Integrated Amp Reviews

When I applied for this fabulist audio-preacher gig, John Atkinson protested, "But Herb, aren't you a triode-horn guy?"

"No, that was decades ago! Today I'm still a bit of a Brit-fi guy, but my mind remains wide open."

However: As a professional reviewer, I am biased toward affordable, lovingly engineered audio creations made by family businesses with traditional artisanal values. I enjoy solid-state as much as tubes—often more!

Full disclosure: Decades ago, I and some other broke, dodgy friends of Walt Bender (Audiomart, footnote 1) and Joe Roberts (Sound Practices, footnote 2) began building directly heated, single-ended-triode amplifiers, mainly in homage to Western Electric's 91A mono, single-300B amplifier and all its WECO Mirrophonic siblings, which many of us had owned and all had deeply admired. We thought these direct-heated SET designs spoke in a less mechanical, more human voice than did their solid-state or EL34/KT88 tube counterparts.

My Japanese friends called these 1930s Western Electric tube creations "talkie amps," for the millions of movie houses, telephones, radios, and PA systems they were designed to power. During the Great Depression, "speech intelligibility" was of immense strategic, economic, and political importance. In America, audio engineering was to the 1930s what the A-bomb was to the 1940s, what NASA was to the 1960s, what railroads were to the 1830s. Just imagine, the world's best minds and $$, all focused on accurate, "mirror-like" audio reproduction.

We all thought the 300A/B triode, created in 1937, was the peak of talkie tube engineering. Even now, 80 years later, the 300B remains unchallenged in the completeness and succinctness of its reproduction of the human voice. (Maybe the 6L6 beam power tube, created in 1936 specifically for push-pull, high-powered, voice reproduction, comes close.) We Walt-and-Joe guys judged our 300B designs on their ability to deliver vocal and instrumental clarity in support of something more intimate and poetic than we were hearing from the high-end amplifiers of the 1980s. We were seeking a new audio humanism.

No horns required
I remember a late night, long ago. My phone rang. I picked up, and a drunk-sounding male voice shouted into my ear, "There are no transducers!" and hung up. He called again. "There are no fucking loudspeakers!" When the phone rang a third time, I demanded, "Who is this?"

The mad caller sounded sheepish. "It's me—Dick."

Dick was Richard Sequerra, of Marantz 10B, Sequerra Model 1 FM tuner, and Pyramid Met 7 speaker fame. He'd just read my story "Flesh and Blood" in the Winter 1994/95 issue of Sound Practices, and had called to remind me of the paucity of suitable loudspeakers to mate with my 5W, low-damping-factor, SET amplifier design. Of course, Dick was right. Even now, 25 years later, there are only a few audiophile-grade speakers that will work properly with SETs—but there are scores of voice-articulating headphones that thrive on precisely the kind of simple SET amps I and those Sound Practices picaroons were striving to contrive. Nowadays, there are more high-sensitivity, benign-impedance "transducers" (aka headphones) than there are high-quality, directly heated triode amplifiers to drive them.

To remedy this disparity, in 2004, Jack Wu and his father, Wei-fu Wu, founded Woo Audio. Their goal: to cater to the growing cult of headphone connoisseurs who understand what simple, directly heated SET amplifiers can bring to the high-end headspace experience. Wei-fu designs all Woo amps—Jack and Wei-fu remind me of all those idealistic guys sniffing solder fumes late into the night. Because the new, rapidly evolving world of high-end headphones picks up where that old triode-amplifier cult left off.

Not a board in a box
There are two ways to recognize a headphone amp: 1) It doesn't come with a remote-control handset. 2) Its power ratings are specified in milliwatts, as are the Woo Audio WA5's: 500mW into 32 ohms, 750mW into 60 ohms, 8000mW into 120 ohms, 6000mW into 300 ohms, 4000mW into 600 ohms (and 8–10W into 8 ohms, footnote 3).

