Pros: Seemingly transparent sound quality, light weight, free quality micro-USB cable.
Cons: No gain switch, not as small as some other alternatives.
If I was to rate products by how easy they are to review, I’d pick amplifiers as an easy first, and headphones and IEMs last. While both have their complexities, with amplifiers those lie with the engineers and manufacturers, and it is fairly easy to determine to what degree the products goals have been met, whereas the complexities of headphones related to the listener, such as music tastes, how loud one listens, comfort and whatnot.
In that regard, the new Rx from Ken Ball at ALO Audio is on the dead easy end of the scale: Do you need an amp that drives IEMs well, especially high-end models with multiple balanced armatures? Do you not mind that it is not the smallest in category, but is a bit bulky? Yes to both? Then the Rx will do the job, all the way up to JHAudio’s new Laylas. That was easy, wasn’t it?
Of course, that is not only what a review is for. You’re hopefully reading this for the fine details, of which there are a few; and to answer a few questions, like “Do I really need this if I have an iPhone?” which is more relevant. “No” is actually the answer to the last question, but because this hobby is not about “need”, because even if an iPhone nowadays is “good enough”, this is about better than good enough, especially if you’re the kind of person who’ll put down a four-figure sum for the best custom, or now universal in-ear monitors.
The Rx is not four figures though, it is $349 at the time of writing. It comes from a line of amps that were first sold in 2009, when a phone really wasn’t good enough for the wave of new multiple-balanced-armature IEMs coming out and required very low output impedances in an amp to avoid quirky results. The Rx line went rather crazy over the next few years, ending up with the monster Rx MKIIIB and its desktop amp-level power. The new Rx goes back to its origins.
It is still, in essence, an opamp-based design, like the original, but every other aspect of the design has been optimised for the lowest distortion possible with IEMs. I have no doubt it was this care and attention to the design that resulted in the experience I had below when I first listened using it with the Laylas. That is why it is easy to review, as the first experience I had told me all I needed to know about amp’s capabilities.
But we still need to cover the physical design, which is a reasonably small nickel-plated case containing a recharchable battery, a power/volume knob, input and output sockets and a USB charging socket on the back. Looking inside the unit, the deliberate choice of a high-quality analogue volume control over a digitally controlled one has pushed the size up above what one might call ideal. Alongside that the sockets and volume aren’t aligned, which ruins the attractiveness a bit too. The case does feel quite classy though.
Functionality-wise, aside from the obvious connections, an orange LED on the front indicates that the power is on, and likewise one on the back next to the USB socket glows orange when charging and green when done. That is as complex as it gets. Holes on the sides allow heat, if any, to vent. Very simple indeed.
An unexpected bonus with the Rx is the inclusion of ALO's Green Line micro-USB cable for charging. It is designed as a high-quality USB data cable, so if you own a DAP or the like which you're using via a regular cheap micro-USB, Ken told me that he put in some serious effort with the design to ensure good digital transmission.
Unlike the original Rx, the actual circuit is more complex. Like we are used to seeing in high-end full-size amps, much care has been taken in the design of the power supply to ensure low distortion and noise in the amplification circuit. Amplification itself is not simply via an opamp as before, but a 2-stage circuit consisting of an OPA49720 opamp feeding a Texas Instruments TPA6120a2 headphone amplifier chip. This chip is a high-bandwidth, low noise design with independent power supplies for low crosstalk, critical for maintaining the sense of soundstage. Designed to maintain a high dynamic range even with high-power output (it can potentially output up to half a Watt) its use in a dedicated IEM amp makes the Rx an interesting design.
I plugged the Rx into my Chord Hugo, which is very much excellent with IEMs, and tried the JHAudio Layla universals both direct and via the Rx. Now I must explain that the Laylas, with their 12 drivers and sophisticated crossover are the biggest pain to drive when it comes to in-ear monitors, possibly more so than any headphones I own. While, say, my FiiO X5 (original model) is pretty fine even with multiple-BA IEMs, it just fails dismally when trying to drive the Layla. Not just less resolution, but wooly bass and mushy sound overall. The Laylas are extremely picky when it comes to amplification, and capable of amazing things when paired well. It is not surprising that they teamed with Astell&Kern to sell them with the AK240. That is the kind of amplification they require.
So I sat down and plugged back and forth between the Hugo direct and Rx. And….no difference! A perfect result! That was most pleasing. The articulation all the way from the bass through to the treble was delivered perfectly, much to my satisfaction. That left me to compare it with other amps and other sources, where there were some interesting discoveries.
I have been using DITA Audio’s The Answer IEMs since their release. Possibly the epitome of single dynamic driver IEM design, they set a high standard, with a price to match. More recently they released a version compatible with the Astell&Kern balanced socket and Danny from DITA kindly conceded to my request for a pair to use for comparison purposes. This proved quite surprising, as the amp in the AK240 is highly capable of driving everything from IEMs up to full-sized headphones with surprising authority. However the Rx had the edge with the easy-to-drive DITAs, even comparing it to the balanced output of the AK240. It wasn’t a large difference, but a sense of slightly more precision and space from the bass through to the treble.
Comparing the Rx to the AK240 with the Laylas was a bigger surprise still. Out of the AK240 in balanced mode they sound quite dark, but the Rx seemed to bring out the treble to a much greater degree than I was suspecting. I rotated through a few amps to try and discover if it was just the AK240 and the Rx still held the result as the amp with the most spacious overall presentation. The Pico Power was a touch more mellow and less spacious-sounding than than the Rx, which can sound slightly brighter in the treble. The Sound Potion Monolith seemed to put a bit more emphasis in the bass and was tiniest bit smoother-sounding. Between them they were all hard to pick. The main difference is with the more expensive Pico Power you get full-sized headphone driving ability and gain settings, as well as a leather case, but an amp that weighs considerably more as a consequence.
That leaves the main disadvantage of the Rx being the size and lack of gain selection, though it is quite a light amp and still smaller than the amps I compared it to. While most portable devices do a good job of driving IEMs, if neither of the negatives are an issue, but getting the most out of a pair of high-end IEMs is the primary focus when one's existing source doesn't do the best job, then ALO's Rx could be just what is needed.