Grace Design may be a new name for many audiophiles, but for recording engineers it has long been a well known and respected entity. Michael and Eben Grace have been manufacturing microphone preamplifiers since 1994. Their first model, the 801, began the pro-audio industry trend toward outboard, dedicated microphone preamps that could be used in place of a mixing board’s built-in microphone preamplifiers. My own familiarity with Grace began in 1999, when J. Gordon Holt and I bought a Grace Lunatec two-channel portable microphone preamplifier. After one session, we took it to the Grace factory in Boulder, Colorado, where Michael Grace designed and installed an M/S microphone-matrix circuit and a “Macky Auditorium EQ” created for Macky, which was (and could still be if the University of Colorado comes up with $200,000) a pipe organ auditorium. Provisions in the hall designed for the organ pipes canceled some of the auditorium’s lower-bass extension, so this equalization circuit was created to add a 3dB-per-octave bass boost starting around 40Hz to compensate for the hall’s latticework proscenium. Subsequent versions of the Lunatec preamplifier included the option for M/S matrix based on the original circuit design Michael made for us.
Grace Design’s first “consumer” or “end-user” product is a desktop DAC dubbed the m9XX. Why m9XX? Several reasons: First, all of Grace’s previous pro DAC products have been m9 series beginning with the m901 and ending, so far, with the m920. The xx part is a tip of the hat to Massdrop, a site that specializes in “group buys.” Currently Massdrop is the exclusive retail outlet for the m9XX. Massdrop works directly with manufacturers, commissioning special versions of existing or occasionally entirely new products. In the past, Massdrop has offered exclusive headphones from AKG, such as the K7xx. So that’s where the xx moniker originates.
What is the m9XX? Basically it’s a digital-to-analog converter with a variable line-level RCA output as well as two ¼" stereo headphone outputs. The m9XX has TosLink and USB 1.0 and 2.0 inputs (selectable via menu) in addition to two power modes, one via USB and another higher-current mode via a separate dedicated power connection. Although the m9XX has an MSRP of $799, Massdrop’s “drop” price is $499. The good news for audiophiles on a budget is that the Grace m9XX has the potential to be a reference-level-sonic device at a near-entry-level price.
The digital “heart” of the m9XX is an AKM 4490 chip, which features 256x oversampling, 32-bit processing, and the ability to decode everything from 44.1kHz to 384kHz as well as DSD64 and DSD128 via DoP (DSD over PCM). The DSD64 stream is packaged into a 176.4kHz PCM stream while the DSD128 is packed into a 352.8kHz PCM stream. Special bits are added to the DSD stream to indicate to the m9XX processor that the data is DSD and not PCM. The AKM 4490 has provisions for four user-selectable anti-aliasing filters, including a traditional fast-roll-off linear-phase filter, a slow-roll-off linear-phase filter, a fast-roll-off minimum-phase filter, and a slow-roll-off minimum-phase filter.
Along with these four digital filters, the Grace m9XX has a defeatable cross-feed circuit for its headphone output. According to Grace Design, “The m9XX contains cross-feed circuitry which electronically simulates the signal cross-feed that occurs in a real acoustic space and helps the brain establish instrument locations across the entire soundstage. While it is difficult to perfectly model the very complex level, delay, and frequency response characteristics of the head, the cross-feed circuitry in the m9XX gives the brain some of the basic clues it needs to establish instrument locations.”
The m9XX’s USB input is handled by an eight-core XMOS processor, which operates in asynchronous mode. There is a TosLink input that can handle sources up to 96/24 from Blu-ray or DVD players. The XMOS processor is also responsible for receiving the TosLink data. This recovered clock from the TosLink stream is regenerated by an ultra-low-jitter (50ps) hybrid analog-digital PLL (phase locked loop) circuit. The corner frequency of the PLL is 1Hz, so there is over 90dB of jitter rejection at 1kHz.
Grace spent a lot of design time on its analog circuitry, and instead of a traditional voltage-feedback op-amp the m9XX employs a transimpedance or “current feedback” amplifier design. Transimpedance amplifiers have a nearly constant bandwidth over a wide gain range and are not prone to large-signal slew-rate limiting. The headphone amplifier in the m9XX is the THS6012, which has a slew rate of more than 900V/µS. The tradeoff of transimpedance designs is that they have higher measured total harmonic distortion (THD) than voltage feedback designs. But these harmonic distortion components are correlated with the signal and are much more euphonic than intermodulation distortion. Grace’s designers believe that the sound quality advantages of a transimpedance design outweigh the measurement disadvantages.
The output impedance of the headphone outputs (there are two parallel outputs) is specified at only 0.08 ohms. According to Grace’s published specifications, when driving 20-ohm headphones the m9XX has a damping factor of over 250. By comparison, a typical headphone amplifier with 10-ohm output impedance will have a damping factor of only 3.8. With 300-ohm headphones the m9XX’s damping factor is over 3700. The THS6012 headphone amplifier used in the m9XX is a high-current device specified for driving 500mA of continuous current into a load. In high-power mode, the m9XX headphone amplifier can deliver 440mA peak into 20-ohm loads with both channels driven, which is a momentary power of 1800mW per channel.
The m9XX’s volume control is a hybrid design with most of the volume control duties handled in the digital domain with 32-bit processing. The output amplifier is designed to operate in two gain modes: 0dB and +10dB, which are controlled automatically to create a 99dB volume range with 0.5dB steps. Thirty-two bit processing ensures that any artifacts of volume control operations are at -190dB down from full scale.
Unlike many USB DACs, which rely on a 5-volt USB source to supply power, the m9XX offers two different power options. You can either power the m9XX from your computer’s USB to deliver a maximum of 0.5 amps, or you can use a second USB “power only” connection, which supports a maximum current of 1.5 amps. The m9XX automatically senses when the external 2A USB charger is connected to the high-power DC input. Once detected, the m9XX increases the DC power rails to +/-14.5V. This results in available continuous power for the headphones exceeding 1000mW per channel with both channels driven into 32-ohm headphones. One channel peak power into 20-ohm ’phones is more than 1600mW.