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Grace Design m9XX DAC

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.

Technical Tour
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.

 

According to Grace Design, “noise from the computer USB port can cause jitter artifacts in the sensitive DAC circuits. To prevent this, the m9XX employs multi-tiered noise suppression. The first stage of noise suppression is provided by two high-quality USB cables, which are included with the m9XX. These high-speed cables feature RFI filters that are built-in, as well as extra-heavy 24AWG copper conductors for power and ground.”

USB power is also filtered at the input connectors on the rear panel. Power is separated into five individual supplies: one for the XMOS processor, one for the DAC, and three for the analog circuits. The audio amplifier power-supply rails are produced with an ultra-low-noise push-pull converter that runs synchronous to the digital audio clocks. This high-efficiency, low-noise power-supply circuit was originally designed for use in Grace’s Lunatec microphone preamplifiers, which have an EIN (equivalent input noise) of -130dB. This attention to noise suppression and isolation between digital and analog power yields a system with overall output noise of -109dBV A-weighted and-106dBV 22Hz–22kHz.

Setup and Ergonomics
The m9XX looks like no other DAC. Its extruded sheet-aluminum chassis is only 4¼” by 1″ by 5″ and features a large rotary knob on top. The front panel includes two ¼” stereo headphone jacks and a seven-section LED readout. The back panel has one pair of RCA line-out jacks, a TosLink input, a 5.0V 2A micro-B USB input, and a USB 2.0 input. That’s it. Because it has variable line-level outputs, the m9XX can serve as a DAC/preamplifier. The only drawback is that it only has one pair of outputs. If your system uses a subwoofer, you will need to connect it from either your amplifier’s high-level outputs or use a line-level splitter to supply an additional feed for your sub. I employed the latter methodology with no perceivable sonic ill effects.

I used the m9XX in two configurations—as a digital source with its output set at unity gain routed into a preamplifier, and as a DAC/pre, where it was connected directly to a power amplifier and the volume was controlled by the m9XX. In both situations the Grace m9XX performed without any issues. The only problem I encountered was user-error: One time I managed to switch from USB to TosLink via the menu, and I had to call Grace to discover my mistake (Alex, the customer service person, found the problem quickly and didn’t laugh at me).

Setting up the m9XX is simple, but because its display only supports two characters (due to its size restrictions), you will probably need to refer to Grace’s comprehensive and well-written user’s manual to determine what the display is trying to tell you. In USB 1 mode, the m9XX supports driverless operation on Mac OS and Windows at sample rates up to 96kHz. In Audio Class 2 mode, the m9XX supports driverless operation on Mac OS at sample rates up to 384kHz. On Windows, Audio Class 2 operation requires downloading and installing a free driver, available from Grace Design’s website.

Once the m9XX was hooked up, it was time to go into the set-up menu. Your first option will be whether to employ the cross-feed circuit. “CF” means the cross-feed is off while “C.F.” means the cross-feed is active. Next you can choose between the USB and TosLink inputs. “U” means USB while “To” means TosLink. Other set-up options include a dimmer control to lower the display brightness, a “power-up” option that lets you set the default volume level upon turn-on, the four digital filter options, and finally the USB mode. While it’s a bit scary at first, the menu is easily learnable and once you get the hang of it, going into and out of the m9XX’s set-up menu becomes quick and easy. And, yes, there’s no remote since the m9XX was envisioned as a “within hand’s reach” device.

Speaking of hands, the m9XX handles great “blind.” By that I mean after a day or so, once you’re through fiddling with the set-up menu, you can use the m9XX without ever having to look at it. The large top-mounted volume knob turns easily and the push-to-mute feature makes perfect sense. Another sensible feature is the way the two headphone outputs are slightly different—if you plug into the left one, the line-level output is unaffected, but if you plug into the right-hand one, the line-level output is muted.

Sound
Over the years I’ve spent time with a number of Grace Design DACs and A/Ds. And while there isn’t a pervasive “house sound” to Grace products, they do all share certain sonic attributes. The most important characteristic is a lack of listener fatigue during long sessions. I could listen to the m9XX for as many hours as I wanted (sometimes all day) and I never felt as if my ears needed a rest. But even with its easy-to-listen-to sound the m9XX was also as detailed and resolving as any DAC I’ve used. With my own recordings, which I know very well, the m9XX delivered all the low-level information and soundstaging cues that I’m accustomed to hearing. Part of the m9XX’s sonic appeal stems from its lucid character. This makes it easy to listen deep into a mix without any strain; all the aural information is simply there, clearly and without any loss of low-level details.

The m9XX’s harmonic balance is as neutral as I’ve heard from any DAC, which is surprising only if you also consider the m9XX’s excellent low-bass extension. Usually when a DAC has this much low-bass and sub-bass information its overall balance seems to shift toward “the dark side,” but the m9XX’s harmonic balance still sounds extremely neutral. Perhaps some of this apparent harmonic neutrality stems from the m9XX’s analog amplification section, which remains unfazed by dynamic peaks. This is a characteristic of Grace devices that J. Gordon Holt and I first discovered with our Lunatec microphone preamplifier. It was the first microphone preamplifier we had ever used whose sound did not change even when it was “hammered” by peak-level transients. I hear this same dynamically unflappable character with the m9XX.

