The Usual Suspects: 12 Portable Amps Compared

TINY AMPS: RSA SHADOW, RSA MUSTANG, HEADAMP PICO SLIM

The tiny amplifiers are mostly designed for IEMs, with the exception of the Mustang, which has enough voltage swing to drive the typical full size such as the AKG K701 or Sennheiser HD650 to loud levels. If you happen to use a source that has a high line output level (I used the Hifiman HM-601/602, where the output level is noticeably higher than the Ipod), and your recording level is relatively high (most modern recordings are relatively high in output level), then I find that the Shadow and the Pico Slim can still drive a HD650 to good loudness levels indoors, though with very few headroom left in the volume level. In all fairness, if you plan to use the amp with the bigger full size cans, I would strongly recommend you to get the bigger amps as they do the job with more authority.

The usual suspects for tiny sized amps: (top to bottom) RSA Shadow, RSA P-51 Mustang, HeadAmp Pico Slim.

Front and back views.

RSA P-51 MUSTANG ($375)
The earliest entry into this category was the Mustang by Ray Samuels Audio. Ray Samuels had already built a very prestigious brand name in the industry, and when the Mustang was introduced, it was a ground-breaker as this tiny amp had enough power to drive the Sennheiser HD650 headphone to really loud levels. The Mustang practically made all the other portable amps in RSA’s line up almost obsolete with its tiny amps, and with enough power for anything from sensitive IEMs to big full size headphones. It is not quite as weighty as the bigger Hornet or SR71-A models, but given the size and versatility of the sound, I think a lot of people opted to go with the Mustang as opposed to the bigger RSA amps. Aside from the other, more specialized models like the Shadow, the Predator, or the SR-71a, the Mustang is probably the most versatile amplifier in Ray’s line up, and it happens to have a very likable sound as well. The sound generally follows the signature RSA house sound, though it was a little less dark than most of the other RSA sound. The treble remains soft and unagressive, which is signature of the RSA amps. The bass is not quite as weighty as the other RSA stuff, either the older Hornet/SR71A generation, or the newer Shadow/Protector/SR71-B. However, the midrange is probably the best deal with the Mustang, as it remains to be one of the sweetest midrange I’ve heard on portable amps, even besting the TTVJ slim, and just a tad less lush than the original HeadAmp Pico.

Among the three tiny amplifiers, the Mustang is the only one that comes with a three levels of gain, and with more power to drive big full size cans. This is of course a strong selling point, and despite not having the fancier digital volume control, I think some people would still be better off with the Mustang due to the higher power output. Although the Mustang has one of the most pleasant and most likable sound among the amps in this shootout, the introduction of newer amps that offer newer technology seem to have pushed down the popularity level of the Mustang. Quite a pity to see how this great amp loses its luster simply because there are newer, more technologically advanced products on the market. While the Shadow and the Pico Slim are just as great amps, I think the tonality of the Mustang makes it still the most likable amp of the group.

RSA SHADOW ($395)
It’s ironic to see how the RSA Shadow happens to be one of the amplifiers that drowns the popularity of the Mustang. Never ending innovations from Ray Samuels brought us the first portable amplifier with digital volume control, and the Shadow was eagerly anticipated in the market due to the implementation of digital volume control in a portable amp. The dimension, even tinier than the Mustang, is a huge selling point with the Shadow, as it still remains the smallest, high fidelity, portable amp around in the market.

The sound of the Shadow very closely mimics the warm sound that you get from the Mustang. Both amps are well into the range of the RSA house sound, but with some differences. I had difficulty differentiating the sound of the Shadow to the Mustang at first. But now it’s very clear that the Shadow has quite an edge in separation and articulation. These are understandable, as the digital volume control provide higher level of accuracy and is less destructive to the input signal. Soundstage is slightly wider on the Shadow, but the Mustang has better depth, and the overall impression on the soundstage is more spacious on the Mustang. Busy passages are better articulated in the Shadow, especially in the bass, where the Shadow also  hits lower than the Mustang. Critical listening aside, the Mustang sounds sweeter and less dark, while the Shadow feels a little drier, mostly due to the difference in midrange.

Out of the abundance of portable amplifiers my friend Peter owns, the Mustang is still his favorite when it comes to vocal reproduction, and I really have to agree with him on this one. The Shadow, on the other hand is less engaging and relatively more sterile in the midrange compared to the Mustang. But where the Shadow does have a superior level of articulation, and so it becomes a choice between the two. With the Shadow you get better articulation and a more compact size, where with the Mustang you get a sweeter midrange and three levels of gain for better loudness level with full size headphones.

