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January 2015
Enjoy the Music.com Review Magazine
Woo Audio WA7d Fireflies Vacuum Tube Headphone Amplifier With DAC
I fell in love.
Review By Tom Lyle


Woo Audio WA7d Fireflies Vacuum Tube Headphone Amplifier With DAC

 As music lovers, headphones have always been a part of our lives. Whether this was for privacy concerns, such as listening to music late at night, or for use when traveling on a short commute or a long trip on an airplane, headphones are our escape and salvation. The Woo Audio WA7d Fireflies 192kHz/32-bit DAC tube headphone amplifier answers the call to duty. For those in the music "industry", whether amateur or profession, many of us have had headphones wrapped around our heads for hours at a time in the recording studio. Headphone amplifiers for my personal use were never part of the equation until about fifteen years ago, when I purchased a set of Sennheiser HD-600 headphones. Before that time the best headphones I ever owned were a set of Grado Lab's inexpensive models purchased when Grado initially got into the headphone business in the very early 1990s. But enough about me. At least for now.

These days' headphone amps are ubiquitous, even for many who don't consider themselves audiophiles are even getting into the game and who can blame them? A good set of headphones deserves a good headphone amplifier. A great set of headphones deserves a great headphone amplifier, and as we will learn in this review a great headphone amplifier doesn't have to cost an arm and a leg. But I suppose everything is relative, and with the Woo Audio WA7d Fireflies headphone amplifier with 32-bit/192kHz DAC we will also learn that one can enjoy a great headphone amplifier and upgrade it into a superb headphone amplifier at a later date... or from the get go.


New York
Wie Wu and his two sons started Woo Audio in the outer boroughs of New York about ten years ago. They still design and built their components in the same town where they first set up shop. Their "mission" was to "fulfill the desires of the most demanding audiophiles and delight both listener's eye and ear". The listener's eye? Absolutely -- one look at the WA7d and its matching WA7tp power supply will undoubtedly make clear that Woo Audio put lots of thought into the appearance of the WA7d. Without fail, visitors to my listening room will notice the Woo Audio components, and most will wonder aloud "what the heck are those things?" The WA7d is a 5" cube that is slightly taller than it is wide and deep, the top third of the component is solid "high clarity" glass with room for the tubes that protrude from the top of the aluminum chassis which is available in black or silver. I was sent a sample with a black finish, and regardless of the finish there are no visible screws on the cabinet -- which would surely ruin its sleek, modern exterior.


Woo Audio didn't skimp on quality of the internal components of the WA7d. Many are made by hand at Woo Audio's shop where it undergoes a series of "stress tests" before each is shipped. It is a "pure tube" design which uses two 6C45 vacuum tubes in a Class A single ended design and is transformer coupled. No semiconductors are used in the signal path, and the digital to analog converter section of the WA7d will decode signals up to a 192 kHz sample rate with a word length of 32 bits regardless of its digital input, including its USB 2.0 jack, and uses a CMedia 6631A USB-chip to accomplish this feat. Otherwise, the WA7d uses a Texas Instruments PCM5102A 32-bit DAC chip. There are a pair of nickel-alloy core output transformers, and the amplification circuit's power supply and uses multi-layered military-grade printed circuit boards, and a "custom built" toroidal power transformer. The power supply is remote-controlled, and is external to lessen the amount of noise introduced to the circuit. The WA7d can be used with its standard solid-state power supply, or one can upgraded to the WA7d's matching WA7tp (tube power supply), both of which connect to the WA7d (amp) via an umbilical a custom DC cord. That the upgraded WA7tp power supply adds less than $400 to the price of the WA7d (if added at the time of purchase) is quite reasonable. Also, for $100 one can upgrade the WA7's Sovtek tubes to a matched pair of Electro-Harmonix 6C45s with gold-plated contact pins, I've done a bit of research on these tubes, and Woo Audio is offering them as an upgrade at a very reasonable price.

