Listening #25
By Art Dudley   •   January, 2005 First Watt isn't a real company, and the F1 power amplifier isn't a real product. Consequently, this isn't a real review.

Nelson Pass, who founded the Threshold Corporation in the 1970s and continues to operate the successful Pass Labs, also likes to indulge his more radical side as an audio designer. For some years, his outlet for doing so was Pass DIY", a sort of an online think tank that encourages audio hobbyists to build and elaborate on recent Pass designs that are more niche than needful: ultra-simple and generally low-powered solid-state amplifiers. But the appeal of Pass DIY has been limited to experienced builders. And, in a commercial sense, they've limited themselves to selling the occasional printed-circuit board—for crazy-low prices, it must be said.

For people who want to hear what lies beyond the tired old megabuck megawatt audio experience yet who don't wish to spend their evenings reading schematics and testing MOSFETs, there is now First Watt, which Pass describes as "a kitchen-table company." If the purpose of Pass Labs is to make audio products that answer certain needs, the purpose of First Watt is to ask more questions.

Looked at another way: If Pass Labs were R.E.M., First Watt would be the Hindu Love Gods.

Current events
Given the name—inspired by Dick Olsher's famously correct observation that the most important watt out of a power amp is the first one—one might assume that Nelson Pass created First Watt so he could manufacture low-power amplifiers for hobbyists who, above all else, enjoy vinyl, tubes, and horn-loaded speakers. And that's partly true: First Watt's first product, the F1, is indeed a low-power amp. But it's a low-power current-source amp—and, as such, it's currently (haw) in a class of one, because everything else on the market is a voltage amplifier.

Not that there's anything wrong with that. God knows we've gotten along well with voltage amplifiers for some 80-odd years. But you have to bear in mind that a conventional loudspeaker is an AC motor, and that it thrives on current. (A phono cartridge is the same thing in reverse, of course: a prodigious current source.) When a conventional loudspeaker is presented with a music signal of continuously varying voltage—itself an electrical model of the input signal—and is thus stimulated to draw current, the speaker's electrical complexities cause it to draw a level of current that isn't always proportional to the voltage supplied. The amplifier's high-voltage output may accurately track the low-voltage input, but the loudspeaker's current draw does not—and so the acoustical output is not a faithful replica of the electrical input.

Electrical complexities? Many of those inhabit that relatively small amount of real estate we call the passive crossover—something that has evolved, reasonably enough, over the many years during which the world has been dominated by voltage amplifiers. Think, for example, how the designer of a crossover network tailors the amplitude response of various drivers to achieve a pleasing and uncolored whole: with combinations of high-pass and low-pass filters, each of which depends on output voltage in order to function at all. Filters work in part by selectively throwing away voltage, and it's a lucky break for them that the past 80 years' worth of amplifiers have gone along with the game.

Output current, on the other hand, virtually ignores such passive filters—or, to be precise, ignores the filter elements that are in series with the amplifier's output. Current also more or less ignores the impedance and inductance of the voice-coil, which we hope and presume were taken into account by the loudspeaker designer.

Thus, if a current source is used to drive a loudspeaker with a passive crossover network, and/or a loudspeaker with drastically varying reactance characteristics related to cabinet loading, voice-coil overheating, or whatever else—which is to say, 99% of all loudspeakers on Planet Earth—the resulting sound will be drastically different from that achieved with a voltage amplifier, said difference being safely relegated to the file folder labeled SUCKY.

And what's left in the 1% of loudspeakers not affected by that observation? Pretty much just Lowthers and Fostexes. Now you know why Nelson Pass sent the First Watt F1 amp to me instead of, say, Mikey Fremer.

Five to one with a man on First
Nelson Pass says the idea for the F1 came about when he and his colleague Kent English were experimenting with ribbon tweeters—simple drivers that are simply impossible for a voltage amplifier to drive without an in-line matching transformer, owing to the ribbon element's naturally low impedance. (Said trannie is virtually identical to the kind used between low-output moving-coil cartridges and preamplifiers, only in reverse: High current and low voltage on one side translates into low current and high voltage on the other. To see such a thing at either end of an audio system is to see Ohm's Law at work, which I truly believe is akin to poetry.)

Pass and English wondered, naturally enough, what would happen if they drove the ribbon with a voltage-actuated current source, thus allowing them to dispose of the transformer altogether in the interest of simplicity and, presumably, increased transparency of sound. The output impedance of the current source was so high that the amp exhibited virtually no damping factor vis-à-vis the loudspeaker load (again with the Ohm's Law!), but Pass and English noted that the small and magnetically (if not spiritually) centered ribbon didn't seem to require electrical damping, arguably because a ribbon element is so low in mass. (Cripes, if it's not Ohm, it's that wig-wearing Newton with all his stupid laws. Let's deregulate electricity so we can start seeing some real he-man performance!) Pass and English noted that the ribbon sounded and measured better in every way once its transformer had been removed from the equation. Inventors everywhere will recognize this as the switching on of a cartoon light bulb.

Those events didn't take place in a vacuum, of course. Somewhere in the background, Pass DIY was busy with a design called the Zen—a very-low-powered solid-state amplifier design that has found favor with a great many do-it-yourselfers. In the original Zen amp of 1996, gain was generated by a single MOSFET operating in common source mode, biased by a second MOSFET that was configured as a constant current source, in-line from the positive voltage supply. After the Zen amplifier came Son of Zen, which used a differential pair of gain MOSFETs, but which dispensed with the constant-current source and instead used resistors to power up the two MOSFETs.

The First Watt F1 could be considered the Out-of-Wedlock Grandson of Zen (in which sense it may also be related to Strom Thurmond). The F1 uses a differential pair of MOSFETs for current gain, driven by a trio of slightly different-spec MOSFETs configured as constant-current sources, the latter making the F1 much more efficient than the passive-source Son of Zen. That makes a total of five MOSFETs per gain stage—and one stage is all you need for 10 class-A watts. In the interest of design simplicity and smoothness of sound, the F1 does not have a negative feedback loop—another reason for its tube-like high impedance and low damping factor—although there's a feedback mechanism for keeping DC voltages stable, driven by a pair of bipolar small-signal transistors.