As we all know, it's a new "personal listening" world out there. The majority of consumers, especially younger ones, listen to most of their music on earbuds or headphones, instead of full-range speakers. What speakers they do listen on are often pitiful little things hooked up to a computer or Bluetooth-connected to the smartphone. So, if you're recording, mixing, or mastering music these days, you have to take very seriously the "personal listening" soundscape.

Into this modern reality has come a new generation of high-quality, powerful headphone amps with various feature sets. In the past, I reviewed the Little Labs Monotor [Tape Op #117] and two headphone amps from Hafler [#115]. These three devices, while offering different flavors of excellent amplification, are all-analog, requiring an outboard DAC to take care of conversion.

The new Grace Design m900 combines duties in a compact, worksurface-friendly form, using its USB 2.0 interface to replace whatever suboptimal sound system is built into your computer or mobile device. It can also connect directly to coax or optical S/PDIF outputs, like the pass-throughs of digital monitoring/routing systems, the mini TOSLINK jacks hidden on most Macs, or the outputs of CD players and other hi-fi components. Aside from two 1/4'' headphone jacks in front, the m900 offers unbalanced RCA L/R analog outputs on the back.

Control of the m900 is menu-driven via the large rotary encoder/pushbutton on top. There is a little two-digit LED display to the right of the headphone jacks — a concession to the unit's price-point. (A less cryptic display would have been preferable.) It takes a few readings of the small and well-written manual to master all the menu options — worthwhile because Grace's engineers built in some neat features.

For starters, you have four options to shape the DAC's reconstruction filter: sharp rolloff with linear phase ("Best for recordings that are loud, compressed, and with lots of treble," like pretty much all modern pop and rock music); slow rolloff, linear phase ("Best for acoustic music without compression and artificially high levels of treble"); sharp rolloff, minimum phase (an alternative choice to filter 1, with filter-induced ringing taking place only after transients); and slow rolloff, minimum phase (very similar to filter 2, but with the low-level filter-induced ringing taking place after transients). I heard very subtle differences listening with Sennheiser HD 650 headphones [Tape Op #43], but the filter "personalities" were more pronounced through Audio-Technica ATH–M50 headphones [#63]. Even with loud/crunched music, I preferred filters 2 and 4. Apparently, some distortion artifacts that might have been caused by high-frequency information pumped up to digital zero were less annoying than filter ringing, which sounds to me like a faint spring-reverb "ping." Alas, no filter setting, nor a high-quality amp, could put lipstick on the sonic pigs that are many overly loud modern releases.

As happens when I listen to a bunch of these albums, I reflected on the seeming impossibility that the drummers lost the war. (What? Drummers never lose arguments, do they?) What I call "toothpaste" crunched waveforms hurt percussive transients most, reducing drum hits to something akin to slapping a sheet of newspaper. Modern engineers, please listen back to some of the best rock albums of the ‘90s — you can be loud and have drums that sound like drums. Start with Rage Against the Machine's eponymous first album, and Pearl Jam's debut Ten. But I digress.

Another interesting feature on the m900 is an analog cross-feed circuit. The manual explains that it simulates the experience of listening to speakers in a room, with sound from the opposite channel arriving at each ear with a slight delay, a lower level, and a frequency response altered by the listener's head blocking the direct path. I found this most pleasing with old Blue Note jazz albums, where most instruments are hard-panned to one side or the other. It was also interesting with Beatles and Beach Boys stereo mixes. But beware of a new perspective that may be disconcerting (or falsely enticing) because it's unfamiliar. Obviously, don't enable this feature while mixing if you're trying to estimate what others will hear while wearing their headphones.

The switchable filter and cross-feed features are likely aimed more at audiophiles than audio engineers seeking clarity and neutrality, but even the pros like to do some pleasure listening. Speaking of the world of consumer audio, it's worth mentioning that the m900's DAC handles up to 24–bit, 384 kHz PCM, as well as DSD64/128/256, depending on the limits of the S/PDIF or USB connection. It does not feature the emerging MQA technology, which may or may not catch on.

I used the m900 regularly for a month as my go-to headphone amp, connected via USB to my laptop or to one of my DAW computers. It worked reliably and consistently, indicating its driver software is solid on Windows 7. (The m900 can operate in USB Class 1 mode up to 96 kHz without driver installation on modern versions of Windows, macOS, Linux, iOS, and Android. USB Class 2 mode on the m900 supports up to 384 kHz on macOS without driver installation, and on Windows with a free, downloadable driver.) The m900 had no problem producing all the SPL I desired in my Sennheiser and A–T headphones, but it was unable to achieve super-loud volume with my 600 Ω AKG cans. That said, the AKGs were loud enough to listen to dynamic classical music, and full output level on the m900 did not produce any audible breakup or distortion. All my headphones sounded as I expected them to sound, which tells me the m900 is on the neutral side of things, as one would expect with a professional-grade monitoring device.

To test the m900 against some other good headphone amps, I fed it S/PDIF from my Benchmark DAC2HGC [Tape Op #111], and the Benchmark's analog balanced outputs were connected to a Little Labs Monotor. I used a RadioShack SPL meter to match output levels for my Sennheiser HD 650 headphones, and then spent an afternoon listening to music I love and know, comparing the Benchmark's built-in headphone amp to the m900 and Monotor. The next afternoon, I adjusted output levels for my Audio-Technica ATH–M50 headphones and enjoyed more great music.

With both headphones, I preferred the m900 and Monotor over the Benchmark's built-in amp, but they weren't so much better that I'll never plug into the Benchmark again. The m900 and Monotor reproduced the low end a bit better, making it sound less muddy/boomy. I heard the distinct "fronts" of acoustic bass notes on jazz albums and more "power stroke" impact of well-recorded rock/pop bass. With kick drum, I was hearing more clearly the initial attack. Not a night-and-day difference — but very audible with both headphones.

Between the m900 and the Monotor, the sound differences were subtle but present. My supposition is that I was hearing more difference between the two DACs than the two amplifier circuits. To my ears, it was two flavors of accurate-and-reliable. Things sounded as they should out of both, with the Monotor a bit more "fast" and "present" sounding, but with the Grace perhaps balancing out some mixes more as intended by the original engineer. On complex orchestral music, the Little Labs amp presented clearer detail or points of sound, but the Grace presented a total sound-picture where the whole was the thing rather than a sum of parts. The differences were subtler than these words might suggest — definitely not what my British friends call "chalk and cheese" differences!

With a street price of $499, the Grace Design m900 lands on the value end of combo DAC/headphone amps. It has an interesting and useful feature set, perhaps more intended for pleasure-listening than critical pro audio work; but the transparent quality of the DAC and amplifier, plus the calibrated and precision-adjustable volume control will be appreciated by pros. If the Grace engineers implemented some of the Little Labs Monotor's professional "monitor section" listening options (like reverse-stereo, mono, and left-minus-right), this thing would be an all-time killer-app device. (It is firmware-upgradeable, so maybe some of these features can be added in the digital realm.) The m900 is small, light, and portable. Most importantly, it will drive a range of headphones to comfortable listening levels and beyond, and you can trust what you hear. Take it home, and it becomes the hub of a computer-based music system, feeding headphones and either a power amp or powered speakers. And it's built out of metal and looks good. What's not to like?

Tape Op is a bi-monthly magazine devoted to the art of record making.

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