The WA5 is built on two slim, narrow-but-hefty chassis made from injection-molded, CNC-milled, brushed aluminum, each measuring 9" wide by 8.5" high by 17" deep. It's entirely hand-wired, point to point, and dual-mono from stem to stern. But instead of each assembly being a standalone mono amp, in the WA5 the audio circuitry is isolated from the power supply, which features two large, potted power transformers and a matched pair (one per channel) of Psvane Hi-Fi Series 5U4G/274B full-wave rectifier tubes. The power supply weighs 40 lbs, the WA5 amplifier itself 35 lbs.

The audio circuitry is connected to the power supply via a 12" umbilical terminated in connectors from Amphenol Corp. Besides the rugged Amphenol socket, the rear panel of the amp has left and right speaker binding posts and three line-level inputs: two pairs on RCA jacks, one on an XLR jack. The audio deck has: two output transformers designed in-house; a matched pair of NOS GE or RCA 6SN7 dual-triode tubes; and a matched pair of Psvane Hi-Fi Series 300B tubes.

The amplifier's front panel is unique in several ways. In addition to the ¼" TRS and XLR headphone jacks are five controls: a three-position power-output switch that selects among Spk (loudspeakers only), or, for headphones only, HP Power Hi (8W) and Lo (1.5W). A tiny toggle switch to its right matches the amplifier to headphones of high (H) or low (L) impedance. To the right of the central volume control is a tiny Level toggle that lets the user choose between high (H) or low (L) gain for the amplifier. The knob on the far right is used to select the input.

The WA5's uniqueness and flexibility might best be explained by another story: It's late, I've been playing loud reggae and zydeco all day, and I want to switch from speakers to headphones. So I power off the WA5, then switch the left knob from Spk to HP Power Lo, because I don't want to put the WA5's full 10W into my sensitive and beloved Audeze LCD-X 'phones. I turn the power on but make sure the volume is down; now I can jack in the headphones. Because the LCD-Xes are specced at 32 ohms, I select L on the Impedance toggle and Lo on the Level toggle (with the LCD-Xes, Hi Level also works well). As I turn the volume knob clockwise, baritone Dietrich Fischer-Dieskau begins to sing inside my skull.

I think, I bet Dietrich would sound even better with the Audeze LCD-4s. So I turn the volume down and, because the LCD-4s are 15dB less sensitive and of much higher impedance than the LCD-Xes, I set the impedance to Hi and the Level to H—which allows the volume control to stay in that nice range of 9 o'clock to about noon with both Audeze models. And I can still set the power to Hi if I feel that sounds better.

After only a short listen, I realize: The WA5 amplifier is not an obsolete technology adopted by Woo Audio for euphonic, nostalgic, or marketing reasons—it's a Darwinian step toward a new renaissance of audio humanism in which headphones will become the transducers of choice for a select breed of refined audio connoisseurs.

Jack and Will
The beauty of directly heated SET amplifiers is not only their obvious air and gentle grace, but also their way of showing the listener a complete range of timbral shadings and minute instrumental textures that most other amps gloss over. Listen to some mellow harmonica through a high-quality, directly heated SET to realize how complexly saturated and explosively colored the sound of that simple free-reed instrument can be.

A vivid reminder of that truth was Will Scarlett's harmonica work on Hot Tuna's eponymous first album, recorded live in 1969 at the New Orleans House, in Berkeley, California (CD, RCA 3864-2-R). "New Song (for the Morning)" begins with Jorma Kaukonen building up a quiet but intense groove on acoustic guitar; then Scarlett softly joins him, so tenderly and tentatively, but LSD-colorful and mighty in the way it picks you up and pulls you along. By the time Jack Casady's electric bass joins the momentum and grounds the meditation, you've forgotten your name. The Woo WA5 made my DeVore Orangutan O/93 loudspeakers sound more wine-drunk and kung-fu supple than ever before.

---

Footnote 1: Audiomart was the Xeroxed ancestor of today's Audiogon.

Footnote 2: See www.timebanditaudio.com/300b/WE91A.pdf.

Footnote 3: There are conflicting figures published for the WA5's specified maximum power. The WA5's manual states 10W into 8 ohms, so that is what we have quoted throughout this review, though it turned out that that figure was measured with EMB 300B XLSes, not the Psvane 300Bs.—Ed.