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A few things about the F2 before we join the fun & games. It's black, it's plain, it runs hot and looks virtually identical to the F1 once you subtract the latter's XLR inputs. The First Watt amps will never win any beauty contest. These are short-run specialty products designed for extreme performance into limited-use applications. The emphasis is on circuit execution rather than sex appeal. While we'll take a look at the innards in a moment, one aspect remains invisible to the eye - reliability. Dr. Peter Poltun has held the post of Director of Archives for the prestigious Vienna State Opera and Philharmonic Orchestra since 1992. They've used Pass Labs electronics for more than 25 years. Failure rate? Zero. 'Nuff said.

Just as reliable proved synchronicity when Ed Schilling from The Horn Shoppe asked whether I'd like to take a crack at his Fostex-based Model 1. Below are his "Hornettes" and Little Miss Triode. The name The Horn for the back-hornloaded Fostex 108E Sigma driver is wildly tongue-in-cheek. It dates back to a beer-stained christening ceremony at Ed's. His little speaker sat in front of stacked LaScalas, outperforming those to his and his friend's ears. Irreverently naming the tiny tots The Horn stuck, both to howling laughter and subsequent ire at the Asylum. Ed would have changed the name to Hornette. Alas, the heated vitriol already spent on the subject convinced him that the audiophile world does
need far more humor than is often demonstrated [see boat for another good example of humor in action]. Hence it's been The Horn for the last five years. Its innards are a complex 6-foot folded exponential affair mouthed aft. This diminutive Bushhorn-inspired design was first discovered by Art Dudley who then still headed the much missed and never matched Listener Magazine. Ed had sent one of his first pairs to Don Garber whose amp he owned. "Don, you probably don't know me from Adam but your amp loves my speakers. You gotta hear it for yerself." Don concurred and promptly introduced Art to the horns. Art found out that Ed had only been in business for 3 months at the time, with only 3 pairs sold. "To heck with that, they're too good not to write up," countered Art and penned the first review on The Horn Shoppe's first product. Art's the man. Once at Stereophile, he revisited another pair for an early 2004 issue as part of his review series on single-driver loudspeakers. JA kindly waved some of the usual review policy requirements when he learned that Ed offered an unconditional satisfaction guarantee. Don't absolutely love his speakers? Ed sez you can't keep 'em. No sir. Send 'em back for a full refund - but not until you've lived with them for a full month. That's how long the drivers take to break in and indicate what they can do. [Incidentally, Ed's amp of choice is the F1.]

Harmonious and not so harmonious harmonics?
With all of that out of the way, what's the deal with 2nd and 3rd-order harmonic distortion anyway? Does it warrant a second low-power current-source amp from the same designer? Harmonics are multiple integers of a fundamental. The 2nd harmonic is two times the fundamental frequency, the 3rd harmonic three times its cycles in Hertz. The 4th harmonic of 100Hz is thus 4 x 100Hz - two octaves up. Have you ever played with one of those floppy sound pipes kids swirl around? The faster you spin 'em, the higher of a tone they emit. The first tone above the slowest one is the octave-doubled fundamental (i.e. C over C if your pipe was tuned to C). That's the 2nd harmonic. Speed up the pipe and you'll get to G over C over C. That's the 3rd harmonic - octave-doubled fundamental plus an open 5th.

If you can twirl the sound pipe faster again, you climb up the harmonic ladder and get to the 4th harmonic - C over C over C i.e. octave doubling squared. From the 5th harmonic on up, overtones begin to correlate to the lower harmonics in increasingly dissonant proportions. The fifth adds a major third (E over C over C over C), the 6th a diminished third over that and so on. By the way, you can duplicate the experiment of the sound pipe by overblowing an empty bottle. However, your lungs will give out before you get very far. Higher and higher air velocities are required to keep over-blowing. It's easier to use the physical motion of the sound pipe for the same effect.

If you could design an amplifier that produced 2nd and 4th-order harmonics exclusively and no 3rd? Heck, you'd be in lushville. That's like having an alto and soprano sing in unison with a tenor. Each sings the same tone but in their respective registers, spaced even octaves apart. In this example, the tenor of course stands in for our recorded fundamental. Remember, that's the only tone our amplifier is supposed to reproduce. The extent to which the other two singers or ghost tones remain audible behind and above the fundamental? That's simply a function of the amp's distortion figures. Those tend to vary with frequency and amplitude. The higher the percentile distortion figure, the louder our additive harmonics become (say 1.5% for the second harmonic/alto, 0.4% for the forth overtone/soprano).

Remember that music very rarely consists of solo notes that lack any accompaniment. Say a pianist played a left-handed D-minor chord of four keys: D - F-flat - A - D. Total harmonic distortion impacts each of those four notes simultaneously. Each one gets a few phantom tones added. As long as we're dealing exclusively with 2nd-order harmonics, that's reasonably harmless. The pianist simply puts down his right hand and duplicates D - F-flat - A - D one octave higher. We're still in D-flat land - except now it's a two-handed rather than one-handed chord. As I said, lushville.