Woo Audio WA7d Fireflies Tube Headphone Amplifier With DAC

A look at the rear panel of the WA7d speaks volumes about not only the capabilities of this headphone amp/DAC, but its flexibility. On the rear panel are two digital inputs -- USB 2.0 and optical TosLink -- and an analog output via a pair of RCAs, which enables the WA7d to be used as a standalone DAC in one's system. These same pair of RCAs allow the user to connect an analog source to the WA7d such as a CD player or phono preamp, which essentially turns the WA7d into a an outboard DAC/preamp so one can connect three sources, one analog and two digital. There is a small toggle switch which chooses between high and low headphone impedance, and therefore the WA7d can accept headphones with an impedance range of 8 to 600 Ohms, which pretty much means any one is likely to use. There is a power output jack for connection of the power supply's umbilical DC cord, and another small toggle to power on and off the unit. Lastly, is a three-way multi-switch to choose between the optical, USB, or RCA input/output. When using the optional matching WA7tp tube power supply one can turn on both unit's power from its large front control push-button. On the WA7d's front panel are two headphone outputs, a 1/8" mini jack for use with in-ear monitors and the like, and a standard 1⁄4" headphone output for full-sized headphones.

The WA7d that Woo Audio sent me for review came with the matched set of the Electro-Harmonix 6C45 tubes with gold-plated contacts. I was also sent the matching WA7tp tube power supply along with a standard power supply for the WA7d for comparison's sake. Plus, my review sample came with the Sovtek "stock" tubes so I could compare the two. Woo Audio was also nice enough to send me their HPS-R aluminum headphone stand finished in black to match the WA7d/WA7tp system. The stand's height is adjustable (11" to 14" high) and is made from aluminum and which makes for quite a nice looking setup. Next to my listening seat is a heavy duty end table where I placed the WA7d/ WA7tp system and the headphone stand with my headphones de jour suspended on its outstretched arm. The stand can be adjustable from 11" to 14" high, and is made from aluminum. With a o.75" thick bass, its rubber feet, and its substantial weight at about four pounds it isn't likely to slide off the table or wherever one parks it. Its base is only about six inches in diameter, so rather than one of those dummy-head type headphone stands it should take up less room since the headphones are hanging above the rest of the equipment on the desk or table. Nice.

The faceplate of WA7tp vacuum tube power supply looks practically identical to the WA7d, except what looks like the large volume control on the WA7d it is instead a heavy-duty on/off switch. Beside the absent of headphone outputs, WA7tp tube power supply looks practically identical to WA7d amp. What looks like a volume knob on WA7tp is actually a heavy-duty power ON/OFF button. Woo Audio recommends one use the power ON/OFF button on WA7tp tube power supply for daily use; leaving the main power ON/OFF flip switch on the back of WA7d amp in the ON position unless the system is not in use for an extended period of time (such as when you're on holiday). If one leaves the power to the WA7d in the on position, switching on the WA7tp will switch both units to the on position.

Of course the headphone jacks on the front of the WA7d are absent on the WA7tp, leaving just its sleek front panel. The two tubes used in the WA7tp power supply are a pair of omnipresent 12AU7s, so a tube swap might be hard to resist. For the review I left the Russian tubes in place for most of the review period, but near the end substituted a set of military-grade Sylvanias I had lying about. The sonic benefits were a subtle increase in all that makes a good power supply a good power supply; which was quite audible when using the best headphones I have in- house.


I spent most my time with the WA7d/tp pair with flipping between two sources, my main system and a mobile device. I love using my trusty 160GB iPod Classic when reviewing headphone amplifiers. I can bring transport the headphone amp anywhere in the house I wish, and have high-end quality sound with me wherever and whenever whether I'm relaxing (or not) and listening to music. I have the iPod on a Pure i20 dock which bypasses the mediocre DAC and audio section of the iPod, so I connect an optical cable between the i20 and the WA7d's TosLink input.

About 95% of the files on my iPod are uncompressed 44.1k/16-bit AIFF ("CD quality") files, so when listening to them there was little difference in the sound of these files when sourced straight from my server or the iPod. Bypassing the DAC of the WA7d and running straight, high-resolution files from my main system through the Auralic Vega I reviewed in July is another story, this had the best sound quality by far, but only because this DAC costs roughly four times as much as the WA7d. I also used the WA7d as an outboard DAC. Since the WA7d has two digital inputs plus an analog input and output, if one doesn't mind switching some cables around every once in a while the WA7d could be used as a killer centerpiece of a very nice desktop system.

