provides a completely asynchronous data transfer mode which allows for bit-perfect USB playback with zero interface induced jitter. This new interface affords standard driverless USB Class 1 operation on Windows and Mac up to 24bit/96kHz and USB Class 2 audio for 24bit/192kHz operation on Windows and Mac (drivers needed for 192kHz operation under Windows). This is computer-based audiophile playback, perfected.

New crossfeed circuitry is even more useful and includes a gentle compensation to eliminate the perceived loss of low frequency content. Both balanced and unbalanced line outputs are now standard, with individual volume controls for each. We have even included a mono mode for checking a mix for phase issues or simply monitoring in mono. These improvements cement the m903 as the perfect compact, audiophile grade monitor controller, even for larger scale production environments with more than one set on monitors.

A full complement of analog and digital inputs are provided (balanced, unbalanced, AES3, S/PDIF, TOSLINK and USB), which are selectable via a precision rotary front panel switch. And we offer an updated optional infrared remote control, which provides essential control for anyone working out of reach of their m903.

Advancing the tradition of unmatched audio performance and functionality, the m903 is now the ultimate solution for high resolution audio playback. From the very top mastering engineers to the most discerning audiophiles – it is a must for anyone searching to discover new depths in music and sound.

24bit/192kHz digital stereo inputs- AES3, S/PDIF, TOSLINK and USB via Burr Brown PCM-1798 Dac Chip

High speed USB interface - completely asynchronous transfer mode allows for bit-perfect playback with zero interface induced jitter

Driverless operation on Windows & Mac up to 96kHz/24bit

Supports USB Class 2 audio specification for 192kHz/24bit operation on Windows & Mac (drivers needed for 192kHz operation under Windows)

New generation DAC's and ultra low noise balanced current to voltage converter yields more musical performance compared to the m902

Mono mode for checking mix phase issues (or listening to Pet Sounds)

s-Lock™ phase lock loop sample clock regeneration for ultra-low jitter and rock solid digital stability

High-current transimpedance headphone amplifier circuitry built to effortlessly drive low impedance headphones

Balanced and unbalanced variable level analog line outputs

Improved power supply architecture isolates headphone power amplifiers, line amplifiers, DAC's and digital logic

Precision level control with a 99.5dB range in 0.5 dB steps

Channel level matching accuracy of 0.05dB

Improved x-feed circuit eliminates perceived loss of low frequency content

Optional infrared remote control available

Internal linear power supply with custom wound toroidal transformer

Sealed gold contact relays used for all signal switching

Five year transferable warranty on parts and labor

SPECIFICATIONS ANALOG IN

Gain

Headphone output, volume at max, trim at +9.5 +15dB

Balanced Line output, volume at max, trim at +9.5 +14dB

Unbalanced Line output, volume at max, trim at +9.5 +11dB

Frequency Response

@ 0dBu out +/- .25dB 22Hz 120kHz

@ 0dBu out +/- .5dB 12Hz 260kHz

@ 0dBu out +/-3dB 4Hz 600kHz

Maximum Input Level

Balanced Input +22dBu

Unbalanced Input +22dBu

Maximum Output Level

Unbalanced output @1kHz, 100k Ohm +22.5dBu

Balanced output @1kHz, 100k Ohm +27dBu

Headphone output @1kHz, 50 Ohm load +20.4dBu

Impedance

Balanced Input 106K Ohms

Unbalanced Input 53K Ohms

Balanced Output 95 Ohms

Unbalanced Output 47.5 Ohms

Headphone Output 1.2 Ohm

Dynamic Range

@ 0dB gain TBD

@ -10dB gain TBD

THD+N

Headphone +10dBu out, 50 Ohm load, SMPTE 4:1 <0.01%

Headphone +10dBu out, 50 Ohm load <0.008%

Unbalanced Line Out +10dBu out TBA

Balanced Line Out +10dBu out <0.002%

Attenuator

Channel matching <0.05dB

Attenuator Range 99.5dB

SPECIFICATIONS D/A CONVERTER

Input Sample Rate 32, 44.1, 48, 88.2, 96, 176.4, 192kHz

THD+N

44.1kHz, 24bit, 1kHz, +20dBu out <0.002%

Noise Floor

0dB gain, 22-22kHz -94dBu

0dB gain, A-weighted -97dBu

Output level

Unbalanced Output @ 0dBFS TBD

Balanced Output @ 0dBFS TBD

Power Requirements

120VAC .16A

230VAC .08A

Dimensions and Weight

Dimensions H1.7 x W8.5 x D8.25

Weight 5 lbs (2.2kg)

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