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ALO Audio Rx MK3-B Fully-Balanced Portable Headphone Amp (Playback 61)

ALO Audio Rx MK3-B Fully-Balanced Portable Headphone Amp (Playback 61)

Playback has covered two earlier ALO Audio portable headphone amplifiers—the original ALO Rx (click here to read our Rx blog) and the ALO Rx MK2 ($449, click to read our Rx MK2 review) with favorable comments on both. But now, we have on hand the Rx MK3-B fully balanced portable amplifier ($649), which is by far the firm’s most ambitious design to date. What, you might ask, would make a portable headphone amp not much bigger than a deck of playing card worth the princely sum of $649? We’re glad you asked.

ALO’s Rx MK3-B is a no-holds-barred attempt to build the most versatile and one of the most powerful portable headphone amplifiers ever brought to market. The target market for this amplifier consists, as you might expect, of very serious earphone/headphone enthusiasts (ones who might typically own multiple sets of top-tier earphones, custom-fit in-ear monitors, and full-size headphones), and ones who sometimes like to travel with more than one set of ‘phones. For these listeners, what’s plainly needed is a portable amplifier that offers performance comparable to a top-tier desktop amp, but in a pocket-sized format. The Rx MK3-B was created for just such listeners, and in fact Ken Ball, the founder and owner of ALO says the MK3-B is “arguably the most sophisticated portable amplifier design in the world,” one that includes a combination of features not found in any other portable amp.

Understand, then, that we are talking about a portable amp that must attempt to be all things to all listeners—an amp quiet and sonically refined enough to drive extremely sensitive custom-fit in-ear monitors, yet powerful and robust enough to drive the most power-hungry full-size ‘phones available today. In short, we are expecting the Rx MK3-B to cover not only the performance extremes but also all the performance bases in between. Does the ALO amp successfully pull off this daunting feat? That is one of the key questions we will attempt to answer in this review. We will also take a look at how the Rx MK3-B compares with the conceptually similar and comparably priced Ray Samuels Audio SR-71B portable amp as recently reviewed in Playback (click to read our SR-71B review).


•The Rx MK3-B is a full balanced portable amplifier that provides both unbalanced (single-ended) and balanced inputs and outputs.
•Switch selectable single-ended and balanced stereo inputs.

The single-ended inputs are implemented via a 3.5mm input jack, while the balanced inputs are implemented via a miniature, square-shaped, four-pin connector. Users select between the two inputs via a rear panel-mounted mini-toggle switch
A number of specialty cable makers (including ALO Audio, Moon Audio, and others) offer stereo XLR-to-miniature 4-pin adapter cables that make it easy to connect high-end home or pro-sound source components to the Rx Mk3-B.


•Single-ended and balanced stereo outputs (implemented, respectively, via a 3.5mm mini-jack and the aforementioned miniature 4-pin balanced connectors). Whenever the amplifier is powered up, both the single-ended and balanced outputs are live.
•Switch-selectable master gain switches, with settings for low, medium, and high gain.

This feature lets users adjust the overall gain of the amp to match the sensitivity of their earphones and headphones. For instance, you might use the “Low” gain setting with high-sensitivity custom-fit in-ear monitors, the “High” gain setting with hard-to-drive full-size headphones, and the “Medium” setting for ‘phones falling somewhere in between.
The gain switch also effectively serves as a means of controlling the amplifier’s noise floor (choosing the “Low” gain setting gives you a lower noise floor but also reduces maximum available gain, while choosing the “Medium” or “High” gains settings will give you more available gain, but with an attendant increase in noise).