Headphones used for this review included the fifteen year old Sennheiser HD600 that I spoke of earlier, recently restored to minty-fresh condition by Sennheiser USA. Most of the time, though, if I really wanted to hear what the Woo Audio gear was made of using the top-of-the-line Grado PS1000 headphones was the way to go. I also had on hand the excellent Oppo PM-1 planar magnetic headphones reviewed in August, and the modest but certainly fine sounding Grado SR80e which is a relatively new upgrade of their affordable yet impressive SR80i.


At first I let the WA7d settle in for a spell with its standard solid-state power supply connected, but after listening to this setup for a while I couldn't resist hearing the Full Monty the WA7d paired with the matching WA7tp tube power supply and plus the Electro-Harmonix 6C45s with gold-plated contact pin tubes for the amp. Was there an improvement in the sound quality of the WA7d with each upgrade? You bet there was! But the sound of the WA7d was not inherently changed I know that sounds paradoxical, but the sonic traits that make the WA7d so attractive in the first place were just amplified when adding these upgrades. Those who purchase a basic WA7d can rest easy, they are not being deprived. But the upgrade to the tubes with gold plated gold-pins upgrade tubes and especially the upgrade to the WA7tp tube power supply are just that upgrades. The changes to the sound quality will not change one's life. And it makes sense that these changes were more noticeable with higher quality headphones than with an average pair. Therefore, to really truly appreciate what the WA7d capabilities are, I would recommend upgrading one's headphone as well as purchasing a fully loaded an upgraded WA7d system. But I'm getting ahead of myself.

I'm convinced that vacuum tubes are largely responsible for the excellent sound quality of the WA7d. It is one thing to use a tube amplifier to power a pair of speakers in one's listening room, but since it takes much less power to drive a set of headphones the design of the WA7d will not exacerbate any sonic downsides of using tubes. Although there is no denying, and I can speak of from experience, that there is a certain "magic" that many tube amp fans speak of. Still, those stereotypes of old are sometimes difficult to shake. Some might assume that tube amps reproduce bass-shy, midrange forward, and rolled-off treble music through a pair of speakers. Yes, at one point in time tube amps had these characterizes, especially if matched with the wrong type of speakers. The WA7d is a Class A single-ended tube design, yet one is not going to hear a sound that resembles this at all. Tube amps have come a long way, and a quick glance at the WA7d's specifications is evidence of this.

With the WA7d a headphone aficionado can take advantage of the inherent beauty of the sound of tubes sound without the downsides. Still, there is just enough of that old tube sound to use it to one's advantage. Through the WA7 the midrange was the star of the show vocals, whether male or female, had a lifelike, reach-out-and-touch presence. Even though the signal is very transparent through the WA7d and does not homogenize the sound of difference recordings, the treble of the WA7d, even when playing files of "vintage" CDs was reproduced with a natural sweetness that is as attractive as the real thing, but without any graininess or sibilance that can ruin a recording. The bass on the other hand is another story. Yes, it is "fatter" sounding than the typical solid-state headphone amplifier, yet through the WA7d it also goes as deep as the recording demands, is very pitch stable, and has enough transient edge to make its definition pleasing to all but the pickiest of listeners.

Woo Audio WA7d Fireflies Tube Headphone Amplifier With DAC

The "soundstage" rendered by the WA7d is also a pleasure to hear. Of course soundstage is in quotes because the soundfield generated by a set of headphones is nowhere like the soundstage that comes from a properly set-up pair of loudspeakers, and that is why in the ratings section of this review this section is not considered. But the "music-inside-the-skull" sound is often a complaint of many who dislike the headphone experience. My life-long involvement in the art of headphone listening has shown that this is usually the fault of the headphone and the headphone amplifier, and not the fault of the medium. Inexpensive headphones cannot generate sounds that leave the confines of the headphone's earpads, and usually congregate inside one's skull between these two earpads, at best. Listening with a poor headphone amplifier will result in the same effect, and the same thing will happen when listening to headphones with no headphone amp at all. Listening to music with a good (or better) set of headphones, through a good (or better) headphone amplifier will relieve one of these unsettling effects to large degree.

