Woo Audio WA2 Single-Ended OTL Headphone Amplifier

The Amp High-Impedance Headphones Were Made For

If you own Sennheiser or Beyerdynamic headphones in the 300- to 600-ohm range, the Woo Audio WA2 was built for you. It is a Class-A, single-ended triode amp, all tube. The OTL layout, meaning output transformerless, is prized for one thing above all. It puts the tubes in direct touch with a headphone of high impedance.

In an OTL design, the output tubes drive your headphones directly. No output transformer sits in the signal path. That means one less component between the music and your ears. Two 6AS7/6080 power tubes and two 6922/6DJ8 driver tubes do the work. Dual 6CA4 rectifiers give each channel its own. There are no transistors anywhere in the circuit.

Frequency response runs a wide 8Hz to 100kHz (-3dB). Signal-to-noise ratio is 95dB, and distortion is 0.08% or less. Those are unusual figures for a tube amp at any price.

Hand-Built in New York, Point to Point

Like every Woo amp, the WA2 is hand-built in New York with point-to-point wiring instead of a circuit board. A 120-watt toroidal transformer feeds the power supply. Each channel gets its own filter choke and rectifier, which is a thorough layout at this price. An ALPS volume control and silver-plated wiring in Teflon round out the build.

The design is self-biasing. Swapping compatible tubes needs no measurements and no adjustments. The 6922 and 6AS7 families offer decades of production to explore. Four RCA inputs let your DAC, CD player, streamer and phono stage all stay plugged in at once.

The chassis is anodized aluminum. It measures 13 inches wide, 9 deep and 5.5 tall, and weighs 17 pounds. It takes global voltage from 100 to 220 volts, so it travels without a converter.

What Reviewers Are Saying

Linden Park of Enjoy the Music, in a July 2009 review, wrote that with the WA2 "the music sounded SET-liquid, silken, and ultimately guileless in expression," praised construction that "is impeccable," and called it "one of the most musically involving amplifiers" he'd heard and a "phenomenal value." Lieven of Headfonia, in a September 11, 2013 review, described a "clean top end that maintains extension without being bright, neutral smooth mids and wonderful tight bass," naming high-impedance dynamics like the Beyerdynamic T1 and Sennheiser HD 800 among its best matches.

Who Is This Amplifier Best For?

• You own high-impedance headphones — the OTL design is at its best with loads above 100 ohms, like classic Sennheiser and Beyerdynamic models.
• You want the purest tube signal path — no output transformer, no semiconductors, just tubes driving your headphones directly.
• You have multiple sources — four selectable RCA inputs keep your whole system connected.
• You want to explore tube rolling — self-biasing operation and widely available tube families make experimenting easy.
• Your headphones are low-impedance planars or sensitive IEMs — an OTL amp is the wrong tool for that load, and a transformer-coupled design from our headphone amplifiers will serve them far better.

How Does It Connect to Your System?

Connect up to four line-level sources, such as a DAC, CD player, or streamer, to the RCA inputs and select between them from the front panel. Headphones plug into the 1/4" jack.

Give the tubes a few minutes to warm up before serious listening, and leave open space above the amp for ventilation, since a Class-A OTL design runs warm by nature.

Pairs Well With

Building your system? Here are some great companions for the WA2:

• Headphones: High-impedance Sennheiser and Beyerdynamic models are classic matches — see our Headphones.
• DAC: A quality source makes the most of this amp — see our Digital-to-Analog Converters.
• Cables: Quality RCA interconnects complete the chain — see our Interconnect Cables.
• Audio Rack: Give it a stable, ventilated home — see our Audio Racks.

Frequently Asked Questions

What does OTL mean?

Output transformerless. The tubes drive your headphones directly with no output transformer in the path, a design prized for its immediacy with high-impedance headphones.

Which headphones work best with it?

High-impedance models, above 100 ohms, are the ideal match, such as 300-ohm Sennheisers and 600-ohm Beyerdynamics.

What tubes does it use?

Two 6AS7/6080 power tubes, two 6922/6DJ8 driver tubes, and two 6CA4 rectifiers. The self-biasing design makes tube swaps simple.

How many sources can I connect?

Four, via selectable RCA inputs.

Does it run hot?

Like all Class-A tube amps, it runs warm in normal use. Leave open space above it for ventilation.

What's the warranty?

Woo Audio covers the WA2 with a 3-year limited warranty.

See It in Action

The Source AV Design Group shows why the WA2 is a high-impedance headphone's best friend:

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Learn More

Want to learn more about headphone amplifiers, tube audio, or building a headphone system? Visit our Learning Center for guides, tips, and expert advice.