Many portable audio enthusiasts invest in high-quality earphones that they plan to use with their iPods, iPhones, iPads or other personal digital audio players. But the further you go in this hobby, the more like you are to encounter serious, high-performance earphones or custom-fit in-ear monitors that cry out to be powered by high-quality portable amplifiers. To be perfectly clear, let’s acknowledge that you could drive most of these high-end ‘phones or monitors directly from an iPod or other portable device—if you really wanted to do so. It’s just that past experience has shown that high-end earphones can and do sound better—often a whole lot better—when driven by high-quality portable amps. Think of the step up to a separate portable amp, then, as a “graduation day”-like experience, where you can expect to take a big step up in quality from the sound of a box-stock iPod-powered system.
Which portable amplifier should you choose? Frankly, there are many good models on the market, a number of which Playback has reviewed or will explore in the future. But one of the nicest we’ve found, and a model that we have used as a reference in doing many of our high-end earphone and in-ear monitor review, is the ALO Audio Rx Mk2 ($449).
ALO Audio (the acronym stands for “Audio Line Out”) is an Oregon-based firm headed by Ken Ball that manufactures a very high quality portable headphone amplifier (the Rx Mk2 model under review here) plus a range of associated specialty audio cables and accessories geared for high-end headphone enthusiasts. In addition to building its own products, however, ALO Audio also serves as an online reseller that offers a broad array of home audio, portable audio, headphone, specialty cable, and iPod/iMod audio products.
In addition to ALO Audio, Ken Ball also operates a Portland, Oregon-based, high-end headphone-oriented specialty retail store called 32 Ohm Audio. To check out this gorgeous facility, which describes itself as “Portland’s only Headphone Boutique and Digital Audio Salon,” click here.
While ALO Audio quite reasonably asserts that it own amplifier and cable products are carefully designed and extremely well-made, it stops short of claiming that its products are “inherently superior to all others” or “the only valid choices for discerning listeners.” Instead, ALO Audio takes a welcome and thoroughly refreshing “live and let live” approach toward its respected competitors, whose hand picked components are offered for sale directly alongside ALO products through the eclectic and inviting ALO Audio web site.
ALO Audio’s first portable headphone amplifier was the lovely and great-sounding Rx (or “Prescription”) model—an amp that Playback first encountered in June of 2010 and that I began preparing to review in the fall of 2010. Click here to read my detailed “First Listen” blog on the original ALO Rx amp.
However, just before I was ready to finalize my Rx review, I met up with ALO Audio’s Ken Ball at the Head-Fi.org Can-Jam event held at the Rocky Mountain Audio Fest in October of 2010, and learned that a new and more highly-evolved version of the amp—called the Rx Mk2—had entered production and was ready for review.
As you might expect, the Rx Mk2 incorporates many of the signature features of the original Rx, along with several seemingly small but decidedly worthwhile changes. I’ve listed technical highlights of the Rx Mk2 below:
• A slightly larger brushed aluminum chassis with revised (and in my view, improved) cosmetics.
• A significantly larger, beefier, dual Li-ion onboard battery pack.
• A version of the “smart charging” circuit pioneered in the original Rx amp, which “constantly monitors the batteries’ voltage and current capability” and that also “determines the optimal current and voltage to feed the batteries that will best charge and preserve the life of the batteries.”
• A version of the special power supply used in the original Rx amp that features, says ALO, “a real bi-polar power supply not a rail splitter...” and that can deliver “a minimum of 5.25V peak to peak and over 200mA of output current.”
• A new “illuminated on/off ‘smart’ LED switch, indicating charging and battery status.” The color code for the switch works as follows:
- Power switch off, no LED = amp is turned off.
- Power switch off, LED is solid green = amp is plugged into charger and battery is actively charging.
- Power switch off, LED is blinking green = amp is plugged into charger and battery is fully charged.
- Power switch on, no LED = battery is exhausted.
- Power switch on, LED is solid green = amp is turned on.
- Power switch on, LED is bright green = amp is on, but also is plugged into charger and battery pack is charging.
- Power switch is on, LED is blinking green = amp is one, plugged into charger, and battery pack is fully charged.
- Power switch is on, LED is Red = amp is on but battery is low.
• Improved heavy-duty 3.5mm input and output jacks.
• A version of the very low distortion analog audio circuitry from the original Rx amp, with claimed frequency response of 10Hz – 20kHz ±0.1 dB @ 1 V output, with THD + Noise of 0.004% @ 1 V output into a 24 Ohm load.
• A version of the distinctive “digitally controlled dual stepped attenuator” volume control circuit from the original Rx amp, said to be accurate to within ±0.2 dB. The control features a tiny, spring-loaded volume up/volume down switch rather than a typical control knob. Upon initial power up, the volume is automatically set to a moderate middle level (so you can tell from the outset that everything is working properly), with users of course having the option to adjust levels upward or downward from there.
• A compact 100v-240v charger that comes fitted with a US-type plug.