Headphone listening, often referred to as "head-fi," has changed radically during the past decade. Headphones used to be devices that were employed only when speakers weren't practical. But for many younger audiophiles, headphone listening has developed into a unique and specialized category of audio enjoyment. Also, the fact that headphone sales is one of the few product categories that's been showing any growth over the past couple of years has gotten every audio manufacturer's attention. By the time the 2015 CES is over, the question may be not who makes headphones and headphone amplifiers, but who doesn't.
Woo Audio is one audio manufacturer that specializes in products for the Head-fi market. Since 2004 Woo has been making headphone amplification products. Currently Woo has 13 different amplification devices ranging in price from the $499 WEE electrostatic headphone converter to the $15,900 WA234 mono-block amplifiers. One of the most exciting and affordable amplifiers is the WA7 Fireflies single-ended tube headphone amplifier with a built-in USB DAC. Available with either a solid-state power supply for $999-$1,199 or with a tube-based power supply for $1,398-$1,598, the WA7 Fireflies headphone amplifier offers headphone aficionados a tube-based sonic perspective for top-echelon headphones.
The WA7 Fireflies headphone amplifier offers a unique cosmetic and technical approach to headphone amplifier design. Physically the WA7 is a smallish cube measuring around five inches (it varies slightly depending on whether you measure the width, height, or depth). The lower two-thirds of the cube is the WA7's all-aluminum chassis that features no visible screw holes, while the top third is a solid piece of "high-clarity" glass with two holes in the top. The holes are for the two 6C45 tubes that Woo employs in a Class-A, single-ended topology with transformer-coupled outputs. The front of the WA7 has a single, large, centrally located volume knob, as well as two headphone output connections: one for standard quarter-inch headphones and the other for smaller mini-stereo headphone connections.
The back of the WA7 has one pair of RCA single-ended analog inputs, as well as a USB digital input. (A second version of the WA7, called the WA7d, adds a digital Toslink input and costs an additional $200.) The basic WA7's back-panel controls include a three-way input/output selection switch for choosing RCA analog input, USB digital input, or RCA output of the USB D/A conversion. (The WA7d does not offer the USB D/A conversion or RCA output option; rather, the three-way switch lets you choose between the three input options.) The rear panel also has two switches: one for on/off and one that lets you choose either high or low impedance for the headphone outputs. The last item on the WA7's rear panel is the five-pin DC power connection.
Woo makes two power supplies for the WA7 headphone amplifier. The solid-state linear power supply that is furnished with the base model WA7 is a small, black metal box with a five-pin DC connection on one end and a standard IEC AC connection on the other. The WA7tp tube power supply costs $399 more when purchased with the WA7 (it costs $649 if purchased as an add-on) and looks very much like the WA7's main chassis, with a similar clear glass top. The tube power supply uses a pair of fairly common 12AU7 tubes that can be swapped out for other brands if owners wish to try some alternatives. The included Sovtek 6C45 tubes that are used in the WA7 are also swappable; Woo offers one alternative on its site: a pair of Electro Harmonix 6C45 tubes for $100 (the stock tube replacements cost $55 per pair). Controls on the tube power supply consist of a single large on/off button on the front.
The WA7 utilizes a single-ended Class A circuit topology with transformer-coupled outputs. With no semi-conductors anywhere in the signal path and an open-load circuit protection so that amplifier is protected when no headphones are plugged into it, the WA7 employs a purist topology that still allows for the foibles of its human operators. Other technical highlights of the WA7 include a pair of handmade nickel alloy core output transformers and multi-layer "military-grade" PCB boards.
The digital circuitry inside the WA7 features an asynchronous USB 2.0 digital interface. The digital converter supports sample rates as high as 32/192 via either digital connection. Woo has developed its own dedicated USB driver for Windows 7/8, and for Apple OS X 10.6.4 the WA7 is supported natively. The WA7 also supports Apple iOS devices with an Apple Lightning to USB adapter and Android devices with an Android OTG USB adapter. At the current time, the WA7 does not support any DSD formats.
The WA7 spent most of its time in my system functioning as an analog headphone amplifier, connected via its analog inputs (although I also tested the USB input). Using the WA7 was a simple matter of turning on my USB DAC, un-sleeping my MacPro desktop computer, switching on the WA7, opening up a music playback app, waiting about 20 seconds for the WA7's tubes to warm up, pushing play in the app, and then listening to music.
During its time with me, the Woo Audio WA7 operated without any glitches or bugs. The only time I had any performance issues with the WA7 was when I accidentally knocked the protective glass tube cover with my knee and it pushed the tubes sideways, which unseated the tubes slightly and led to some audible crackling. Reseating the tubes eliminated the crackling. My advice: don't do that.
