Art Dudley Listening

There's one thing I enjoy about single-ended triode amplifiers, although it isn't so much a characteristic of a good SET as the combination of a good SET amp and an appropriate loudspeaker: They load the room convincingly when the music demands it. It's a hard thing to describe, but I know it when I hear it, like open fifths or pornography. And it can be subtler and less bombastic than you might guess—like the first piano chord in Schubert's "Trout" Quintet: There's a manner in which the sound of that chord reacts with the room, any room, in a real performance, and a manner in which a reproduction of that performance tends not to. A good combination of SET amp and speaker is closer to the former than most everything else.

I'll come back to that later on. First I'll tell you about a single-ended amp that's as different as can be from the Viva Solista I wrote about last month—and that's a new single-ended transistor amp designed and built by Nelson Pass. You'll remember Pass from the January 2005 installment of this column: In recent years, the founder and president of Pass Labs has come up with a few new creative outlets for his more experimental tendencies, including Pass DIY (which specializes in supplying plans and parts for low-power solid-state amplifiers) and now First Watt, a "kitchen table" company set up to create commercial versions of certain Pass designs in limited editions. Limited but not overpriced: First Watt's first product, the F1 power amplifier, was downright cheap at $2000.

So it goes here, with the latest First Watt creation: a power amp called the F2, which seems poised to make a much greater impression than its price ($2500) would lead you to believe. Like its immediate predecessor—and nothing else I know of—the First Watt F2 is a transconductance power amplifier, a technology I wrote about in the January issue. To recap:

Most amplifiers are designed as voltage amplifiers. They start with a low-voltage AC signal at one end and produce a high-voltage version of (we hope) the same thing at the other. But owing to the electrical complexities within an amplifier-loudspeaker system—and especially the reactive elements in a passive crossover network—the relationship between voltage and current within the system as a whole is far from linear. Therein lies the problem: What the loudspeaker uses is not the same as what the amplifier provides. The answer, then, is to devise an amplifier that produces signal-modulated current: a current amplifier. That's what Nelson Pass has done with the F1 and F2.

A key difference between the F1 and F2 is that the latter uses a single-ended topology. In each channel, the music is amplified with only three active devices: a MOSFET configured as a constant-current source, a bipolar transistor used to regulate it, and another MOSFET used as a current-gain transistor. That's it. One gain stage and just 35 parts per channel, not counting the power supply—and that, too, is simple.

Compared with the F1, Nelson Pass went for a lower output impedance this time. By putting a trio of 47 ohm resistors in parallel with the F2's output section, he endowed it with a nominal impedance of just over 15 ohms (compared with the F1's 80 ohms), for a bit more electrical damping with low-mass, full-range drivers. If you don't think you need the damping, simply remove the resistors and you'll be left with a source impedance of about 700 ohms. But don't be surprised if you're also left with more bass overhang than you'd care to hear.

What to listen for in tuning the F2's bass with your speakers? Try XTC's "The Meeting Place," from Skylarking (LP, Geffen GHS 24117). For the second half of every verse (coinciding with the line "Whistle will blow..."), bassist Colin Moulding switches from a rhythmically complex line to simple whole notes; when you've got the bass damping right, you'll hear more nuance in the former—and you'll hear that, in the latter, the second note is a half step below the first. When the damping is poor, they tend to sound like the same note.

Now to the sound in my system. First, the F2, like the F1, was uncommonly quiet. Driving my very sensitive Lowther Medallions, there was no hissing or humming at all: It was impossible to tell whether or not this amp was switched on just by putting my ear up to one of the Lowther drivers—something that the Lowthers telegraph lustily with every other single-ended amp I've tried, including those with rectified filament voltages.

Second, the F2 did sound more like a single-ended tube amp than the F1. The F2 sounded meatier and more colorful, all the while preserving the F1's good way with pitches and rhythms, its ability to convey musical nuance, and its utterly remarkable sonic clarity.

Third, it did sound richly textured, like a single-ended amp of the tubular sort. I listened to János Starker's stripped-down and somewhat dry performance of Bruch's Kol Nidrei (LP, Mercury SR90303, which should be available as a Speakers Corner LP reissue by the time you read this): The plucked notes in the double basses toward the end of the piece each stood alone as a richly colored event in time, with remarkable weight, even through my Lowthers.

My Fi 2A3 stereo power amplifier remains a different-sounding thing, and while it and the F2 can deliver more or less the same degree of musical and emotional involvement, neither renders the other irrelevant by any means. The Fi works with the Lowther Medallions to load my medium-small room in a very pleasant manner (as do my Lamm ML2.1 SETs, with either the Lowthers or my largish Quads)—and that's a trick that the First Watt F2 never quite pulled off, for whatever reason. But that doesn't trouble me. The F2 wasn't designed to sound like a tube amp, single-ended or otherwise. It was simply intended to sound like what it is: a well-made, minimalist, low-power amplifier that just happens to have not a tube in sight.

The First Watt F2 is a one-weight fly rod, a mando-cello, a drawknife with a flexible blade—in other words, a fascinating tool with the potential to make you rethink everything you know, even if you don't wind up using it on every job (although you may). I dislike making windy pronouncements even more than reading them, but I can't resist saying: If this amplifier had been available 25 years ago, the high-end audio landscape would look quite different today.

A final note: A great bonus that comes with any Nelson Pass product is the quality of the accompanying manual. He's a gifted writer with a voice not unlike that of Kurt Vonnegut, Jr., and his many insights are as entertaining as they are true. My favorite: "Just as corn chips are an excuse to eat salsa, Lowthers provide a reason to build big back-loaded horns."

Trials at home
I devoted a portion of my April 2005 column to equipment-reviewing policies— Stereophile's and my own. I'd like to return to that subject for a moment, as I'm sure I'll do again from time to time.

Today I'm struggling with the whole idea of allowing manufacturers to visit me—to install, adjust, or simply take stock of the performance of their gear in my house. On the surface it seems reasonable enough: A magazine review can make or break a product (it can also have no effect whatsoever, but that's a topic for another day), and I shouldn't be surprised if the people involved in making and selling it want to ensure that the review sample is performing up to par. Besides, it's their property.