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...