•Four-color indicator light:

Blue = play

Green = charging

Yellow = low battery

Red = very low battery/charge now

•Bass boost control: The Rx MK3-B provides a distinctive, variable-level bass boost control that can be defeated by rotating the control knob fully counter-clockwise. By design, the effects of the boost control are subtle, for the most part applying low-frequency boost below 80Hz with maximum impact down below 40Hz. Used in moderation, the control helps provide a touch of low frequency foundation at the bottom end of the audio spectrum, while maximum boost setting also add audible mid-bass and upper-bass enhancement.
•4400 mAh Lithium-Polymer battery pack that provides up to 10 hours of balanced mode playing time or up to 14 hours of unbalanced mode playing time. Charge time: 3 hours.
•Accessories include a wall-wart-type 9V battery charger and three ALO-labeled rubber straps for attaching portable DACs or music players to the Rx MK3B.
•Available in satin black and matte silver finishes.


Background: In our listening tests, we evaluated the Rx MK3-B with a wide variety of ancillary components as outlined below.

Source Components:

•Apple iPod Classic loaded exclusively with lossless CD-resolution files, with signals fed via high-quality line out dock (LOD) cable.
•Apple iPod Classic loaded exclusively with lossless CD-resolution files, with digital signal fed from the iPod to a Cypher Labs AlgoRhythm Solo portable DAC, with digital signals fed to the AlgoRhythm Solo and analog signals fed, respectively, through high-quality digital and analog cables from ALO Audio.
•Oppo BDP-95 universal/Blu-ray player, using the player’s balanced outputs with signals fed via high-quality balanced analog cables from Moon Audio.


Earphones and Custom-Fit In-Ear Monitors:

•Audeo by Phonak PFE 232
•Westone Elite Series ES5
•Ultimate Ears In-Ear Reference Monitors and Personal Reference Monitors.


•Audeze LCD3
•HiFiMAN HE-400 and HE-6
•Shure SRH1840
•Sennheiser HD800

The Sound: The ALO Rx MK3-B delivers overall sound quality that is fully comparable to that of an accomplished, full-size desktop amp. By this I mean that it shows all the hallmarks of larger amps: robust and well-controlled bass, authoritative and well-defined sound, and plenty of dynamic clout. What’s more, it also shows a thoroughly refined quality of smoothness so that even when reproducing small, low-level details it never exhibits the sharp, piercing, edginess that spoils the presentation of some transistor amps.

One area of particular strength is the entire bass region, where the ALO sounds taut and detailed, provides very fine low-frequency pitch definition, yet is also appropriately weighty, powerful, and punchy. Use the ALO to drive ‘phones with first-rate bass (e.g., the Audeze LCD-3 planar magnetic headphones) and you’ll find it easy to get lost in the sheer beauty and inner detail of low frequency instruments such as pipe organs, tympani, acoustic and electric basses, kick drums, etc.

Another welcome touch is the ALO’s Bass Boost control, which provides really subtle touches of low-end reinforcement without imposing a big, overblown, mid-bass sound the way some conventional “tone controls” do. The control can be a godsend for those who basically appreciate their current headphones, but yearn for a little more low-end weight. With the ALO, you can apply tasteful bass reinforcement at the twist of a control knob (or switch the bass boost circuit off altogether if that’s your preference).

Another key area of strength involves the Rx MK3-B’s midrange, which is highly transparent, well detailed, and highly dynamic, and yet consistently delivers that oh-so-elusive quality of refined smoothness I mentioned above. After listening to challenging orchestral passages first through the ALO and then through competing amps, I found the ALO was typically able to equal or, more likely, to surpass the performance of like-priced portable amplifiers in terms of the sheer amount of musical detail and nuance it could reproduce. When I tried this comparison between the Rx MK3-B and the also superb Ray Samuels SR-71B Blackbird, I found the outcomes were very close, that—to my ears—the ALO eked out a narrow but consistent performance edge over the SR-71B.


The bottom line is that the Rx MK3-B is one portable amp that sounds much bigger than it looks; it’s a pocket-sized powerhouse that can belt out music with serious gusto.

