ALO Rx Portable Headphone Amp in Review – Double the battery, double the fun

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ALO, a name highly respected for the manufacturing of hi-end audio interconnects and iPod line-out cables has firmly stepped into the world of analogue headphone amplifiers. Already, they have partnered with Red Wine Audio to produce the high-end solid state battery-powered Amphora headphone amp, and now, partnered with GR9 Technologies, are introducing the Rx, which in their own words, is your ‘prescription for sound’. I say, ‘touché’.

Specifications
Frequency response: ±1dB; 10Hz – 20kHz ±0.1dB @ 1V out
Maximum output: 7.45V Peak to Peak
Output source impedance: Less than 1Ω
THD+N: 0.004% @ 1V RMS out into 24Ω
Broadband noise: <10μV RMS, unweighted, integrated over 20Hz – 20kHz
Output DC offset: <3mV
Input impedance: 40kΩ
Maximum input level: 5V RMS
Channel tracking (gain difference between channels, all volume steps): <±0.2dB
Gain: 1.5X /6X —5V (switch-selectable), ±<0.5dB
Dimensions: 4.2″ long x 2.5″ thick x 5″ wide

Build and Packaging
“Those who like it, like it a lot!” – if you are not an IPA-drinking Canadian, this quote will mean little to you; but with a certain cache of blogging liberty, and no contempt for Mr. Keith, I will apply it to the Rx. The slim and sleekly-polished palm-sized aluminium chassis says $$$ from the first glance. Make no mistake though, ALO constructed the Rx with one thing in mind: performance. Thus, forget weeny-widget miniaturisation; forget malleable construction – this is a solid, yet surprisingly light machine. Because of its footprint, it fits nicely with an iPhone or iPod touch, but when attached with almost any sort of interconnect, isn’t the compactest of pocket rigs. That isn’t to say that it is big. It isn’t. But, if portable means a matchbox, you may have to look elsewhere.

The Rx’s highly polished surface also attracts fingerprints and unfortunately, scars. When the first scratch scrawls along the Rx’s shiny exterior — trust me, this _will_ happen — it is heart-wrenching. But apart from Apple-esque good, yet scratchable looks, the Rx is a great design. The aluminium walls shield audio from radio interference and do a lovely job in protecting the innards. At the same time, while the Firestone Audio’s products could be thrown through a wall, the ALO remains aloof to violence, preferring to be kept safely on a desk or in a pouch. It’s chassis is strong, but it isn’t bullet proof. And, keeping it beautiful will require an extra step, as the included canvass pouch is about as protective as cooking wrap. But then again, who spends 350$ on a piece of portable equipment only to throw it at a tree? The Rx is a design which will attract certain customer very much, and others, who prefer either smaller, or squarer designs, will probably ‘ooh and ah’ at the shiny exterior, but in the end, move on.

The front of the amp sports all the connections: power, headphone output, and headphone input; and two small blue LEDs indicate if the amp is ON and/or charging. The ON/OFF switch flips easily and the audio ports are spaced perfectly for both large and small interconnect cables. Because the volume and power switches are nestled between the IN/OUT and mains power input, turning the amp on and off and/or changing volume can be difficult. If the power toggle was vertically oriented, this wouldn’t be the case.

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For many years, ALO have sported tasteful, if sometimes gaudy designs. No where else is this duality presented better than in the juxtaposition of the Rx’s wholesome packaging to its pimp exterior. It comes safely packaged in an easy-to-open cardboard box which apart from its size, looks like a pizza-box. Pragmatically styled and sized, its design doesn’t spell ‘cute’ as well as Firestone’s does, but it works perfectly.

Features in Review
ALO’s Ken and GR9’s Matt have worked hard to build a great piece of technology. There are literally hundreds if not thousands of portable headphone amps competing for your money and most of them cannibalise the designs put forth by the good Dr. Chu Moy. God knows we don’t need another amp – fortunately, the Rx doesn’t do the same-old in the same way. It employs several exciting technologies which help to distinguish it from its laconic peers. The first is the supervised power/charging circuit which is protects and services dual lithium ion rechargeable batteries which ALO say are good for 24 hours. Ken was adamant about including a dual power supply and Matt drew up the charging circuit which is purported to prolong the life of the lithium batteries. In my giddy stupefaction, I am more excited about is the dual stepped volume attenuator, which to put it bluntly, bloody rocks. Most portable amps use volume pots, or single stepped attenuators which can offer good control and balance, but often, add too much gain between steps. Headphones and earphones are either too loud or too quiet, and amps based on these technology can be cumbersome to use. The Rx, on the other hand, works perfectly with any sort of headphone; and with impeccably balanced left and right channels, maturely steps up and down through volume settings. I haven’t used a better system.

