Gramophone Dreams #26: Nelson Pass & Harmonic Distortion

Every time I fly to California, my brain gets stuck on the lyrics of that Arlo Guthrie song: "Coming into Los Angeles / bringing in a couple of keys . . ." Even landing in San Francisco, I'm always smiling, because I've never been busted in California.

Which means that I'm a lucky guy. In this life I have acquired nothing of material value, but I did see Howlin' Wolf and Little Walter in Chicago in 1962, at Theresa's. I spent the whole Summer of Love (1967) in San Francisco listening to music. And because I lived only a few blocks away, I witnessed the Ramones' first gig at CBGB, in 1974.

So you see, I've frequently been in the right place at the right time, standing on the right corner wearing a hat of the right color. Every time a black car pulled up and the driver said, "Get in!," I was brave enough to comply—no questions asked. To my surprise, audio has sent more than a few (metaphorical) black cars to corners I was loitering on. The latest was an invitation to the 2018 edition of the Burning Amp Festival, "DIY Audio's Premiere International Festival," at the Fort Mason Firehouse, under San Francisco's Golden Gate Bridge. This time, instead of sleeping in the bushes of Golden Gate Park, I stayed in a genuine Fisherman's Wharf hotel that was conveniently attached to an In-N-Out Burger. My assignment was to look, listen, and learn—and walk the beach with Burning Amp supporter and Pass Laboratories founder Nelson Pass; his vivacious wife, Jill; their wolf-eyed dog, Jack; accomplished circuit designer, Wayne Colburn; and PR magician, Bryan Stanton, of J.B. Stanton Communications.

On September 29 it was raining hard. I got in late, but the hotel clerk and an associate at In-N-Out were still up, waiting for me. The next morning I went for a walk and realized I hadn't been on Fisherman's Wharf since 1967. I saw some of the same burnt-out hippies I saw back then. But what a beautiful sunny morning it was: Alcatraz to my right, and Fisherman's Grotto #9 directly in front of me. Burning Amp would open at 11am, and I knew it was somewhere nearby. I typed "2 Marina Boulevard" into Google Maps and began my wonder-filled walking tour of San Francisco Bay.

I spent so much time gawking at fit hipsters and petting hound dogs that I was almost late. Burning Amp Festival's new manager, Mario R. Yrun, greeted me at the Firehouse door and hooked me up with a T-shirt and a badge. Admission is $20—unless you've brought along a component you've built yourself to be auditioned or simply admired, in which case admission to the one-day event is free. A tasty, healthy lunch is provided, as well as three lectures by distinguished audio designers.


According to Nelson Pass of Pass Labs (footnote1), "Burning Amp was started in 2007 by some of the guys hanging out at—far as I can make out, they were Mark Cronander, Vladimir Simovich, and Stewart Yaniger. Mark Cronander went on to manage the event until 2017."

The Burning Amp Festival is a siren call to audio tech heads and DIY audiophiles to come out of their basements and share their discoveries. The beauty of such forums and gatherings is that more experienced builders can mingle with and inspire neophytes. I've been a DIY amp builder since the 1980s, and every one of the supercool audio wizards I've met, including Amp Master Pass, has entered my life through participation in the worldwide DIY community. Ever think about joining them?

Burning Amp's trio of DIY project descriptions (footnote 2) were MC'd by the erudite Kent English, who works for Pass Labs and is a seasoned amateur audio builder.


The first lecture was by Wayne Colburn, Nelson Pass's close friend and design partner since the days of Threshold. Colburn described a DIY line stage using four n-channel JFETs. I laughed when he mentioned that the output stage was biased by an LED that, he added, "Lets you know when the unit is turned on." At the end of his talk, Colburn mentioned that there were free circuit boards of his design for each of us to take home.

Roger A. Modjeski, of Music Reference and RAM Tube Works fame, made a huge shout-out for vintage RCA Tube Manuals and F. Lanford Smith's venerable Radiotron Designer's Handbook, Fourth Edition—the Old-Testament Bibles for every builder of tube amps. He explained vacuum-tube plate characteristics (graphs of plate current vs plate voltage) and how best to establish operating points (grid-cathode voltage). I asked Modjeski if he ever went beyond voltage and current and considered transconductance in determining operating points. "No," he said. "I use my own RAM Labs curve tracer and plot for minimum distortion."


Not surprisingly, Amp Master Nelson Pass continued the discussion of load lines and operating points in his description of his H2 harmonic generator and LX Mini active crossover. According to Pass, the H2 is designed around some "cheap-ass JFETS that are almost not good enough to carry signal."