What’s the benefit of multiple gain settings? As mentioned above, the Rx MK3-B’s three gain-level settings allow users to match the amplifier’s gain levels to fit the sensitivity of their chosen earphones or headphones. I found the best approach was to experiment by ear and then to choose the lowest gain settings that would still allow adequate volume levels with the earphones or headphones at hand—an approach that gives the best overall combination of low noise levels with satisfying volume levels. In practice, this meant I used the amp’s “Low” gain setting almost exclusively for high-sensitivity in-ear monitors, the “Medium” gain setting for the majority of listening scenarios, and the “High” gain setting in those instances where I needed/wanted to drive ultra-power-hungry headphones.

What about noise levels? I found the Rx MK3-B was absolutely quiet enough to use with high-sensitivity in-ear monitors, provided I first engaged the lowest of the amp’s three available gain settings. With the gain switch in the “Low” position, the ALO produced no discernible hiss or hum when used with either my Westone Elite Series ES5 custom-fit in-ear monitors or my Ultimate Ears Personal Reference Monitors. Indeed, the Rx MK3-B gave better results with the Westone monitors than other portable I’ve yet heard.

Interestingly, the Ray Samuels SR-71B might be just a smidgeon quitter than the Rx MK3-B, largely because the SR-71B offers slightly lower gain settings (and thus potentially lower perceived noise floors) than the ALO. Even so, I felt noise differences between the amps were so subtle that I doubt most listeners would even notice them.

Got Power? With its balanced outputs brought into play, the Rx MK3-B had sufficient power to drive even the brutally demanding HiFiMAN HE-6 headphones, although my sense was that the HE-6 pretty much used up every ounce of power output the ALO amp had to give. For all other headphones, though, I found the ALO amp had more than ample power, typically serving up expressive and, where appropriate, explosive dynamics.

Compared to the Ray Samuels SR-71B, the ALO Rx ML3-B seems at first to be the more powerful amp, although in theory the SR-71B offers somewhat more power output because it features higher voltage battery packs than the ALO does. What tips the balance in the ALO’s favor is the fact that its higher available gain settings help users tap the power at hand, whereas the SR-71B sometimes begs to be run with its volume control wide open.


Do balanced inputs and outputs make a difference? I found the ALO’s single-ended input sounded fine, but that sound quality took a subtle but audible step up when the balanced inputs were brought into play. I don’t know whether the improvement I heard was attributable to the Rx MK3-B’s balanced input circuitry, or to the fact that source components with balanced outputs often perform better in balanced mode. Either way, think of the ALO’s balanced inputs as a “turbocharged” version of the amp’s already very good single-ended inputs.

Balanced outputs give the ALO (and the Ray Samuels SR-71B) significantly higher power output than the single-ended outputs do, as the SPECIFICATIONS table, below, reveals. What is more, many headphone aficionados claim that balanced outputs afford superior control over headphone drivers, and thus yield a more nuanced and revealing sound over all—results that my listening tests solidly confirmed.

Rx MK3-B vs. full-size desktop amps? When push comes to shove, the Rx MK3-B is not quite as good as the best desktop headphone amps we have tried (e.g., the Apex Peak with Volcano power supply, the Burson Audio Soloist, the Cavalli Audio Liquid Fire, the HiFiMAN EF6, or the Woo Audio WA22), but it certainly is not embarrassed in their company. This is saying a lot given that these desktop competitors are much larger than the Rx MK3-B and cost anywhere from hundreds to thousands of dollars more.

I found the performance gap between the ALO and premium priced desktop amps was narrower than I expected, meaning that the tiny Rx MK3B gave me a generous taste of what top-tier sound is like, while also conferring the benefits of effortless portability.