Thanks to its great stepped attenuator, it works well with all interconnects so that the source can be set to optimal volume levels. At my disposal, I have three brilliant line out dock cables: the ALO Cryo and two docks from a small Australian manufacturer, Twisted Cables. But, even a cheapo 3,5 mm headphone mini jack does the trick. Apart from that, the Rx doesn’t hide any extra features. Quite simply, it is an amplifier whose innards are constructed for technically sound and strong audio amplification.

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Audio Performance
The Rx is among a handful of production headphone amps which output ~1 ohms and can sustain signal for the most demanding of inner earphones to power-hungry headphones. Many amps are rated for 16 or 32 ohms and when faced with the constantly switching impedance of earphones such as the JH13Pro, suffer tumbling audio performance in a variety of audible and measurable criteria.This is where the Rx sings. There simply isn’t a headphone that can stump it; whereas some headphones are made for inner earphones, and others for studio/DJ/home hifi headpones, the Rx maintains an excellent, flat frequency response with every headphone I have thrown at it. But, as with all things, this too has its blessings and curses.

Firstly, the praise. Because of the flat frequency response, no headphone is held back. If the JH13Pro sounded good from an iPod touch, it gains new legs with the Rx; bass texture and speed is improved and the treble which hitherto has alternated from lungy singing to stifled steps is eased. Earphones which employ balanced armatures really stand up with the Rx. Surprisingly, however, the Victor FX500, an earphone which isn’t hard for the iPod touch to drive, is so much better with the Rx. Its highly resolved, chalky bass is even tauter, uncovering new PRaT checkpoints.

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Where the iPod touch loses neutral response, the Rx picks up. Apart from reaffirming the strengths of the JH13Pro, the Rx reigns in the Sleek Audio CT6 which at times, suffers from a voluble, meandering bass. While it isn’t as precise as the Jerry Harvey’s top end monitor, its acoustically amped bass and treble — my version has ‘+’ treble and bass ports — gain definition and better control. Neither custom monitor suffers any sort of discernible roll-off when driven by the Rx, though, each pick up on of the Rx’s weak points: white noise. It isn’t a big problem, and in fact, the Rx hisses less than a 1st generation iPod Nano. Still, with sensitive earphones, background noise is noticeable as soon as the power switch is engaged. For headphones like the DJ1Pro, Phonak PFE, and to a lesser extent, the v-Jays, the background is perfectly black. The second issue too, only applies to inner earphones. The Rx has ‘ignition thump’; when engaged, its circuitry will thump audibly into the earphones. Fortunately, it isn’t painful, or in comparison to other headphone amps, loud; and headphones can be removed and inserted whilst engaged without any similar adverse affects.

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No portable amp I have ever used can stand up to the sheer bone-numbing bass and impressively ‘3D’ sound space of a good home headphone amp; there simply isn’t enough gusto in the power supply and output. The Rx, however, does a very good job despite its battery-powered engine. And, even at top volumes, it isn’t over-stretched. At no time do any of my headphones exhibit audible distortion even when driven at peak volumes. And, under hardware tests (as my ears wouldn’t abide it), it is obvious that the Rx doesn’t have a ceiling. In contrast, the Graham Slee Voyager (another expensive portable amplifier) runs out of steam, losing power and signal after its volume pot has been turned to about 80%.

Even the somewhat sensitive DJ1Pro which can accept very powerful input doesn’t distort, and can be driven with aplomb even at the Rx’s full volume. Still, such volumes are unsafe; the message then, is to translate that it is possible to use the amp even at 100%. Continuing with the same headphone, I am surprised by the bass which gains new texture from its drivers when paired with the Rx. It sounds great from the lowly iPod touch, and even from a MacBook Pro, but is much better from the Rx. That headphone, along with a couple of favourites: the Beyerdynamic DT-880 and DT-770, are better with my home amps, but not embarrassingly so.