I can hear you thinking: Why would anyone need or want a harmonic distortion generator?

The simple answer: Maybe a little distortion of the right kind could make our recordings more intelligible. Maybe adding just the right amount of second-harmonic "information" will assist our neural sensors and brain in reconstructing a recorded sound event.

In a recent interview, Pass explained something I'd been waiting 45 years to understand:

"Having previously designed with SIT (VFET) single-ended amplifiers and also the Korg Nutubes, I have worked up some performance targets which deliver an effect that is usually well-liked. This effect is described as having negative phase 2nd harmonic at approximately 1% of the amplitude of the original signal . . . but after you invert the output (to get absolute phase correct) you will find that it is now a negative phase 2nd harmonic. I know this might be confusing—I have occasionally awakened in the middle of the night and thought 'That can't be right.'"

Pay attention now, because here is where it gets really interesting:

"So why is the phase important? Well, it's a subtle thing. I don't suppose everyone can hear it, and fewer particularly care, but from listening tests we learn that there is a tendency to interpret negative phase 2nd [harmonic] as giving a deeper soundstage and improved localization [of images] than otherwise. Positive phase seems to put the instruments and vocals closer and a little more in-your-face with enhanced detail."

You knew this, right?

"Your results may vary, but when I first explored this with the SIT-1 amplifier at First Watt, I had a knob on the front of the amplifier which varied the amount and phase of the 2nd harmonic. It was easy enough to lend the amplifiers to listeners who didn't know what the knob did and gather their comments. Roughly speaking, they tended to prefer about 1% negative phase 2nd harmonic, so it became my standard setting for that knob."


Right away, Kent English raised his hand: "Why would we want to add distortion?"

Pass smirked. "Because this is entertainment, not dialysis! . . . and I am giving these things away!"

Pass reminded everyone that "The H2 works best with simple material. With complex material, second harmonic may be more of a burden than an asset. When you turn the knob to the left, which is negative-phase second harmonic, the room gets bigger, more atmospheric, you start to see images of musicians. Turn the knob right, you get positive-phase second harmonic, which brings the musicians closer to you: drier but more intimate, more apparent detail. But of course," Pass reminded us, grinning hugely," this is all illusion!"

A man in the audience cleverly explained how the second harmonic is exactly one octave up from the fundamental and is "like a chorus of backup singers. As H2 distortion rises, the backup singers get louder."

My objectivist friend Mr. O calls H2 "second-harmonic sauce." I've always called it "second-harmonic echo." I think a little of it makes voices fuller toned and more intelligible, spaces more spacious and tangible. To my ears, modest amounts of H2 fill in some blanks of the illusion and make reproduced music more comprehensible.

Trickster Pass reminded disbelievers: "You are welcome to take my remarks as entertainment."

Footnote 1: Pass Laboratories Inc., 13395 New Airport Road, Suite G, Auburn, CA 95602. Tel: (530) 878-5350. Fax: (530) 878-5358. Web:

Footnote 2: See videos of all three lectures here.


Allen Fant's picture

Excellent piece of writing- HR
I like the pictures of the "Master" at work. Or is is play?
Nelson Pass has that whole Rick Rubin (a different Zen master) vibe.

wojciech.froelich's picture

"otherworldly, superscenic, supernarrow, death-at-every-curve Coastal Route 1 is dangerous"

Come on, this is a fantastic route. I had the opportunity to drive south and I enjoyed every minute of it :)

JimAustin's picture

One interesting aspect of this: If you flip just the sign/polity of the second harmonic distortion, the only difference between the new waveform and the old is absolute phase/polarity.

prof's picture

If you flip just the sign/polity of the second harmonic distortion, the only difference between the new waveform and the old is absolute phase/polarity.

All the more interesting, then, that the subjective effect depends on the polarity of the 2nd harmonic distortion that you added.

Of course, if you look at the waveforms posted here, you can see that his box adds some higher harmonics as well (so that the net result of flipping the polarity of his added distortion is not quite an overall polarity-flip).

It's those higher harmonics that presumably are responsible for the perceived difference when you flip the polarity of the distortion.

prof's picture

Sorry, wait, no.

If the input signal is sin(ωt), adding some second harmonic yields an output signal

sin(ωt) ( 1 ± 2a cos(ωt) )

where the amplitude of the 2nd harmonic is "a" (a ~ 0.01 for Mr. Pass) and the ± corresponds to positive/negative polarity.

The choice of ± is not just a choice of overall polarity.