The most important question that any prospective WA7 buyer will have is the inevitable, "Will the WA-7 drive my difficult-to-drive headphones?" or "Will the WA-7 work with my highly sensitive in-ear monitors?" Although I don't own every difficult-to-drive headphone or sensitive in-ear monitor on earth, I do own enough to give most headphone amplifiers a thorough workout. With my least efficient headphones, which included the new 90-dB 50-ohm HiFIMan HE-560 planar headphones and new 90-dB Mr. Speakers Alpha Prime headphones, the WA7 had adequate drive to push either headphone past my maximum-volume-level comfort zone even with my own live concert recordings, which I typically record at a lower level than commercial recordings to allow for their extremely wide dynamic range.
With highly sensitive in-ear monitors, the WA7 was quiet enough so that the 115-dB Westone ES-5 and Jerry Harvey Audio Roxanne (also 115-dB efficiency) custom in-ears had no background hums or buzzes. Even with the volume knob turned up to the max, there was nothing but inky blackness. With efficient headphones and in-ears, the only issue was that the volume knob needs only be turned about a quarter of its full rotation to achieve near-deafening volumes; so, if you go from a pair of inefficient cans to hyper-sensitive in-ears, you should turn down the volume on the WA7 to prevent the first notes from over-driving your in-ears or, worse yet, deafening you.
If you intend to use the WA7 as your DAC for USB sources and have any DSD material, you will be disappointed to learn that the WA7's digital section does not support DSD-formatted digital music files. To play DSD material through the WA7, you will need a separate DAC that does support DSD to feed the WA7's analog inputs.
The overall build quality of the Woo Audio WA7 is excellent throughout. I was especially impressed by the solid-glass tube protector, which not only looks deluxe but also serves as a most efficient heat dissipation device. The WA7 volume knob turns smoothly without any sideways wiggle; and instead of the usual "audio taper" where the first quarter turn has a massive volume increase and then the amount of gain diminishes, the gain increase with the WA7 is gradual and even throughout its volume range.
The worst ergonomic nit I could find to pick on the WA7 is the location of the input switch. Putting such a small three-way switch on the back almost guarantees that it will not be easy to use. It's placement infers that the WA7's designers didn't expect users to regularly employ multiple inputs; so, if your plans require that your headphone amp/DAC will be often switched from analog to digital, be aware that you're going to be doing a lot of reaching around and hunting for that little three-way switch.
Click over to Page Two for Sonic Impressions, High Points, Low Points, Comparison & Competition, and Conclusion...
Comparing the sound of the WA7 coupled to its tube supply with any solid-state headphone amplifier, regardless of price, will almost immediately make a listener aware of fundamental sonic differences between the two technologies. The standard cliché about tube sound revolves around "tube warmth" and the euphonic harmonic colorations that tube implementations are known to produce in many components. But if you suspect that the Woo Audio WA7 is merely another sweet-sounding euphonic sonic Band-Aid that will make your headphones more "musical," you would be wrong. No, what you get from the WA7 are harmonically neutral yet dimensionally mind-blowing sonics. If you think that headphones can't produce a solid, three-dimensional image with outstanding separation, you've obviously never heard a pair of good headphones tethered to the Woo Audio WA7. Also, if the tube headphone amps you've experienced have always seemed too sweet, dynamically underwhelming, and lacking the ability to retain low-level detail, the WA7's sonic abilities will come as a welcome surprise.
Compared with the best solid-state headphone amplifiers I've used, the WA7 delivers a greater sense of three dimensionality on every musical source, whether it's the latest mixes from Fatboy Slim via TIDAL streaming or my own live concert DSD 128X recordings. Not only was the soundstage larger than the Oppo HA-1's headphone amplifier section, but there was also better separation between instruments and a greater sense of where each instrument and vocalist was specifically located within the soundstage. Low-level detail, which is often obscured by tube noise and hum on lesser designs, was as well defined by the WA7 as it is with some excellent solid-state amplifiers, such as the Bryston BHA-1.
Although the WA7 isn't overly warm harmonically, it can still help "warm up" dry analytical headphones like the classic AKG K-701 (Austria-made original version). Even listening to electronic dance music from the Crystal Method via TIDAL, my old AKG K-701s sounded more harmonically lush and had some serious kick on the synthesized bass. The WA7's combination of low-frequency weight coupled with superior bass definition below 200 Hz makes even the AKG K-701 sound bouncy and beefy. Couple the WA7 to a more bass-centric earphone like the Jerry Harvey Roxanne custom in-ear monitors, and you'll be surprised by the amount of clean, controlled, and well-defined bass you'll experience. The WA7 and Roxannes combo even re-created that "puff of air" pressurization around the bass's leading edge that I've previously associated only with room-based system's low-frequency response.
The WA7's treble response, while not as noteworthy as its bass, was respectable without any noticeable loss of air, top-end extension, or harmonic control during dynamic peaks. While not quite as forward as I'm used to hearing from solid-state designs, the lower treble was still well defined, but not quite as in-your-face on aggressively mixed music. For music lovers whose tastes lean toward loud, occasionally rude music, the WA7s treble response may be just the sonic bromide needed to make the barely listenable more palatable.