Sure, there are going to be some sounds that are panned to center that still end up sounding like they are geminating from one's cerebral cortex, but one will be surprised when listening to a record recorded in mono, such as the new Beatles In Mono set, that sounds are not confined to a single area even though the program material is just one channel. The amount separation of voices and instruments that occurs is quite amazing through the WA7d. Separation of instruments and voices is a hallmark of a good high-end system, regardless of whether it is coming from a pair of speakers or from one's headphones, or its coming from a tube or solid-state powered component. Admittedly, even though there is a separation of instruments that result from listening to just about all the material that comes through the WA7d, it is not by a long shot a representation of the original event's positioning of the instruments. It is what it is. But it is one of the great things about listening to a set of high-quality headphones such as the Grado PS-1000 and a great headphone amp such as the Woo Audio WA7d Fireflies: The music isn't just lumped together in an amorphous blob, but the voices and instruments are separated so one can hear the musicians, producers, and engineer's original artistic intent.

Another trait of the WA7d is the ability to reproduce real instruments and voices with a very lifelike sound. This is one of the greatest characteristics of the WA7d, so when playing orchestral and jazz my headphone setup became a sonic time machine, if you will. When I spun my LP of Le Sacre Du Printemps recorded by the Leonard Bernstein's New York Philharmonic in 1959, I was "there". In part due to the excellent Grado PS1000, it brought me to another place and time. With this record I bypassed the WA7d's digital section and listened through the analog input of the WA7d. Wow! It is a great recording, made even greater due to the beefy sound quality of the WA7d. The right side of the orchestra (the double basses and to a large extent the cellos), the percussion, and tympani rattled my brain. In a good way. I could see the string section in my mind's eye, each playing as a whole but separate, the sound as smooth as silk via this re-issue on 180 gram vinyl. Each section of the orchestra occupied a different space in the soundfield, yet there was no discontinuity between them, ever. Although the "soundstage" was nowhere near realistically laid-out and scaled, but still, the separation of each section let me bathe in Stravinsky's complex masterpiece.

Those who are aware of my tastes in music (or lack thereof) know that I hardly spend most of my time listening to music that features real instruments and voices recorded in a real space. So, what about music that isn't "real"? That is a subject where I'm tempted to waste lots of space with questions such as "is a recording of a microphone placed in front of a guitar amplifier "real"? Is a multi-track recording of a drumset with the microphones inches away from some of the drums "real"? Is a recording studio where a band is set up to record their basic-tracks a "real" space?" I think we've come far enough, and the audiophile demographic has changed enough, to answer these questions as "yes".

Woo Audio WA7d Fireflies Tube Headphone Amplifier With DAC

Therefore, the WA7d can reproduce the 2013 self-titled debut album by Ensemble Pearl, a band comprised of members of Sunn O))), Boris, Jesse Sykes & The Sweet Hereafter and Earth with as much "realness" as one who is listening to the band  in the studio (wearing earplugs, I hope) as they record the album, and then attend the mixing sessions and hear it playing back through headphones with the rest of the band, engineer and producer -- although when we're listening it's through a much better headphone amplifier, with (hopefully) much better headphones then they are. When listening to this album the heavy sounds that emanate from my PS-1000 headphones connected the WA7d/WA7tp set up are mesmerizing. The guitars don't sound like "normal" electric guitars playing through a loaded pedal board connected to stacks of their amplifiers, but droning ghosts (a description I read in a listener review). The cymbals on the drumset sound as if they are sometimes being struck by something other than drumsticks, and the added percussion isn't the off-the-shelf variety, that's for sure. But I hear the end result of what they were setting out to accomplish, and because of the WA7d setup can hear everything that was recorded not only because of the stupendous amount of separation between the individual sounds, but the "realistic" sound that the WA7d/tp is reproducing of each instruments. Screw the "I know what a real violin sounds like so I can judge this component" mind-set. This is some heavy stuff, and it is obvious why this album made my and many other's best of 2013 list, and it is made so, so enjoyable through the Woo Audio gear.


