As has been my practice in the past, I try to keep AVguide.com readers informed of promising products that I’m in the process of reviewing for Playback, and one such product is the ALO Audio Rx (Prescription) Portable Headphone Amplifier, which sells for $345.
Perhaps the first question I should address is the inevitable, “Who are these guys?” ALO Audio (where ALO stands for “Analog Line Out”) is a Portland, OR-based manufacturer that focuses on building amplifiers and specialty signal cables targeted toward the high-end headphone community. In turn, ALO products are sold through a sister retail store also based in Portland, called 32 Ohm Audio. A gentleman named Ken Ball heads up both firms, and if you ever have the chance to meet him you’ll soon discover that he’s an enthusiastic and very knowledgeable proponent of all things having to do with high-performance headphones. Apart from the 32 Ohm Audio sales channel, ALO products are also sold through a select handful of retailers in the U.S., China, South Korea, Japan, and Thailand.
The ALO Rx is a little gem of an amplifier that is housed in a custom-made, extruded aluminum enclosure with beautifully made and precisely fitted aluminum endcaps. The Rx is powered by two 3.7 V lithium-ion batteries said to provide “tons of power for headphones of all flavors, from the ultra-sensitive IEM to more power-hungry circum-aural headphones.” At first glance, the Rx might seem similar to any number of other good portable headphone amplifiers, but the closer you look the more certain subtle but critical details stand out. I’ve highlighted some of these below.
Unlike many portable amps, whose charging circuits can arguably shorten battery life over the long haul, the Rx uses a purpose-built intelligent charging circuit that “constantly monitors the batteries’ voltage and current capability” and that “determines the optimal current and voltage to feed the batteries that will best charge and preserve the life of the batteries.” One upshot of this, says ALO, is that it’s perfectly safe to listen to the amp with it plugged into the charger to keep the batteries topped off. At full charge, the Rx offers between 17 – 30 hours of playing time per charge when used at normal volume levels.
ALO stresses that the Rx has “a real bi-polar power supply not a rail splitter...” One implication of this is that the amp can deliver “a minimum of 5.25V peak to peak and over 200mA of output current,” meaning it can comfortably drive headphones that demand fairly high levels of current—something not all portable amps can really do well.
One the of Rx amp’s signature features is its distinctive “digitally controlled dual stepped attenuator” volume control circuit, which features a tiny, spring-loaded volume up/volume down switch rather than a typical control knob. Interestingly, the amp automatically powers up with volume set a moderate middle level (so you can tell from the outset that everything is working as it should), giving the user the option of adjusting levels upward or downward from there. Channel tracking is said to be accurate to within an excellent ±0.2 dB.
ALO quotes impressive real-world specifications for the Rx amp, including 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.
Since the amp is named the Rx, one might well ask, “What’s it a prescription for?” Let’s just put it this way. If thin, shrill, glassy, or painfully analytical sound is the disease (and with some portable amps it is), then the Rx is the cure. I say this because the Rx manages to convey the natural weight, warmth and ease of well-recorded music, yet without throwing a blanket over low-level details or quashing dynamic contrasts or transient information. If I had to describe the Rx’s sound in just three of four words, the words I would pick are “natural,” “unforced,” and “at ease.” There’s a certain sonic gracefulness about this little amp that just “clicks” with a wide range of headphone designs, and its bass-drive capabilities are very impressive in light of its diminutive size.