JimAustin's picture

There's also a time shift, which of course makes no difference. Wanna see the math?

prof's picture

Unless your argument is that the phase is irrelevant (in which case, the polarity of the added 2nd harmonic distortion should have no audible effect), then I don't see how that changes things.

An arbitrary phase shift, ϕ, just interpolates between the two polarities (as ϕ ranges between 0 and π). But, as long as the amplitude of the added 2nd harmonic is small, there's no choice of its phase, ϕ, that has the effect of just inverting the resulting total waveform.

JimAustin's picture

OK, I'll spell it out then. The time-shift I mean is (again, like the polarity inversion) absolute: The whole thing together is shifted by pi. Which obviously doesn't matter.

The only identity you need is sin(x) = - sin(x - pi). So, sin(wt) + a sin(2 wt) = - sin(wt - pi) - a sin(2 wt - pi)) = -sin(wt - pi) + a sin(2wt - 2pi) = -sin(wt - pi) + a sin(2(wt - pi)) = -(sin(wt-pi) - asin(2(wt-pi) ).

So you've shifted the whole spectrum together by pi and inverted it. QED.

prof's picture

Yes, an extra π phase shift in the 2nd harmonic is (obviously) equivalent to flipping the sign of a. I didn't realize that was what you meant.

I'll say again, though, that either relative phase doesn't matter (in which case, the sign of "a" should make no audible difference) or it does -- in which case your (frequency-dependent) time-shift matters.

JimAustin's picture

I still feel like we're not communicating, but after this I've got to let this go. What I'm saying is that when you flip the polarity of the second harmonic, the only significant difference is that the overall sign of the waveform is reversed. (The whole waveform is also shifted by pi, but that obviously makes no difference.) There's nothing relative in any of this: It's a change in absolute polarity accompanied by a shift (of the whole waveform, NOT of the harmonic relative to the fundamental) by half a period of the fundamental, or a whole period of the second harmonic. Since, for pure tones, a time-shift of the whole waveform doesn't matter, the only significant difference is that absolute polarity is flipped. Here, I've reversed the sign of the second harmonic at 5 seconds.

Be Well,


prof's picture

We agree on the mathematical identity

sin(ω(t+π/ω)) + a sin(2ω(t+π/ω)) = - [sin(ωt) - a sin(2ωt)]

I'm sure we also agree on the even simpler identity

a sin(2ω(t+π/2ω)) = - a sin(2ωt)

If you believe the standard psycho-acoustic research that says that frequency-dependent time shifts of this sort are inaudible, then either of these identities is sufficient to conclude that flipping the polarity of the 2nd order harmonic should not make an audible difference. (In fact, the latter identity is "better" in that it applies to any harmonic distortion.)

Clearly, those who hear a difference between "+a" and "-a" cast doubt on the inaudibility of the frequency-dependent time shifts in either of these identities.

I was initially confused and thought you were claiming "+a" and "-a" were related by a frequency-independent time-shift (or no time-shift at all). Sorry.

JimAustin's picture

This--that flipping the sign of the harmonic only affects absolute polarity (plus a phase shift) is true for all even harmonics.

Bogolu Haranath's picture

"I'm trying to bring a little bit of every type of sauce into one type of sound ..... something that's really fresh." ............ Post Malone :-) .........

Ortofan's picture

... a deeper soundstage and enhanced detail? Can you connect two of the H2 modules in series, with one set to add positive-phase distortion and the other set to add negative-phase distortion?
Is that an "audiophile approved" electrolytic capacitor in the signal path? Where's the film cap bypass?

ok's picture

..but not quite

NeilS's picture

Is there a reason why the addition of desired harmonic distortion to a digital audio input signal can't be instead generated mathematically by DSP?

misterc59's picture

I was expecting a flurry of responses to this piece.
Maybe they're coming, or, as I have been wrong many times in my life, this article hasn't yet stirred up the emotions/discussion I had thought...


Ortofan's picture

... old news:

FredisDead's picture

"I was expecting a flurry of responses to this piece.
Maybe they're coming, or, as I have been wrong many times in my life, this article hasn't yet stirred up the emotions/discussion I had thought...


After carefully reading the piece twice, I think HR was trying to play provocateur but we readers can see through it. There is really nothing new here. Everything in audio is a trade-off. Pick your poison.


Biil J's picture

I was wondering if the author could provide additional comment regarding the open baffle experience with the SIT3, CUBE Audio 8" driver and 15" Eminence woofer in the slot position in terms of imaging and bass response. Thank you in advance. I really enjoyed this article and enjoy your writing at large.