Since one of the m9XX’s primary functions is to serve as a headphone amplifier, I connected more than a dozen different earphones of various types to try to find any mismatches. Beginning with the most sensitive in-ears I have, the Westone ES-5, I found the m9XX did have a slight amount of low-level hiss that was difficult to hear once music began playing. With the Westone ES-5, my usual listening levels were near the lower end of the m9XX’s output, between 20 and 30 (on a scale of 0 to 99). With Ultimate Ears’ newest custom in-ear, the In-Ear Reference Monitors Remastered, there was no hiss and normal listening levels were between 30 and 40. Moving over to the other extreme, the m9XX had no problems driving my least efficient and most power-hungry cans, the Beyerdynamic DT-990 600-ohm version,  to what I consider loud levels while still having some power to spare, at a usual setting somewhere between 70 and 80.

 

As I read through “headphone pairings” threads on headphone sites I’m often surprised by how two excellent products when connected together can be perceived as a major mismatch by listeners. This was certainly not the case with the Grace m9XX. Here’s an example of the m9XX’s wide-ranging compatibility: The AudioQuest NightHawk and Sennheiser HD 700 are two radically different-sounding headphones. The NightHawk has a dark character while the HD 700s are bright and vibrant, yet both sounded as neutral and uncolored as I’ve ever heard them sound through the m9XX. It’s not as if the m9XX minimizes or reduces their intrinsic sonic personalities, yet the most sonically pernicious aspects of their innate characters were minimized. With the m9XX, the NightHawks weren’t overly dark and the HD 700 wasn’t peaky or lacking in dynamic control.

One of the headphone pairings that offered the most sonic value was when I connected the AKG K553 headphones to the m9XX. This relatively inexpensive over-ear closed design ($199 street) can sound somewhat matter-of-fact with many amplifiers, but with the m9XX they had a dollop of extra dynamic verve and their bass extended lower than I’d heard before. Another headphone that proved to be a nicely balanced pairing with the m9XX was the Oppo PM-3 ($399 street). Once more, the m9XX brought out the PM-3’s strengths, especially in soundstaging and image placement.

As a DAC/preamplifier, especially for a nearfield desktop system, the m9XX performed beautifully once a solution was found for its paucity of outputs. I used a splitter to give me the two pair of RCA single-ended outputs I needed to connect both my subwoofer and power amplifier. Since it was a nearfield system, interconnects were only one meter long. I also tried driving two 25-foot runs and I heard no audible fidelity losses at the frequency extremes. If you need to drive an even longer run of cable you might want to check with Grace first. Of course you can always resort to “subwoofer plan B” and connect a second pair of speaker cables from the outputs of your power amplifier to your subwoofer’s “high-level” or “speaker level” inputs. The disadvantage to this is that you need both the extra set of speaker cables and have to pass the subwoofer’s signal through your power amplifier. Unfortunately this method allows the amp’s colorations and noise to be added the original signal.

Sonically the Grace m9XX performed with flying colors when used as a DAC/pre. The well-controlled and extended bass that I heard through headphones was readily apparent when auditioned in a nearfield or room-based system. The m9XX’s unfatiguing upper-frequency presentation made listening through my all-solid-state and Class D amplification system a very natural and relaxed affair. My only quibble is that if you have built a system that is already soft and forgiving the m9XX might be too much of a good thing. However, if you want a neutral, revealing, but still listenable DAC, the m9XX checks all the right boxes.

Does the Grace m9XX have any competition? Does Rihanna wear short skirts? However, nothing in its price range has the ergonomic elegance and simplicity of the m9XX. It reminds me of the Aurender Flow, but at half the size and price. If I could stretch my budget an extra $200, I’d still be tempted by the Audeze Deckard due to its analog input. But currently I’d say the Grace m9XX is a major contender for best-buy top honors at its price point.

Conclusion
Digital audio, especially with DACs, has its own particular version of Moore’s law—every two years the price halves or the capabilities double. Two years ago you had to spend at least $1000 to get the sonic performance and features included in the $499 Grace m9XX. So, why not wait another two years and get m9XX’s capabilities for $250? Because, silly rabbit, you’d be forced to spend two more years without experiencing the sonic and ergonomic elegance of the Grace m9XX.

The m9XX is U.S.-made and its build-quality is such that Grace Design offers a five-year warranty in an age when most manufacturers’ guarantees for similar products are only good for a year at most.

Simply stated, the m9XX DAC/pre is something special—so special that it deserves to turn up on a lot of savvy audiophiles’ desktops.

Specs & Pricing

THD+N: 1kHz, 22Hz-22kHz BW, @1.0V out, no load <0.002%; @1.0V out, 32-ohm load <0.010%
Frequency response: 0.5Hz–45.9kHz (96kHz input)
Dynamic range: 112dB (20-22kHz)
Output impedance: 0.08 ohm (headphone) 47.5 ohms (line)
Power consumption: 8.0W (high-power mode); 2.5W (low-power mode)
Dimensions: 4″ x 1.8″ x 5.25″
Price: $799 ($499 street)

GRACE DESIGN
4689 Ute Highway
Longmont, CO 80503
(303) 823-8100
gracedesign.com

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