HEADAMP PICO SLIM ($399)
The HeadAmp Pico Slim is the latest entry to the tiny amplifiers market. Most of my friends who’ve heard the Pico Slim left feeling cold and unimpressed with the sound that they hear out of it. The original Pico had this sweet lush midrange that’s even better than the Mustang’s, and it still remains one of the most loved portable amplifiers today (Why did I miss including it for this article?). The Pico Slim, however, had a neutral vanilla colored sound to it, and to most who’ve heard it, it’s simply too plain. Actually, vanilla ice cream still have some sweetness to it. Instead, think low fat, low sugar vanilla ice cream, then you’re getting closer. In term of features, it’s quite similar to the RSA Shadow. Both are tiny, though the Shadow is tinier at almost half the size. Both comes with a high resolution digital volume control, and both are limited in gain levels and is optimized for IEMs. But if the RSA Shadow follows the similar sound signature as the bigger Mustang, and relatively to the other RSA models, the Pico Slim is a big departure from the sound of the original Pico. The vanilla color reminds me heavily of the desktop HeadAmp Gilmore Lite amplifier, relatively plain and “as it is”. Among all of the portables in this comparison, I find the Pico Slim to have the least amount of coloration, and closest to the popular adage “wire with gain”.

I realize that the previous paragraph may have sounded like a so-so impression of the Pico Slim. And yet, here is the twist, the Pico Slim happens to be my favorite amplifier of the bunch, and definitely my favorite pairing for my JH customs. I think the words to use for the Pico are articulation, resolution, bass, and soundstage. All impressive words, but I’m not just throwing them out there. When people ask me what the Pico Slim sounds like, those are the words I give them. In fact, people in my local forum probably knows me for being the biggest fan of the Pico Slim (good thing HeadAmp is not our sponsor, though they have good reasons to be one now).

When I was talking about the Mustang and the Shadow, one of the thing I mentioned was how the Shadow, benefiting from the digital volume control, brings up an even better level of articulation over the Mustang. The Pico is similar to that, but to an even greater degree. Simply put, I find the Pico Slim to have the best level of articulation among all the portable amplifiers in this comparison. This is definitely aided by the step less digital volume control on the Pico, though circuit and power supply design also contributes a big deal to the Pico’s overall level of articulation. Most of us today listens to fairly fast-paced, “busy”, and complex music, and articulation is really important when playing those kind of music. Even more, if you’re using a JH16 like me, where the task of controlling those quad-bass drivers becomes quite a challenge to the average quality amplifiers. This is where the Pico Slim really shines. Crisp, tight, articulate, well defined bass, even with the JH16 and the most complex passages. Added to the fact that the Pico Slim can punch quite hard on the bass, though it’s relatively not-dark signature tends to make people think it’s not good with bass. I actually love the bass punch on the Pico Slim. It’s powerful, tight, and articulate, leading to one of the best PRaT factor among the amps here.

Compared to the big portables such as the SR-71A and the i-Qube, I still think the level of articulation and resolution of the Pico Slim to be ahead those two highly touted amps, while closer to the articulation level of the Stepdance. While the SR-71A has one of the highest level of resolution, the Pico manages to match it, and even better it in the area of bass control and articulation.

Compared to the portable balanced amps, the Slim doesn’t sound as impressive, as it doesn’t boost up the lows and widens the soundstage as the balanced amps does. Nor does it have the kind of power output that you get with the portable balanced amps. But as with the case of the comparison with the SR-71A, the case with the Pico Slim is about quality and not quantity. The bass is punch is actually quite good, but it’s the bass definition and detail that beats the balanced amps handily. And though the soundstage is less wide, the overall imaging performance and coherence on the Slim is definitely superior. For IEMs, the Pico Slim is probably as good as an amplifier can get for now, and will remain to be my reference amp to judge the technicality of other portable amps.

I happen to love the Pico Slim very much, not only because it’s amazing articulation, but also the fact that it synergies beautifully with my JH16 IEM. However, I think the low coloration sound of the Pico is mostly what turns people off after they audition one at the meets. Although I find the Pico Slim to have better technicalities than the RSA Shadow, most people would prefer the warmer and darker sound of the Shadow, as a lot of people tend to be allergic to treble these days. The Pico Slim adds a little boost on the lower treble, and while it’s perfect on the JH5/10/16Pro IEMs, most other IEMs have plenty of treble to begin with, and for them I generally would recommend them to go with the RSA sound to give a better tonal balance. If you happen to be a JH user, you definitely need to find a way to audition the Pico Slim.