m903

About the m903

The Grace Design m903 is a stereo reference headphone amplifier, DAC, and compact monitor controller housed in a sleek 1U half-rack enclosure. It provides balanced and unbalanced analog inputs (XLR and RCA) plus digital inputs including AES3, S/PDIF, TOSlink, and USB, all supporting 24-bit/192kHz operation, with the USB interface running asynchronously and offering driverless Class 1 operation on Mac and Windows up to 96kHz and Class 2 with drivers for 192kHz on Windows. The internal DAC is based on a Burr-Brown PCM1798 converter paired with Grace's s-Lock phase-lock loop for ultra-low-jitter clocking, feeding a dedicated I/V stage and separated power supplies for the DAC, line outputs, and headphone amplifier. The headphone amp uses a high-current, current-feedback (transimpedance) output stage designed to drive low-impedance headphones without audible degradation. Monitoring functionality includes two headphone outputs, balanced and unbalanced variable-level analog line outputs, and a precision volume control with 0.5dB steps over a 99.5dB range and 0.05dB channel-matching accuracy. Additional features include mono mode, crossfeed circuitry to simulate loudspeaker listening over headphones, and sealed gold-contact relays for signal switching. The unit is powered by an internal linear supply with a custom toroidal transformer, and an optional infrared remote control is available. The m903 was discontinued around 2014 and remains a reference-grade headphone amplifier, DAC, and monitor controller for high-end audiophile and professional studio monitoring use.