Are there any downsides to using the Woo Audio WA7d Fireflies? Sonically, I really can't think of any. Some, if they have been accustomed to the sound of solid-state headphone amplifiers might think that the WA7d is a bit rolled of in the treble, and has its transients are a bit slower to respond than "normal". Yet, I think if one did as I did, and A/B'd the WA7d with a good solid-state headphone amp to recordings of jazz or classical they'd realize that the WA7d sounds more true to life on much more material than they'd expect. This is made more obvious when the WA7d is compared directly to the sound of many solid-state units, which are, let's face it; sometimes take detail to the extreme. The only problem I had with the WA7d in normal day-to-day listening was that it has been designed withouta coax (S/PDIF) digital input. I asked Mike Liang at Woo Audio why it didn't have an RCA coax digital input and all he said was "optical TosLink is commonly found on CD/DVD players and portable music players like offerings from Astell&Kern". Fair enough. But that doesn't mean that I didn't miss having an RCA coax input, mostly because all my high-quality digital cables are 75-Ohm coax cables. There are excellent optical cables made by quite a few high-end cable companies, and I was even tempted to call MIT to see if I could borrow one of their AVT 2 Optical cables that cost less than $100. Before I knew it the review period was nearing its end, so I never was able to get my hands on one. The many generic optical cables I have all fit the jacks on both on my device and the WA7d rather poorly and were constantly being jogged lose. I assume a high-end cable would be terminated with better connectors on its ends.


The Woo Audio WA7d Fireflies headphone amplifier/DAC is a fantastic product. Not only does it sound fantastic, but it looks fantastic, with its cube-like appearance and its glass upper portion, no one who entered my listening room did so without making a comment regarding its looks. Although one comment was: "it would look much better if it was in my listening room". Even before the WA7d was fully broken in; I fell in love with its meaty and lifelike sound. One thing I did not mention in the body of my review was that this is a tube amp that doesn't have the background sound of most tube amps in that its background is completely silent plus, it never misbehaved not a click, hiss, pop, or anything that took away from the muscular sound that was being fed to my headphones. It is as versatile as it is functional, with two digital inputs and an analog input it can satisfy all but the most fastidious among us which means me. I've heard more than my share of headphone amps, and more than my share of headphone amps that have an onboard DAC, and if I had to choose which one I would have to put down my own money to buy the WA7d/WA7tp combination would be at the top of my list. Therefore, I recommend one consider purchasing a WA7d Fireflies, or better yet a WA7d Fireflies with its matching WA7tp tube power supply. I can imagine that many will thank me for this recommendation.



Sub-bass (10Hz - 60Hz)

Mid-bass (80Hz - 200Hz)

Midrange (200Hz - 3,000Hz)

High Frequencies (3,000Hz On Up)



Inner Resolution


Fit And Finish

Self Noise

Value For The Money


Type: Vacuum tube stereo headphone amplifier with DAC
Frequency Response: 11 Hz to 27 kHz
Amplification: Single-ended Class A, transformer coupled
Tubes: Two 6C45 driver/power tubes
Power Supply: Linear external high-performance power supply (remote-controlled switching)
C-Media 6631A USB chip
Headphone Input impedance: 100 kOhms
Headphone Impedance: 8 to 600 Ohms
Sampling rate: Up to 32-bit at 192kHz
Inputs: Optical TosLink, asynchronous USB 2.0 and S/PDIF via RCA
Outputs: 1/4" and 1/8" headphone outputs
Maximum output: approximately 1 Watt @ 32 Ohms
Distortion: <0.03%
Signal To Noise Ratio: 95dB
DC Cord Length: 12" with WA7to tube power supply or 4' with solid state power supply
Dimensions: AMP: 4.8" x 4.8" x 5.1" (LxWxH)
Weight: Amp is 5.3 lbs., glass 3.3 lbs., WA7p PSU 2.8 lbs. and WA7tp 9.4 lbs.
Price: WA7d $1199 with standard power supply or $1598 with WA7tp tube power supply.
Add WA7tp power supply $398, add Electro-Harmonix 6C45 with gold plated pins $100


Company Information
Woo Audio
Phone: (872) 222-9667
Fax: (718) 764-6939
Email: info@wooaudio.com
Website: www.WooAudio.com














































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