When you ask a tube fancier what it is about "the tube sound" that they find most appealing, the majority will answer with one word: midrange. The WA7 won't disappoint any tube connoisseur. The mid frequencies have that particular organic rightness that sounds more like a live microphone feed from a top-flight microphone preamp, such as my Grace Lunatec V3. Like the V3, the WA7's midrange character doesn't change due to high volumes or sudden transients. Zero-dB levels were as detailed, involving, and without stress as -30dB. I hesitate to call the WA7's midrange character "warm" because that implies some variations from a neutral harmonic balance, which is not the case, but the WA7's tube-based single-ended circuitry does a superb job of eliminating glare and the additive colorations that make music sound harder and less natural.
Dynamic control and contrast from loud to soft was well controlled by the WA7. Some headphone amplifiers/headphone combinations seem to over-emphasize dynamic contrasts. Occasionally on my own very wide-dynamic-range recordings (often having more than 50 dB between the quietest and loudest passages), it's difficult to find the right volume level with some headphone amplifiers so that I don't need to gain-ride during the peaks to save my ears. But with the WA7 I found the "right" levels easily and only rarely felt the need to lower or readjust them. Sure, some headphones, such as the Audeze LCD-2 bamboos, still needed some volume adjustments due to their wide dynamic capabilities; but, with most headphones, including some sensitive in-ears such as the Westone ES-5, the combination with the WA7 produced wide dynamics without any need for gain adjustments during peaks.
• The Woo WA7 sounds exceptionally good compared to other headphone amp/DAC products in its class.
• The WA7 runs cool for a Class-A amp design, which are known to be able to often cook an egg.
• The WA7 works successfully with a wide variety of headphones.
• The WA7 does not offer a balanced headphone connection.
• The WA7's digital section does not support DSD formats.
• The WA7 does not offer preamp features.
• The input selector switch is on the back of the WA7.
Comparison and Competition
For much of the review period, I had the Woo Audio WA7 Fireflies headphone amplifier tethered to the analog outputs of one of its primary competitors, the $1,200 Oppo HA-1 DAC/pre/headphone amplifier. The Oppo is a more full-featured component, with analog preamp and DSD DAC options, but its solid-state headphone amplifier is no match for the Woo's tube-based Class A headphone amp. The WA7 had more spatial information with a greater sense of depth, a larger soundstage, and a more organic and natural overall sonic character compared with the HA-1's headphone amplifier.
Both Burson's $1,500 Conductor DAC/headphone amplifier and Lehmann Audio's $1,399 Blackface USB DAC/headphone amplifier are more expensive than the Woo Audio WA7 and have similar feature sets, but both are solid-state rather than tube-based amplifier designs. If you want a tube-based single-ended headphone amplifier in the $1,000 to $1,500 price range, the Woo Audio WA7 is the only game in town. There are quite a few inexpensive headphone amplifiers that employ tubes on the market, but none of the lower-priced amps have similar circuit topologies or a similarly high level of construction quality. If you want to spend substantially more money and move up to the price ladder, the Red Wine Audio Isabella ($4,500 without DAC), the $3,500 Leben CS300X, and the $2,950 Cavelli Liquid Glass are all well-regarded, premium-priced tube-based headphone amplifier options.
Since I've connected the Woo Audio WA7 Fireflies headphone amplifier to my desktop audio system, I've found myself spending more time listening to my headphone collection than any time in the past. There is something so right about the WA7's overall sonic picture, so involving, that it's hard not to devote upwards of several hours tethered to it. While you can't expect one headphone amplifier to be the best choice for all varieties of headphones, from sensitive in-ears to power-hungry planar designs, the WA7 comes closer to being an all-around, all-purpose headphone amplifier than any similarly featured or priced headphone amp I've experienced.
If you've always enjoyed headphone listening but felt that headphones can't or at least did not deliver the level of imaging specificity and three-dimensionality purveyed by the best loudspeakers, you owe yourself some time with the WA7 tethered to the best headphones in your collection (you do own more than one pair of headphones, don't you?). Put on a recording you know well and prepare to be delighted by the WA7's ability to retain and illuminate the low-level information and depth cues buried deep in the mix.
Given the sheer number of new headphone amplifiers on the horizon, with many more promised for the 2015 CES, I can't imagine that the Woo Audio WA7 Fireflies headphone amplifier won't see some stiff competition in the upcoming year. But as of right now, the WA7 is the best-sounding, best-looking, and most outstanding headphone amplifier I've heard priced under $1,600. Give it a listen, but I warn you: after a serious listen, it may be very hard to take off your headphones and leave a retail establishment without the Woo Audio WA7 Fireflies tucked under your arm.