Management, not extermination. Our curvy topic is perfect to present an important oft misunderstood concept. Be it Audiopax, Lamm, Pass or Tenor, deep thinkers on the subject are adamant. In hifi, distortion can't ever be eradicated completely. Tech limits won't allow it.

Plus, if ultra-low distortion really were boss, Halcro would be our apex predator, Japanese receivers from 40 years ago not far below it in the food chain. Yet in that scenario, nobody would hear squat since speakers—by suffering their own very significant THD profiles—would be categorically outlawed. As Bob Marley said so famously, no speakers, no smile.

With that setup, the alternative is clear. Since one can't exterminate THD by pest control, one must manage it. Inherent in that concept is the notion that certain harmonic distortion is more benign than are other forms. Vladimir Lamm is famous for claiming that he doesn't ever listen during his design process. He 'simply' manipulates a circuit until it exhibits the precise distortion curves which he, quite a long time ago, determined correspond to ideal sonics.

As Mike Malinowski chronicled in his Tenor 350M HD review, its designers went to great lengths to compensate for a significant power increase of their core circuit with global adjustments elsewhere. This was to insure that their Harmonic Distortion Integrity mirrored the lower-power version precisely. Clearly this is a very complicated topic. It must account for a higher than rudimentary grasp on how human hearing actually works. The operative soft term is psychoacoustics. People like Vladimir Lamm have conducted their own R&D into it since the usual text books are rather mum on the subject.

Compare the voltage figures between graphs, from the 2nd to the 10th harmonic, to appreciate how seemingly insignificant amounts are in fact audible.

The late Audiopax designer Eduardo de Lima allows the user adjustments which manipulate the load-line curves between his multiple dissimilar output transformers and their associated tubes which only get combined at the very speaker terminals. Quite self-explanatorily, he dubbed this feature TimbreLock. The Nelson Pass SIT1's bias adjustor does something similar, shifting the THD distribution between remaining 2nd and 3rd.

AURALiC's Wang Xuanqian, for their Hypex-based transformer-coupled Merak monos with linear power supply, supplied these two graphs to show how their proprietary modification to the UcD module changed their THD spectrum. "Above is the test result from the original Hypex module, below that of our improved version. Whilst we did sacrifice THD by 6dB in the lower harmonics, we reduced all 4th to 9th higher-order harmonic distortion products by a remarkable level. This is a key reason why Merak sounds different from other switching amplifiers." - 王轩骞

Grounded inside that brief statement hide the seeds for any serious THD consideration. Higher-order THD is progressively more troublesome; and higher amounts of low-order THD are quite preferable to even minuscule amounts of higher than 4th.

Related factors question how a circuit reduces distortion where the cure (negative feedback) could be worse than the disease; how THD shifts with power (as things get louder, how does THD distribution alter); and how THD between circuit stages and components sums, subtracts or gets more complex. Because amps and speakers suffer their own THD, de Lima's contention was that strategic or fortuitous interaction between them could result in partial out-of-phase cancellation, creating lower distortion at the ear than either component would measure in isolation on the bench.

Here I confess that the actual math behind the subject is beyond my own security clearance. When I was born, the gods decided that I had no need to know. Here is what Sasa wrote during R&D on my Kaivalya amplifier commission a few years back. "I have two ideas but am still thinking. I am making some harmonic analysis between ECC99 and ECC81. The ratio between 2nd and 3rd harmonic favours the ECC81. This ratio directly influences the sound and is critical. It's about how the choice of driver tube for the output tubes influences the sound not just with its ability to drive via current, impedance and headroom but harmonic interactions." As that design progressed, Sasa informed me that besides impedance conversion and perfectly matched phase splitting to turn his single-ended driver stage into a push-pull feed for the output bottles, the very complicated interstage transformer would also be responsible for the harmonic matching between ECC81 and 6P14P. As the months passed whilst Sasa whittled away at this custom project, things kept changing. "We're into our third IT now and very close to perfection but it must still ace a 10kHz square wave. This IT is so different from the one in our 300B monos that prior experience was no help at all."

Next time I checked in, Sasa was on fifth base. "Yesterday I finished the 5th version. Now I've capriced to go to the end of all possibilities. The 4th one was excellent but I found that I could still improve it a bit with a slightly different core geometry that's changed over the first four. This was a nice opportunity to see what happens with different ways of winding an IT when you already have a superior OT. I conclude that the IT must be at minimum 30% better than the OT to predictably produce a superior sounding amplifier. This really maxes out the sound of the EL84. By now I like it more than any 300B/2A3 amplifier I've heard; and I listened to it CD direct without preamp. I'm surprised and you will be too I think."

A bit later, "I'm just now drawing up a new type of IT which is actually a new approach to the problem - and hopefully even better if I can trust my mathematical predictions. The problem is in the ultrasonic band. Everything is fine up to 70kHz but beyond it I have 90° phase inversion between the left and right sides of the push/pull outputs of the transformer; and 180° rotation at 100kHz. It's not a real concern since this is well out of band but now is a good chance to make this IT perfect since the market offers nothing that's good enough for my tastes. Leakage induction should be three times lower than with the earlier design. I am on the right track with the new winding system. I now have perfectly symmetrical halves of secondaries but a new problem, albeit smaller than the symmetry of secondaries. It's endless work but will be very good in the end." Then, "the final one has 9 separate horizontal windings for 38 separate windings total. The sound is unbelievable for an EL84 construction. I am happy." But that still wasn't the end. The production interstage transformer ended up with 11 horizontal windings and a very small amount of embedded feedback to tweak the harmonic interaction between single-ended triode driver stage and push/pull pentode power stage.

From this, we extrapolate. 1/ Sasa pays attention to THD tuning. 2/ He is resourceful and unafraid to tackle new projects that challenge him with a steep learning curve. 3/ He is a master transformer designer who will prototype again and again in his own shop until he's content. 4/ He's rather experienced with tube design and given 1-3, has established himself as a go-to guy for one-up custom projects which involve tubes and transformers of even grotesque dimensions where a giant bottle might put more than 3'000V on the rail. This should explain why I was so excited about taking Sasa's new hybrid for a spin. About its name, "in ancient mythology, Belus was the son of Poseidon and Libya, one of the demigods and rulers of Egypt. Being half human, half celestial seemed a proper name for a hybrid." To me it also sounded like a male form of the Italian "bella". A strong name either way.