One of the well-known disadvantages of portable amps has always been, simply put, a lack of power available, being that they are limited to using batteries. While with in-ear monitors and more easy-to-drive headphones this isn't such a problem, it limits their capabilities when it comes to either high-impedance headphones (which sound best with amps that can consistently swing a greater voltage) and headphones such as lower-sensitivity orthodynamics (which sound best with amps that can deliver high amounts of current). While it is readily possible to cram a larger battery and more capacitors (to hold more reserve power for peaks in the music) in a case, the result quickly becomes ungainly in size.
It's not for want of trying though, with the recent crop of portable amps such as the Ray Samuels SR-71B and ALO Audio RXIII and International overcoming this with a balanced circuit -- basically double the electronics for greater power. The results with those have been much closer to desktop capability. However, once you have to consider re-terminating your headphones with the unusual plug (a regular TRS plug is no good) the cost starts heading towards 4-figures before one even looks at headphones (and well over for the top set-ups with a high-quality digital source).
Since coming out with what is very probably the world's smallest IEM amp, the Pico Slim, Justin Wilson of Headamp set his sights on making an amp that was both small, yet packed enough punch to use with full-sized headphones. That took him 2 and a half years, but the result, after painstakingly getting everything perfect is quite an accomplished amp.
Case quality, as always with Headamp products, is excellent. Switches for the power and 3-position gain are tiny, but that prevents them easily being bumped though they are easily manipulated. The headphone sockets are quality metal items the volume knob is short, more so than a right-angled headphone plug, but deeply knurled. Behind it is a teflon ring which makes it deliberately stiff (to prevent accidental adjustment), but smooth to turn. Along with the beautiful anodising and laser-engraved labels, a good leather case completes the picture, holding the amp firmly inside.
Unlike both the Pico, which contains a DAC, and the Slim, which contains a digitally-controlled volume and charging circuit, the Pico Power is purely an amp and nothing else. That allows it to hold 2 9V batteries while only taking up a volume pretty much only the case wall thickness over the size of about 4 of them. Compare that to the vastly larger O2 or Triad L3, both of which use the same batteries. This makes it far more reasonable in size, even with the included leather case.
Given the use of 9V batteries rather than a battery pack, the Pico Power allows some flexibility in what trade-offs can be had. Higher milliamp hour (mAh) re-chargeable batteries will give longer battery life at the expense of peak power, whereas lower mAh ones sacrifice usage time for higher power delivery. By default, the Pico Power comes with a set of Duracells. I found these lasted quite a while and I could still play music with the amp even after the power LED stopped lighting, letting me know I was due to change them out. While there isn't a charging port on the amp, changing out the batteries is a fairly straight-forward affair. Flip the amp over and undo the nuts, which you have to be very careful not to lose as they come off suddenly. A spare pair is usefully included with the amp. The battery springs are stiff and a firm grip on the amp is needed while removing the batteries. Some slightly over-size re-chargables are going to be tough to get in and out. With the batteries out you can admire the artful machine marks inside before inserting another pair.
Speaking of the case, it fits the amp very snugly, and I saw no reason not to include it when attaching the amp to my (trans)portable rig which includes a Fostex HP-P1. However, while the Fostex has a headphone amp included, I felt I got far better results when using it with the Triad Audio L3 driving headphones. Likewise, with the Pico Power, while I cannot compare both amps directly, since I sold the L3, I do feel I get similar results, with the Pico Power sounding less like it is making an effort to play the music than the HP-P1.
A recent addiction ..err.. I mean addition to my transportable rig has been the iRiver AK100. While the inbuilt headphone amp is ok, high-impedance headphones sound a bit muffled directly out of its headphone port and IEMs are somewhat limited by the higher than ideal output impedance. Adding the Pico Power in the chain to the former, music comes through more relaxed and spacious with instruments more clearly delineated and the soundstage wider. Sensitive, multi-BA IEMs likewise are controlled perfectly. The only disadvantage the it may have is that standard volume controls are not always perfectly channel balanced, unlike the more noisy electronic volume control used in the Pico Slim.
Using the gain switch and selecting a suitable volume level on the Pico Power, I can use the volume on the AK100 to precisely dial in the level I want. The slight stiffness of the amp's volume control means that it stays in place firmly. The amp is also dead silent, critical for playback with such IEMs.
Similarly, the Pico Power is a good match with devices such as the Audioquest Dragonfly and Meridian Explorer, which are limited by the amount of power that they can draw from a computer's USB port and thus their limitations when it comes to powering headphones. Even using a pair of highly resolving Tralucent Audio 1plus2 IEMs or my JH-13 customs and feeding my Dragonfly better power via an Aurorasound USB Bus Pro, I felt the Pico Power still managed to get a bit more out of fast-paced music such as Paco de Lucía, John McLaughlin and Al Di Meola playing on Friday Night in San Francisco. Each guitar note seem a very slight bit more clear and less harsh. Not surprisingly, comparing the ability of my iPhone 5 with full-sized headphones with and without the Pico Power there was no contest.
Some time ago I DIY'ed an Objective 2 amplifier to see what all the fuss was about. I brought it out to see how it compared to the Pico Power. With careful volume matching there wasn't a lot of difference, with the O2 seeming to be a tiny bit more detailed with my best recordings and the Pico Power giving the music a bit more "body" and being more overall listenable. What was interesting for me was to later learn that Tyll Hertsens over at Innerfidelity had measured the Pico Power and it measures phenomenally well.
Probably the only place the Pico Power falls down a bit in my experience is with orthos, which work best with high-current amps. My LCD-3s sounded rather asleep from Pico Power, unfortunately, though given my purpose in buying the amp, I'm not likely to be using it with them. The LCD-X and LCD-XC fared quite a bit better, given their higher sensitivity and more lively presentation, however, so while not a portable rig as such, as a somewhat heavy transportable rig, the LCD-XCs can easily be used with it and one's choice of source.
Overall, the Pico Power is a fantastic amp for most headphones, only excepting those with very demanding requirements, such as some orthos. If measurements mean a lot to you as well, you'll be happy to know that it measures exceeding well also. Cutting down the design to the basics combined with Justin's pursuit of perfect execution has resulted in an excellent amp.
Audioquest Dragonfly (with power from an Aurorasound USB Bus Power Pro), Pico Power and Symphones Magnums (Grados with custom drivers)