With the M902 already lauded as one of the best high-end headphone amps, what could Grace do to make it better?
Grace Design are well known for producing a range of high-end pro-audio products including microphone preamps, channel strips and monitoring controllers. The M903 is the latest addition to their range, replacing the previous M902 model which has set the benchmark for the last six years or so. I use an M902B myself as a reference converter, monitor controller and headphone preamp — and it is a very impressive performer. The new M903 looks very similar to its predecessor, and the case is the same sturdy, 1U, half-rack size (measuring 216 x 43 x 210 mm and weighing 2.2kg). However, the new design has two large rotary controls instead of a large volume and smaller input selector, the sample-rate LEDs have been moved to the centre, and the level display window features a crisp, cool, white illumination that makes the M902's blue display look positively old-fashioned.
More importantly, the new M903 has been blessed with some extra facilities and even better technical performance. Apparently, all the key performance areas have been addressed, starting with the power supply, to provide better isolation between the headphone and line amps, the DAC circuitry, and the control logic. This tweaking alone has wrought significant improvements in noise and distortion. The electronic volume-control circuitry has also been upgraded, to increase headroom and reduce both noise and distortion (while retaining the 99.5dB adjustment range in 0.5dB increments), and the latest generation of DACs and associated current-to-voltage output drivers have been employed. Grace Design's proprietary 's-Lock' jitter-suppression circuitry has been improved, too.
In terms of the new facilities, the two most obvious are a second independent analogue line output and a revised high-speed USB interface. The M902 was originally supplied with only one set of unbalanced stereo line outputs, but it could be modified to replace the RCA-phono outputs with balanced TRS sockets instead (the M902B version). The new M903 has both balanced TRS and unbalanced RCA-phono outputs as standard, which is a very useful feature. In case you're wondering, the extra rear-panel space needed to accommodate the additional balanced output sockets was found by using a smaller IEC mains input module!
Also, while the old M902 USB port only supported base sample rates and 16-bit word lengths, the new M903 handles both base and double sample rates with 24-bit word lengths, for USB class-1 operation without the need for drivers on OS X or Windows platforms. It can also accommodate 24-bit/192kHz (USB class-2 mode), although drivers are then required for Windows computers (it's still driverless for Macs). USB audio data is transferred asynchronously, with the D-A's clock controlling the data transfer directly rather than the computer, which ensures bit-accurate transfers and no interface jitter. The USB interface is also isolated from the M903's audio ground to avoid ground loops with the noisy computer ground.
Less obvious but still worthy improvements include revised 'crossfeed' circuitry and a very handy mono check mode. The crossfeed mode is intended to overcome the 'sound inside the head' feeling when using headphones, and it works by introducing some spectrally-shaped crosstalk between the left and right ears. When in use, it sounds a little more obvious than the M902's version, and it also lifts the bottom end a tad more too.
The input connectivity remains the same as previously, with balanced analogue line inputs on XLR sockets, unbalanced analogue inputs on RCA phonos, and digital inputs in the forms of AES3 on XLR, S/PDIF on both RCA phono and Toslink optical, and USB (using a mini-USB socket). All of the digital inputs support sample rates up to 192kHz and 24-bit word lengths. Signal switching is performed by sealed gold-contact relays, and the channel matching is guaranteed to within 0.05dB!
There are three independently controlled outputs, comprising the two analogue line outputs at the rear, and two paralleled headphone sockets on the front panel. The headphone amplifier is Grace Design's revered high-current 'transimpedance' design, which can drive low-impedance cans brilliantly, as well as mid- and high-impedance models — and all to silly levels, if necessary. Users of the old M902 who were hard of hearing had to navigate the menu system to switch in an extra 10dB of output level, but the new M903 effectively operates in the 'high gain' mode all the time, and that option has been removed from the configuration menus.
Technical specifications for the new M903 appear broadly the same as its predecessor, with the audio bandwidth extending between 22Hz and 120kHz (±0.25dB) and 4Hz to 600kHz (±3dB), although it boasts an extra 0.5dB of input headroom, with a maximum analogue input level of +22dBu. The maximum output level on the balanced sockets is +27dBu (4.5dB less for the unbalanced outputs). The headphone output impedance is just 1.2Ω, and the maximum drive level is a very healthy +20.4dBu (with a 50Ω load) — although, in direct comparison, the old M902 did seem able to drive headphones a little louder, at the expense of some obvious distortion.
The published specifications for the M903's harmonic distortion and D-A performance are strangely identical to the old M902, but measuring both versions side by side with an Audio Precision test system proved that the new model enjoyed fractionally (0.0005 percent) lower THD+N and a 1dB better signal-to-noise ratio via the balanced analogue inputs and outputs. It could also drive the balanced outputs 2dB louder than the previous model. With an AES3 input, the M903, again, enjoyed fractionally lower THD+N, and a 2.25dB better signal-to-noise ratio than the old M902. The AES17 dynamic range measurement came out at an impressive 111.2dB for the M903 (the old M902B scored 108.9dB), so that's more evidence of small but worthwhile improvements.
For anyone familiar with the M902, the new model is immediately familiar, and its operation is just as intuitive. However, the more advanced features are still hidden in menus, accessed by holding down the volume knob for three seconds until the volume display changes to show the current menu-level abbreviation. The different options are accessed by rotating the wheel, and then entered by pressing it again, and you exit the configuration menu by holding down the knob for three seconds again. Configuration options include stereo balance adjustment (±6dB in 0.5dB steps), power-up volume level (the default is 0), and active output locking to prevent accidental switching. Other menus configure the USB input for class-1 or class-2 operation, enable the optional infra-red remote control, adjust the brightness of the volume display, and allow relative offsets (up to ±9.5dB) to be introduced between the three outputs (phones, line 1 and line 2).
The new mono mode is activated via another menu option, along with the headphone crossfeed circuit, and whether the two line outputs are selected and adjusted separately or together. There's also a mode that makes the headphone and line outputs mutually exclusive, so that activating one automatically mutes the other. The optional infra-red remote control is essentially the same unit as for the M902, but has some extra labelling on the buttons to access the additional functionality. One particularly useful new feature is the ability to toggle the mono mode by holding down the balance button.
The new M903's additional output and high-performance USB input probably justify the redesign all on their own — the latter, in particular, making it very tempting to trade in the older model for the new one — but Grace Design have also managed to advance the audio benchmark in small but very worthwhile ways. The M902 was no slouch in the performance stakes, but the M903's incremental circuitry improvements undoubtedly enhance the sound quality and 'resolution', subtly but audibly. This ensures that the M903 restates its role emphatically as a highly desirable reference headphone preamplifier and an excellent monitor controller, with very high-quality D-A conversion and all the I/O options you could possibly want. This is well worth taking a hammer to the piggy-bank for! .
The nearest equivalent to the M903 is probably the Benchmark DAC1 Pre. This unit shares a similar half-rack case format, has the same output configuration and similar input options to the Grace Design product, and costs rather less. However, it lacks the balanced analogue inputs and has two additional coaxial S/PDIF inputs in place of the M903's AES3 input.