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"I believe that the Pico is the first portable USB DAC/amp with a top-level 24-bit/192kHz DAC chip and the first to use an upsampling design whilst still being one of the smallest if not the smallest in size. Almost all other USB DAC/amps use all-in-one integrated 16-bit chips for their DAC. The one chip I won't identify for you is in the output stage of the DAC. This op-amp is a unique MOSFET design which eliminates input crossover distortion which traditionally plagued op-amp performance.

"The 1000uF capacitor next to the battery remains charged up to absorb the turn-on surge from the headphone amp. This prevents the charging circuit from reading the surge current from turning the amp as an empty battery. This cap is of no use in the power supply where it would merely lower the battery's impedance. The impedance of a li-ion battery already is so low that this capacitor would do nothing.

"Replacing the battery simply requires taking the circuit boards out of the enclosure. The battery is not soldered to the board or permanently attached in any way. I stock replacement batteries and also stay on top of increasing battery capacities. Each time I order more battery packs, I make sure to get the largest one I can fit into the amp

"A linear regulated power supply runs the amp with wall power. The amp will switch to wall power whenever plugged in, allowing the battery to charge without its controller being triggered by peak amplifier currents. The battery does not charge continuously. It tapers off and stops when the battery is full. Active charging is indicated by the red LED on the front. One could run the amp on AC power continuously. If the battery doesn't need charging, this will just power the amp. I can't hear any difference in performance between AC or DC power nor during charging.

"The headphone amp section of the Pico uses the AD8397 chip, a very linear high-output current op-amp. The Pico's Lithium-Ion battery is actually a twin cell in a single pack. This gives 8.4V—twice the voltage of most li-ion batteries—and delivers at least twice as much sound to the headphones.

"This voltage and high current will properly drive even the most power-hungry headphones, hence the headphone amp is never driven from the lower 5V USB bus. I offer a 3-year warranty on the amp and battery."

As to what the Pico can drive, Justin Wilson owns "Sennheiser HD650s and AKG K701s. The latter are harder to drive. Thus I used them on the Pico which caused many changes in the prototypes. The final Pico works just fine with the K701s." Of the various headphones I own, the AKG K-702s are indeed the toughest loads. I could run the same test vis-à-vis my ALO Audio Rx amplifier whose foot print and slims duplicate the iPod (Justin has the Pico Slim for that).

The Pico ships with a 12V/1A universal 100-240V charger; a USB standard-to-mini link; peel-off rubber bumpers; and a black pouch embossed with the Pico logo. There are various color options as detailed on Justin's website and fit 'n' finish truly set new standards in this genre.

For a sonic reality check of souped-up portable vs. top-class stationary, I still had the Burson Audio HA-160 and Corda Concerto amps. Headamp's target audience and design brief for the Pico are obviously different. Portability is key. Still, it's always instructive to know what—if anything—the pursuit of extreme miniaturization sacrifices in ultimate performance. For a like-meets-like comparison, I had my treasured ALO Audio Rx amplifier and for a really power-hungry load, Fang Bian's review loaner HifiMan HE-5LE planar magnetics.