To hear both the ALO’s power and refinement in action at the same time, I put on the track “O Vazio” from Jim Brock’s album Tropic Affair, which is represented in the Reference Recordings sampler, Jazz Kaleidoscope [HDCD]. I like to use this piece as a test track for several reasons, partly because it has some unbelievably loud, clear, low-percussion moment near the opening and close of the piece, and partly because all the other instruments in Brock’s jazz ensemble sound so rich, clear, and sumptuous—especially the delicate cymbal work heard throughout. The catch (or potential catch, in this case) was that I chose to listen through my terrific-sounding, but almost painfully difficult to drive, HiFiMAN HE-6 headphones, ‘phones that sponge up amplifier power the way camels in the desert soak up every drop of water they can find. Let’s be clear; asking most portable amps to drive the HiFiMAN HE-6 properly would be an exercise in frustration, if not a bad sonic joke. But not the ALO Rx MK3-B. It simply latched on to the HiFiMAN ‘phones and drove them with the kind of depth, punch, power, and articulacy typically associated with big, muscular desktop amps. As the low percussion notes arrived, I marveled at the way the little amp caused the headphones to unleash big bass pressure waves I could both hear and feel, and I noted with satisfaction that the amp caught the subtle modulation and gradual decay of those mammoth notes with terrific articulacy and precision.


But the ALO went much further on “O Vazio”, revealing the sonic craftsmanship with which Reference Recording’s “Prof.” Keith O. Johnson put this album together. In particular, I was struck by how beautiful percussion instruments sounded, so that I could hear the discrete parts of each individual note—attack, sustain, and decay—as they unfolded with remarkable realism. It’s one thing to talk about relatively costly desktop amps delivering this level of performance, but it was breathtaking to hear a $649 portable step up and sing at this level. Who knew? One further point I would draw from my experience with Brock’s “O Vazio” as heard through the ALO amp and the HiFiMAN HE-6 headphones is that the Rx MK3-B’s uncanny smoothness and refinement was everywhere in evidence. I my experience, many headphone amps tend to make the HE-6 ‘phones sound detailed, but also a bit “hot” and edgy. Through the ALO, however, I was able to enjoy gobs of inner detail in the music, without suffering any edginess or glassy-sounding distortions. I can’t speak for you, but I love it when audio components offer gain without pain, as the ALO happily does.

A big part of the ALO’s appeal involves its delicacy and finesse, qualities that came into play in major way when listening to Gordon Getty’s Overture “Plump Jack” Ancestor Suite as performed by Sir Neville Marriner and the Academy of St. Martin in the Fields [PentaTone SACD]. Though only a little over twelve minutes long, the “Plump Jack” Overture presents a thoroughgoing orchestral workout for any headphone or headphone amp. For this particular listening session, I used Shure’s new flagship SRH1840 headphone, a headphone that, while not particularly difficult to drive, is extremely revealing both of recordings and of headphone electronics (honestly, these Shure ‘phones could serve as the “poster child” for the benefits of extreme openness and transparency). I reveled in the way the ALO/Shure pair captured the pure, distinct timbres and sonic signatures of each orchestral section and of individual instruments within the section. In particular, the Shure’s perfectly captured the elusive, melancholy sound of the French horns, and the uniquely luminous, liquid tonality of the harp. On this track and with this amp and the Shure’s working together, you have the sense of living within a sonic cornucopia—experiencing a literal horn of plenty as you move from one moment of sonic richness to the next.

If there’s any catch, and there is one, it would be that Shure’s SRH1840 headphones tend to sound overly lightly balanced from about the middle of the mid-bass region on down (the SRH1840’s can go quite low, but not with as much authority as you might want). Happily, though, the ALO supplies an answer in the form of its distinctive Bass Boost control. On Getty’s “Plump Jack” Overture, I found it was child’s play to dial in some much-needed low-end reinforcement, yet without making the sound muddy or overwrought. I simply turned up the boost control until I found a point where the Shure’s began to exhibit appropriate weight and depth on low strings and, and stopped when I felt the sound had reached a “sweet spot” in terms of overall balance. Happily, the boost control is designed for subtlety, not for excess, so that it is pretty hard to get the settings wrong. Better still, the control lets you add bass reinforcement as needed, yet without undercutting the inherent clarity or lucidity of the ALO amp’s sound. Once again, the ALO gives you sonic gain without pain.



