Retail price: $135
Practical Devices, based out of Canada, has been in the business of making dedicated portable headphone amplifiers for a couple years now. Their current product, the XM4, was originally released in 2006, but was updated in January with a lithium-ion battery, and then again very recently to solve an issue with the headphone jack, as the first batch of units had loose headphone jacks. No issues at all came up with the headphone jack on the review unit, which is good news indeed. I was eager to find out how it stacks up in today's portable amp market so I put it to the grinder with some of its latest competition - Xenos' 1HA-EPC, RSA's Emmeline The Hornet "M", and the very recently discontinued Go-Vibe V6 w/ AD8620.
Sources: Arcam FMJ CD33, iAudio X5 (using FLAC) w/ line-out cable
Power cord on CDP: Signal Cable Silver Resolution Reference
Interconnect: Signal Cable SilverMini
Headphones: AKG K701; Audio-Technica ATH-AD2000, ATH-ES7, & ATH-CK7; Grado SR325i; Koss KSC75; Sennheiser HD650
Comparison Amps: Go-Vibe V6 (w/ AD8620 op-amp), RSA Emmeline The Hornet "M", Xenos 1HA-EPC, HeadAmp Gilmore Lite v2 w/ DPS
Retail price of review component: $135
Jewel - Spirit Alison Krauss - Lonely Runs Both Ways
Eva Cassidy - Songbird Laika - Sounds of the Satellites
Massive Attack - Mezzanine Norah Jones - Not Too Late
Porcupine Tree - In Absentia The Crystal Method - Tweekend
Portable amps are in general tough to pull off well, for two primary reasons: (1) they need to pack a lot of electronics into a tiny chassis, inevitably creating the dilemma of putting enough space between the input/output jacks and volume knob (and any other optional features), and (2) they need to pack as much sound quality as possible into that tiny chassis.
With that said, the first factor was clearly in play when I saw the XM4. While the input jack, output jack, and volume knob have plenty of space between each other, two push-buttons (one for bass boost, the other for crossfeed, both described below) get in the way and make everything feel crammed. It's not so bad with a compact input interconnect and headphone plug, but there's definitely a problem when using a 1/4"-3.5mm adapter on the headphone plug (for headphones terminated to 1/4" plugs like the Grado SR325i, AKG K701, and Sennheiser HD650), and not all RCA-mini (or mini-mini as the case might be) interconnects are terminated with slim connectors either, as some Neutrik, Canare, and Cryo connectors can get pretty big. As it is, the front panel of the XM4 simply shows a lack of foresight in this aspect.
That takes care of the physical, but what about the sound, right? when I fired up the amp off of the Arcam FMJ CD33 (using the Signal Cable SilverMini as interconnect) with no idea of what to expect, I was pretty surprised at the sound. It was better than what I was expecting - the full audible frequency spectrum was clear and open, there was some soundstage, and most of all it simply sounded clean with no major sonic information missing. Practical Devices recommended 8 hours of burn-in, so I left a CD on repeat and came back to it approximately 8 hours later.
Before moving on to the amp's sound, it should be noted that the XM4 is probably one of the most (if not THE most) feature-laden portable amps in existence. It has bass boost, variable crossfeed, adjustable gain via jumpers, and automatic power settings - achieved through the use of a microprocessor. These automatic power settings allow you to set an amount of time for the amp to turn on before it turns itself off, ranging from 1 hour, to 3 hours, or 6 hours, or "forever" (until the battery runs out). That might be handy for people who tend to be forgetful, but I didn't use these settings and just set the amp on "forever" mode, partially to test battery life. The XM4 uses an internal lithium-ion battery, and it ran quite a long time too before finally being depleted - easily over 30 hours, as it went on for days across many listening sessions with various current- and voltage-hungry headphones, and I only had to recharge it once during the time I listened to it.
Bass boost worked surprisingly well and added just the right amount of extra oomph in the approximate 40-60 Hz range to make bass hits satisfying. It won't turn bass-light headphones into bass monsters but it definitely added an appreciable kick on the K701 which is certainly a headphone that needs this sort of kick. The variable crossfeed can also have surprising results, provided you use it with the right headphones. It's variable in the sense that you can control the level of signal crosstalk, all the way to a mono-channel signal if desired. It didn't work all that well with headphones that already have layered soundstages, like the K701 or ATH-AD2000, but it fared much better with the ATH-ES7, ATH-CK7, KSC75, SR325i, and HD650. A caveat there though, because if it weren't adjustable, it wouldn't have been able to sound good with these headphones - it's really only due to the adjustable nature of the crossfeed that it was able to realistically simulate soundstage on them. Also, while enabling crossfeed does slice a chunk out of the mid-range, increasing the volume does partially work to offset things.
