Review: Single Power Audio Square Wave Headphone Amplifier

4/21/09 Update: It recently came to our attention that Singlepower Audio has received many customer complaints about both its service and the quality of its products within the past 1-2 years. More info can be found at, where many customers have been documenting cases of Singlepower delivering products with questionable build quality, failing to ship or service products in a timely manner, not delivering products at all, or even not responding to communication: - Steve Ham

[ Intro ]

Single Power Audio, based in Aurora, CO, has long made a name for itself with its line-up of large, up-scale tube amplifiers for the high-end audiophile crowd, starting in the low-end with the ever-popular PPX3 and going all the way up to multi-thousand-dollar, customized-order units that can be given either gold or chrome finishes upon customer request. Most of these amplifiers are primarily headphone amps for either dynamic or electrostatic headphones, but some can also be tailored to function as pre-amps too upon customer specification.

Single Power Audio and tube amps are practically synonymous, so when the company unveiled its latest product, the Square Wave, at the 2007 International Head-Fi Meet in San Jose, CA, back in April, there was almost a collective gasp, as it was not a tube design, but a solid-state one - and not just solid-state, but also transportable in size! Solid-state AND small enough to take with you around the house or between home and office? It's a bold new move from Single Power, which has built its reputation on large ornate tube amps that sit on a component rack to probably never move again, but it's also up with the times, as more and more people want to be able to take their music with them as much as possible.

The review unit is a final prototype, "March 2007 Rev A," that was a display model at the 2007 International Head-Fi Meet. It is not a final production model as the amp still has not been released yet and there may be a few minor tweaks between the review unit and the production models once they start going out the door.

[ Associated Components ]

Sources: Arcam FMJ CD33, Arcam DiVA CD73
Power cord: Signal Cable Silver Resolution Reference
Interconnects: Audioquest Python, Signal Cable Silver Resolution Analog (w/ Eichmann silver bullets)
Headphones: AKG K701 & K601, Audio-Technica ATH-AD2000 & ATH-ES7, Grado SR325i, Sennheiser HD650
Comparison amplifiers: HeadAmp GS-X, HeadAmp GS-1, Heed Audio CanAmp
Retail price of review component: $999.00

[ Test CDs ]

Alison Krauss - Now That I've Found You
Jewel - Spirit
Howard Shore - The Lord of the Rings: The Return of the King [OST]
KT Tunstall - Eye To The Telescope
Laika - Sounds of the Satellites
Massive Attack - Mezzanine
Norah Jones - Not Too Late
Orbital - Middle of Nowhere
Porcupine Tree - Deadwing
Renee Fleming - Thais (Massenet, 1894)
Sarah Brightman - The Andrew Lloyd Webber Collection
The Crystal Method - Vegas
Thievery Corporation - The Richest Man in Babylon



[ Fit & Finish ]

The Square Wave packs a lean mean frame with its small but durable brushed aluminum body, and it feels rock-solid in the hand too. The chassis is approximately 6x4x1.5 (inches) and is entirely brushed aluminum in solid black, with the front and back screw-mounted transparent panels each adding 0.25" onto the length dimension for a total 6.5" in length. The transparent panels don't exactly give the amp a professional look, but Single Power has stated that alternate, non-transparent panels may be an option in the future. As these dimensions (and the pics) indicate, the amp is very compact, and very transportable on its own. However, the "floor-wart" style AC adapter it's powered by adds a fair bit of bulk to the package - not too much that it doesn't become non-transportable, but it's an extra piece to deal with regardless.

The amp has two noteworthy features that add versatility, one being the Pre Out jacks on the back for pre-amp duties, and the other, a second headphone jack for 3.5mm mini-plugs. The mini-plug jack is pretty small though and won't fit all mini-plug terminations - it doesn't acommodate the ATH-AD2000's mini-plug, for example, and it doesn't work with standard gold-plated 1/4"-mini adapters either, the kind commonly used to re-terminate 1/4" headphone plugs for use with mini plugs. The only headphone in my collection that was able to plug into its mini-jack was the ATH-ES7, which uses an especially slim wire, so the actual practicality of this mini-jack may very well end up being too limiting for most headphones.