Now consider our 3rd-order F1 chap. He'll add an open 5th atop each tone of our chord. The trouble with that? Without getting into music theory, certain of dem subliminal phantom tones simply don't belong into D-flat. Certain of dem tones are like blue notes in Jazz where a player might add an elevated 7th, 9th or 13th into a pure well-tempered chord and spike it up for those "bluesy feelings". (For simplicity's sake, we haven't yet considered how each instrument and voice has its own spectral components. Those create individual timbres and make Jan Garbarek's soprano sax sound distinctly different from Kenny G's even if both played the same note at 800Hz. Clearly, additive harmonic distortions introduced by playback electronics alter timbres of individual instruments and voices if ever so subtly.)

As long as those lower odd-order distortion artifacts remain low enough in amplitude, they're more or less masked and overpowered. They remain a mostly subliminal flavoring. Think spice or tension. Think dash of salt in a sweet. First-rate amplifiers will have very low distortion figures as they must and essentially none of the high-order ones above the 3rd. After all, anything that's not on the recording but reproduced during playback is distortion. THD stands for Total Harmonic Distortion and amplifiers, preamps and speakers all add some. Right now we're merely distinguishing between different kinds of distortion - alterations to the gossamer web of the recorded overtone spectrum. In order to get the F2 to behave like a triode amp with its typical emphasis on octave-doubled components, Nelson Pass had to alter the F1's balanced circuitry. Among other items, this meant halving its output power. It's testament to human hearing acuity that even if these two amps were perfectly identical except for their harmonic distortion spectrum (which isn't technically feasible), they'd still sound sufficiently different. Ergo, two models for two different listeners with different ideas of what's pleasant, life-like, invigorating... however you'll put your reaction into words. Voilà: the F1 and F2 respectively.

A few more words from the maestro: "Driven balanced, the character of the F1 is almost purely 3rd. Driven single-ended, it has some 2nd as well, which accounts for some difference in sound. The harmonics I get are not designed in, they are simply the character of the gain devices in that topology, with that amount of bias and so on. The power ratings of the F1 and F2 reflect a practical characteristic. The F1 benefits from the cancellation effects of a balanced pair of single-ended circuits so it has lower distortion (3rd harmonic) at a given bias. The F2's 2nd harmonic is not removed through cancellation and is a numerically higher value for a given bias, the upshot being that to keep the number more reasonable, I nearly doubled the bias current, giving a single device in the F2 the current that used to go through both balanced stages of the F1. With the same supply voltage, the dissipation comes out about the same so it can use the same chassis and power supply components. The F2 still has somewhat greater distortion numbers than the F1 but that's the way it goes - and this is not really a number's game anyway.

As far as I'm concerned, everything above 2nd and 3rd is headed toward suckage but I think the issue is not solely that of the musicality or dissonance of the harmonics themselves. It is also very much an intermodulation problem. I notice that simple little amps particularly excel at simple musical material but reveal their weakness in complex orchestral passages at higher volumes. It seems that the 3rd order amps do the complex material a little better, but then they usually measure better so there is less overt IM."

And now let's leave math, harmonics and theory to get practical. That'll start with turning the F2 on. Remember, this is an amp highly biased into Class A. Nelson worked hard to reduce in-rush current transients to below one volt and keep thumper in Easter bunny land. Still, very sensitive speakers won't cotton-tail to the amp being turned on and off in rapid succession (why would you do that anyhow?). Wait at least one minute after power-down before you go live again so the standing rail voltage can first drain off. (Incidentally, no Bugs Bunny on my Druids. Not even a whisker.) Give me a few weeks to put the F2 through the various paces indicated by my current in-house choices of single-driver speakers. What I can
already tell you? The F2 does sound different from the F1 on my Druids. It makes the kind of sound tube-infatuated fossils tend to believe transistors are incapable of. Sweetness, very noticeable on bowed strings. Think that's tantalizing? I'd concur with you wholeheartedly. A side effect is a greater sense of relaxation, as though driven music like Fatal Mambo is just a little less - well, driven. It's easy to appreciate how different listeners could go either way. Me? What do you think? Actually, that's a trick question.

With the little listening I've done so far, I prefer the F2's tonality but on certain fare, the subjectively greater tension (or invisible push behind the music) of the F1 is bloody addictive, too. Take The Klezmatics' Brother Moses Smote The Water [Piranha 1896], a live cut from an outdoors Berlin jam that has power belter Joshua Nelson rip into the tunes at gospel rock-out intensity while Kathryn Farmer leans into her Hammond Organ like a just-converted.

"Fascinating," sez Spock of the pointy ears, "how a little bit of harmonic distortion would alter the innate feel of the music." Golly, did the old Vulcan just use the bloody four-letter F word? He sure did. And he's right, too. More on that later. For now, the F2 is shipping and only 100 will be put up for sale. Make of that what you will...