Luca Chiomenti on the previous page: "You absolutely nailed the character of my amp. That was the design goal. About the bass: yes, to stay a little bit in what you call the 'elasticity zone' was a deliberate choice. A damping factor of 16 gives this result. The general question requires a long discussion. Briefly, in my opinion lower Zout gains something only with some speakers which are innately underdamped but loses something else: the richness, articulation, harmonics and speed in the bass. Obviously like any choice, it is a compromise. The Zout in the 0.3-0.6Ω range is mine. And I appreciate how just a mention of any advanced concept could seem like big empty words without providing more detail. My real challenge is mostly having to take time away from production. There is something very concrete about my approach and I'm putting something together for you. As to tech specs, I could provide you with all manner of charts but the real difficulty are the power specs; not because they're secret but because defining and measuring power is one of the key aspects of my design which makes comparisons with traditional circuits difficult. I use 10wpc/8Ω class A as a common reference but it is really more. Just so, it becomes very hard to define power limits when distortion increases monotonically and clipping is so soft as to be difficult to find. And how to define static vs dynamic power in the first place? How short is a transient? How much distortion and what shape of distortion are acceptable during short-term peaks? There are no standardized answers, hence I have difficulty presenting fixed power figures that correlate with the 'norm'.


"The PSU was carefully designed not just for raw power but sound. It's about 200VA; sufficient for a 10wpc amp I think. Five power supplies use two transformers, one for pure audio circuits, the other for control and operational services. The tube branch is pi-filtered and stabilized. The power branch uses another pi filter and a technique I've used for 25 years; distributed capacitance*. Instead of two big slow capacitors, I prefer many small fast caps, with the last one extremely close to each power transistor. And there are no protection circuits, only fuses on the power rails. Key for the signal path is no global feedback anywhere and just minimal local. The single-ended ECC82 voltage amplifier uses none whatsoever and that stage wholly determines the amp's sonic character. The complementary outputs mix BJT and Mosfet and require no phase splitter or transformer. The motorized Alps sits at the very input before the first stage."
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* Here we might cite Steve McCormack's so-called DNA or distributed node amplifiers from some time ago which implemented the same; or very recently, the Denafrips Terminator DAC with its multitude of small capacitors.


The thinking reader has already done the math. If Luca's psychoacoustic model were true—designers from Nelson Pass to Jean Hiraga have said the same things for years—any gain circuit that accounts for correct distortion behaviour should, over neutral speakers, sound congruent, easily accessible and harmonically weighty. Nelson Pass too shares the belief that remaining distortion products should be simple not complex. Still, Pass track their sales. Those show a near 50:50 split between amplifiers voiced predominantly 2nd or 3rd-order. Whether by biological imprint or later overwrites from culture, exposure and musical preferences, half their audience prefers the sweeter lusher sound of low amounts of even-order harmonics, the other half the crisper leaner more separated version of low odd order. Are there two expressions of the basic distortion model? Riviera's clearly was the first. But it either contained some 3rd order spice to avoid the cloying effect of 'deep triode' which Dennis Had of Cary espoused back when; or was simply too low in magnitude to go there in the first place. And by listening to well below 10 watts, I also sat below the 0.2% of specified total harmonic distortion. This so wasn't the noxious perfume-doused stranger sitting down unbidden one table over only to drown out my dinner in a heavy cloud of sickly sweet scent. This was rather more subtle. Now Luca explains more background.


"About 25 years ago I presented my first design in formal production. In its intro I wrote that 'we believe that over the last 20 years, high fidelity ended up in a blind alley. We see a pointless race to technical perfection. For too long the goal has seemed to reach the limits of our test gear. This obsession forgot the real function of an audio amplifier: to reproduce music through an electroacoustic transducer.' 25 years later the situation seems still more confused. Today there is a general consensus. Common tests for audio amplifiers do not correlate with their actual sound. As an indicator of subjective sound quality, audiophiles largely reject technical specs and bench tests. Nevertheless, perfect measurements seem essential in the market. Personally I cannot synthesize 25 years of study and research on the relation between subjective experience and laboratory measurements. I base my studies on direct experiments and hundreds of pages of bibliography covering over 80 years. I hope to soon complete a white paper about this. For now I will try to present certain key points on the foundation of Riviera Audio Labs. Here we go back to the beginning. An audio amplifier must reproduce a signal with the highest fidelity; for the human ear, not test gear. Now it becomes critical to understand certain aspects of how the human hearing system works and consequently, how to define the characteristics of the reproduced signal for the human ear, not for an electronic measuring system. Let’s start at the ear. When we hear a pure tone, many studies verify the creation of harmonics inside the ear, specifically the cochlea. This is no new discovery. First reports on this came from Fletcher (yes, the famous man behind the Fletcher-Munson curve from the 1920s). More precise reports came from H.F. Olson (Acoustics, 1947) and many others later. It is interesting how the ear generates really high levels of second harmonics; about 10% for 90dB SPL (not 120dB or more!). Higher-order harmonics decrease with the order of harmonics. Now we can define a spectrum of the harmonic distortion of the ear. The shape of harmonic distribution is very important. There is a high predominance of lower-order harmonics in a decreasing spectrum which changes with sound pressure but is too complex to examine here. Its key points are: 1/ the high level of distortion the ear generates itself; 2/ that the ear/brain system cancels out those harmonics and the resultant perception is one of an absolutely pure tone.


"In short, our hearing system suppresses its own self-generated distortion. Even more interesting, it suppresses that range of harmonics even if they are of external origin, under the condition that their shape and pattern be the same. Obviously then our biological system is programmed to cancel that shape of distortion. It cannot distinguish whether the origin of said distortion is internal or external (some interesting musical phenomena are related to this, i.e. the missing fundamental note). If the harmonics differ from this shape, the ear/brain system detects them as different tones. From this we believe that an amplifier which generates distortion similar to the human ear will sound extremely transparent and clean even if its THD (measured in isolation) is relatively high.