It's also worth pointing out a couple minor issues with my unit that will most likely be fixed for the production models. First, there are only three rubber feet on the bottom so balance can get tipsy when handling. And second, two of the four color-coded RCA Input jacks are out of place too. The jacks all output the correct signals, Left and Right are still where you expect them to be, it's just that Input Left and Pre Out Right don't have the appropriate color-coded jacks. Again, this will probably be fixed for the final production models.

[ Three-Way Fight? ]

Immediate impression of the amp's sound was that it was on the warm, mid-range-focused side, especially noticeable in comparison against my HeadAmp amplifiers. This was a surprising result, as Single Power had stated at the 2007 International Head-Fi Meet that the amp is fully discrete, so it should have a sound of being very transparent on the input signal, but I found that this wasn't really the case.

Initially my comparison of the Square Wave started with the HeadAmp GS-1, as the two amps are in the same <$1K price bracket and actually pretty close in price - the Square Wave at $999, the GS-1 at $749. Not to mention that the two amps are also similar internally, with fully-discrete, high-current designs. However, running through my first several test tracks revealed that the Square Wave was in the next class above the GS-1. This wasn't very obvious on the Arcam DiVA CD73, but when I moved to the Arcam FMJ CD33, it became strikingly obvious - the Square Wave more easily scaled up with the increased detail and extra musical information that the CD33 provided. As good as the HeadAmp GS-1 is, the Square Wave simply beat it in overall sonics - faster tracking of the upper treble, more articulation with a faster attack, deeper bass, and a more integrated "all-there" frequency response. Not quite neutral though, as is the GS-1, but more focused and purposeful. The one area the Square Wave noticeably fell behind the GS-1 was in the soundstage, as the GS-1 has a very 3D stage and easily scopes through the z-axis to cull out layers of air.

Relative to the HeadAmp GS-X though is where the Square Wave started showing its colors. Now most might say that these two amps aren't comparable - the Square Wave being as small as it is while the GS-X is more than three times its size, almost twice its price, and is a dual-chassis, dual-mono amp designed from the ground up to function as both unbalanced and balanced amp and pre-amp. But the GS-1 was noticeably below the Square Wave's level, and a higher reference point was needed in order to find out just how high the Square Wave could scale. Interestingly, the Square Wave almost competed with the GS-X. It didn't scrape up as much low-level detail, nor present as wide, deep, & open a soundstage, nor have the sound of ease when sifting through layers and making them individually shimmer, but I'd say its sound qualifies it to sit in the >$1K bracket alongside the GS-X. About 90% the performance of the GS-X, to put a number to it.

[ Sound ]

But enough about the amp versus the others I own. How does it sound, right? On Massive Attack's Mezzanine disc with the Audio-Technica ATH-AD2000 (a headphone particularly well-suited for the perpetual low bass lines), the amp added more depth and low-level push. "Angel," "Risingson," and "Black Milk" became even more bass-heavy, with a greater pulsating feel, as if it was digging ultra deep and pushing sideways at the same time. The low bass rhythm on "Man Next Door" felt like it went deeper with greater extension, rather than sounding a bit weak at the way bottom. Switching the headphones to the K701 for "Inertia Creeps," the amp delivered a harder blow to the belly drum making it sound more visceral with a swifter impact. It did seem to take away a bit of the ambient air of the track but it added more impact and energy in the mid-bass frequencies to exude a greater sense of insistence and rhythm, and more attitude as well.

Laika's Sounds of the Satellites is one of my favorite references for evaluating layer separation, treble extension, and attack. "Bedbugs" is a tough track to get right with its opening drum pattern that pervades through the rest of the track, an ongoing layer of cymbal crashes in the left channel, various brass and woodwind instruments that filter in and out, a near-constant stream of effects, and a female vocal layer that spontaneously shifts positions. The amp put up a valiant effort but ultimately didn't succeed - there was an intangible element of it sounding like it was trying too hard, as the various layers didn't quite flow with ease, and its sound wasn't completely clean, as there was the tiniest sense that it was a bit fuzzy on the imaging and couldn't pinpoint the various layers into precise positions. The amp also seemed to compact the track's air and space, leading to a slight perceived loss of soundstage width & depth. On the plus side, it had no problem giving every sound its full attack - every note quickly sprung off the background, and notes with physical impact really felt like they were being hit not just fast, but hard, very hard in fact. On the same disc, "Martinis on the Moon" is a great test for precision of left-right positioning, with its embedded electric guitar that constantly shifts position along a circular path. Unfortunately the amp didn't keep up with the position-shifting nearly as well as I hoped it would - the listening experience should be thrilling as you get to follow this guitar traveling around in a circle, but the amp lost a bit of information around the travel path and made it seem like there were a few gaps.