Finally, while the gain is adjustable, most people probably won't need to adjust it - the default gain turned out to be more than high enough, as it worked great with the voltage-hungry K701 and HD650. In fact, my only complaint about the gain is that the default setting is a bit excessive - you don't get much fine-tuning control on efficient headphones with it. In fact, the Audio-Technica and Grado headphones were a bit frustrating to control, as even the smallest volume adjustments were quite a significant difference.
The XM4 turned out to be a bit of a compromise in terms of achievement towards a completely flat frequency response. However, unlike most portable amps, it's actually surprisingly close to being completely flat, and in fact is considerably closer than the other three portable amps I compared it with, the Go-Vibe V6, RSA Hornet M, and Xenos 1HA-EPC. So as long as we're talking about a flat frequency response being the ideal (instead of colorations for pleasing euphony), the XM4 beats out its competition handily.
Of course that doesn't mean it's completely flat though, and that's where disappointment set in. Not to be over-critical on a $135 portable amp of course, but there are a few areas in which the XM4 is noticeably lacking. First is the treble, which is a tad too aggressive in the upper ranges and can make some music sound metallic and grainy. The metallic nature of the treble pokes its head out way too easily on acoustic music for example and makes instruments like guitar and cymbals fatiguing.
Second is the mid-range, which to its credit is clean and clear on its own, but when stacked up against the competition, and especially the Gilmore Lite, doesn't sound very natural and organic. "Flat" shouldn't mean there's also a loss of musicality, but in the case of the XM4, it misses a chunk of what contributes to musical involvement and sounds more straight-forward and "technical" than anything else. Not necessarily a bad thing of course, this just makes it fall short of being truly transparent in the mid-range.
Continuing down the frequency spectrum, the lower mids and mid-bass retain a clean, accurate sound, all the way down to the lower regions of the bass. More than anything the amp is capable of delivering the kind of bass that's tight and fast, and it can easily keep up with complex rhythms without slowing down the headphones. While the bass doesn't exude true authority and power (usually an area reserved for home amps), it's still plenty enough - it's just that it doesn't add any unnecessary extra boom or oomph. More of a laidback nature with it, as it simply conveys what it's fed instead of adding anything, and indeed, it's a lot more about quality than quantity. Tuneful and textured describe it best, and this makes it a better match for headphones that have more bass quantity than it. It lacks ultimate extension down to ~30 Hz and below though, which was easily heard on the ATH-AD2000, and makes it unable to rock out to bass-heavy music. The bass boost can somewhat rectify this, but the bass boost is just that, it merely adds extra energy, not extension, which is what's really needed. In short, neither bass impact nor extension is satisfying on the XM4, which does hurt it - this kind of response limits the headphone versatility of the amp.
The XM4 doesn't really need its crossfeed to be on to impress with soundstage - there were certainly no major faults with it on either the K701 or ATH-AD2000. Clearly open and wide too, with a very good left/right image that easily spans the width ability of both headphones. Not as wide an image as the Xenos 1HA-EPC though, but that's easily a preferential thing, as the image of the Xenos could be considered artifically wide by some people. However, like the Xenos, the XM4 didn't show much soundstage depth - in fact, it seemed downright compressed compared to the Gilmore Lite, which doesn't even have that much soundstage depth of its own. There was a noticeable lack of openness and air when switching to the XM4 from the Gilmore Lite, with the image much more 2D and in-your-face.
Our recommendation is based mainly on your budget and your amount of patience. The Practical Devices XM4 now sits in a very crowded market of portable amps at all price levels where one product has to exibit something special to stand out. The XM4 does have a strong and compelling feature set and at $135 it's a very good value for the money because there aren't many quality amps in the under $200 range. So, if $200 is your maximum budget, then the XM4 is one of the best options out there for truly portable applications. If you can wait just a bit, it's probably worth saving up for one of its more expensive competitors with better sonics.