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Finally, the Rx is an interesting combination of resolution and silk. It introduces very little distortion into any headphone — a fact which makes electronic, dance, and trance listening simple, pure bliss — and produces a grain-free treble. To a certain extent, I could call it smooth. Transitions from low to high are excellent and space is simply stunning. But, where technically smooth, sonically, it is perplexing. Before I get to far here, I will mention that sonically, it is amazing, but with such low levels of distortion, it is mixed bag for certain genres. Distortion can be pleasing – very pleasing. An example is the simple (and cheap) Fireye I which I can recommend based on price and features. Though it lacks a volume pot and hisses loudly, it produces levels of distortion which transform vocal, jazz, and live recordings into simple pleasures. Is it overall a laudable amp? No. But, where the Rx makes no mistakes, it reveals the sonic tendencies in each headphone. If the headphone is grainy, music will be grainy. If it tends to be hot, it will be hot. Perhaps this is an indication of its purity, of its transparency. Still, those who favour distortion will probably reach for a different amp.

The iPod touch 2G, too, is a technical piece of audio kit. Put under pressure, it can outperform almost any other mass-produced portable audio player in nearly every measurable category. But, it doesn’t excel in all music genres. Players like the AMP3 Pro 1 and 2 which aren’t as technically excellent, but harness distortion to their advantage are excellent for jazz, vocal, and even modern rock. My own preferences tend toward tight, controlled, and well-executed sonics though, and I love the combination of the iPod touch and the Rx.

Chart Disclaimer
This review’s RMAA measurements reflect the performance differences between the naked iPod touch 2G and the same iPod when paired with the Rx and ALO Cryo line out dock. Since they are taken with my equipment, they should not be compared to any other person’s or organisation’s technical data. The data represents the ability of the amplification circuit to drive headphones. It is NOT the headphone response data.

Sound conclusioin
Vivid, lively, and powerful, the Rx is a dream among truly portable headphone amplifiers. It literally can drive any headphone from the smallest, most sensitive inner earphone to power-hungry headphones such as the Sennheiser HD-600. Saying that, it outputs optimal pressure to inner earphones and marginally sensitive headphones such as the DJ1Pro. Harder to drive headphones such as the AKG K701 really need a home headphone amplifier, and the Beyerdynammic DT-880, while a smashing companion, requires more beef. For a battery-powered portable, however, the Rx is probably one of the best at any price. Not prone to distort, or roll-off, it presents details and pushes a wide sound stage. Hard-to-drive IEMs such as the JH13Pro are bottomless, layered, and beautiful. But, you must keep in mind that this amp is quite detailed. Nothing smears; nothing bleeds. It has a good, fatigue-free ambience, but if your preferences tend toward the warm and the fuzzy, your search may not be over. Also, there is a low level hiss in the background, and when the power is engaged, a thump.

All in all, this amp is truly a stellar performer, and among battery-powered portable amps, one of the elite.

Conclusion
Buying the ALO Rx isn’t a small decision. At 350$, it sits rather high up the food chain among portable headphone amps. There are more expensive options in the market, but if you are thinking of buying one, you probably won’t be reading this review! What impresses me most about the Rx is that it can drive everything. It thunders past amps which are marketed toward IEMs, and still has the guts to bust up my DJ1Pro, and put on a respectful performance with the HD-600. For the person who doesn’t care about investing into an amp which has to stay on the desk and plugged into the mains, the Rx is a great option. It also introduces great new technology which has proven benefits to all headphone users. While discerning IEM users may complain of low-level hiss and a power thump which at this price is somewhat perplexing, both of my thumbs are pointed skyward and my proverbial hat is tipped toward ALO and GR9.

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Amp Summary
Title:ALO Rx Headphone AmplifierDeveloper:ALO Audio and GR9 Technologies
Reviewed Ver:Black!
Price:$345
  • Great looks
  • Solid construction
  • Effortless, multi-textured sound from IEMs to full size headphones
  • Excellent sound stage
  • Perfect stepped attenuator
  • Low-level hiss with sensitive earphones
  • Audible thump when power engages

Headphone amps and DACs help your headphones get the most out of their transducers. Take a look through our headphone section for suggestions of good upgrade/sidegrade options, and our headphone amplifier section for suggestions on how to wring out the best performance from your beloved phones.

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