Moving on to Jewel's Spirit and Eva Cassidy's Songbird, both discs are a natural test for how raw and gritty a sound an amp can provide. The soulful, unrestrained voices of both female singers with backing guitar are about as simplistic as music gets, but hard for any component to get truly right - there should be that soul in their voices, the innner emotion, the rawness in their performances, to make you feel what they feel. The Square Wave brought these two female singers closer in the soundstage for a more personal, intimate performance. It also effortlessly tracked the quiet and loud dynamics in their singing, making the crescendos that much more powerful and the quiet parts that much more subtle and low-key. To say it brought them to life would be an understatement - their voices soared as if with unrestricted freedom. The amp also worked wonders for the female singers with other vocal timbres - Alison Krauss gained a level of sultriness in addition to her crystalline silky voice, and KT Tunstall's lower notes received some extra fullness and body. The only female singer in my CD collection that the amp didn't work anything extra for was Sarah Brightman, but this was understandable given her high soprano range.

For orchestral music, Reneé Fleming's recording of Thais and the Return of the King film soundtrack provide a contrast for something from the Romantic era and something very recent. On Thais, the amp didn't do anything of note with Act II Scene 6, as the orchestra takes a backseat role to the operatic performance and doesn't have very complex harmonies. It did make the vocal bursts harder-hitting but didn't do much else. On the film soundtrack, "Twilight And Shadow" was used for its sweeping epic overtones and "Shelob's Lair" for its dynamics and ominous strings. The amp cramped the lateral width of the orchestra on both tracks a bit, and lost a bit of elegance on the violins too, but to compensate it did add more substance on the brass and woodwinds, along with a focused brashness on the ominous rolls to make them sound more oppressive. There was also a greater sense of power as it easily flared up with unrestraint on the sudden bursts in volume.

Oh, I could go on. I could say that the amp provided the most realistic, tactile, and fastest response I've heard yet on Thievery Corporation's "Facing East" with its ongoing layers of Middle-Eastern-styled hard percussion and shakers. It didn't just hit, it rammed into the impacts with such force it was like a hit to the chest every time. I could also say the amp served up the slickest, meanest bass yet on Crystal Method's "Busy Child," "High Roller," "Keep Hope Alive," and "Vapor Trail" from Vegas, making it sound like a mini-nuke was going off every time the bass dropped into the nether regions. But it would all just be reiterating the same point over and over - that the sound of the amp is the kind that captures your attention immediately with a strong, able-bodied mid-range and oodles of bass in reserve. It's not the most adept at retrieval of micro micro-detail or at sinking its hooks into the soundstage to deliver razor-clean imaging with air and space intact, but its sound instantly captivates and more than anything it's extremely capable of delivering an entertaining presentation.

[ Pairing Off ]

The Square Wave proved to work well with all four headphone brands in my collection. It added flesh and weight to the AKGs with more than enough voltage swing to go around, and was well able to tackle the 400-Ohm K340 too, which left my collection during the course of this review. With the HD650, the headphone exuded even more character, more weight, and more inner strength. The Grado SR325i did fare better than the Audio-Technica ATH-AD2000, as its sound was a bit better balanced out, but the AD2000 didn't really receive anything helpful from the amp, other than the high amount of current that it needs. The AD2000 is a finicky headphone though with its unique sonic signature and there aren't many amps, even among only solid-states, that pair well with it.

The amp also proved to be very revealing of interconnects and source, as it ruthlessly revealed some texture, bass, and resolution deficiencies with the Arcam DiVA CD73, and sounded different between the copper-based Audioquest Python and silver-based Signal Cable Silver Res Analog in line with copper and silver expectations. Silver might be more desirable to improve the amp's treble snap. Regardless of interconnects though, a high-end source is almost required for this amplifier, to prevent music from sounding like it's missing information.

Single Power has an undeniable new hit on their hands with the Square Wave. It has a sound that you'd expect from a premier vendor of some of the most exquisite tube amplifiers in the world, with a well-centered mid-range and powerful bass, and specifications high enough to drive almost any dynamic headphones in existence. At $999 it's also a superb value for what it's capable of doing. Single Power's imminent official release of the amp can't come soon enough.