Consider this portable headphone amp if:

•You want an accomplished portable amplifier that is equally at home on the go or in your listening room and that can successfully drive just about any earphone or (non-electrostatic) headphone ever made.
•You like the idea of a truly subtle, variable bass boost control that can add just a touch of low frequency reinforcement for those earphones or headphones that need a bit more low-end weight and punch.
•You want an amp that will pair beautifully with Cypher Labs’ way cool AlgoRhythm Solo high-performance portable DAC. There is close collaboration between ALO Audio and Cypher Labs, as a result of which the Rx MK3-B and AlgoRhythm Solo feature identically sized and shaped chassis. Together, they make what is perhaps the ultimate portable playback system.

Look further if:

•You want to push the edges of the headphone sound-quality envelope, as only a true top-tier desktop amplifier will allow. But recognize that stepping up from Rx MK3-B to something audibly better will cost you portability plus a pile of greenbacks.

Ratings (relative to comparably priced portable headphone amps):

•Design and Features: 9.5
•Tonal Balance: 10
•Timbral Purity: 9.5
•Detail & Resolution: 9.5
•Imaging/Soundstaging: 9.5
•Dynamics: 10
•Value: 9.5 – 10


As earphones and headphones become more and more specialized, the performance envelopes demanded of headphone amplifiers becomes broader and more daunting by the minute. Impressively, the ALO Audio Rx MK3-B not only meets the demands of an exceptionally wide range of earphones and headphones, but also does so in portable format, with beautiful build quality, and at a fair price for what you get. Sound quality is on par with today’s better desktop amps and better than that of many we have heard.

If you could only choose one headphone amp to meet all your listening needs, this one belongs at or near the top your short list. It’s one of the two best portable headphone amplifiers now being produced.


ALO Audio Rx MK3-B Fully Balanced Portable Headphone Amp
Type: Compact, battery powered headphone amplifier with fully balanced circuitry from input to output.
Accessories: As noted under “FEATURES”, above.
Inputs: Two switch selectable stereo analog inputs (one single-ended input via 3.5mm min-jack, one fully balanced input via miniature, square 4-pin connector).
Outputs: Two headphone outputs (one single-ended output via 3.5mm mini-jack, one fully balanced output via miniature, square 4-pin connector).
Other controls: single three-position master gain switches with settings for “Low”, “Medium”, and “High” gain; combination on/off switch and volume control, bass boost control that incorporates a built-in “defeat” switch.
Frequency response: 5Hz – 80 kHz, ± 1dB
Power output, balanced mode:
•640mW into 32 Ohms
•630 mW into 50 Ohms
•150 mW into 300 Ohms
•80 mW into 600 Ohms
Power output, unbalanced (single-ended) mode:
•320 mW into 32 Ohms
•220 mW into 50 Ohms
•40 mW into 300 Ohms
•20 mW into 600 Ohms
Gain setting options:
•High Gain: 12dB
•Medium Gain: 3dB
•Low Gain: -3dB
Bass Boost Control Range (with maximum boost applied):
•<0.5 dB at frequencies above 360 Hz
•0.5 dB @ 360 Hz
•1 dB @ 260 Hz
•2 dB @ 160 Hz
•3 dB @ 80 Hz
•3.5 dB at frequencies below 40 Hz
Battery: 4400mAh Lithium-Polymer
•Play time: 10 hours, balanced mode
•Play time: 14 hours, unbalanced (single-ended) mode
•Recharge time: 3 hours
Dimensions (H x W x D): 0.75” x 2.87” x 5.16” (includes dimensions of control switches, etc.)
Weight: Not specified.
Warranty: 30-day Money Back Guarantee
Price: $649

Manufacturer Information
ALO Audio
(971) 279-4357

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