THE BRAND – THE TEAM
THE RIVIERA WAY
- Zero Overall Feedback
- “Class A” amplification at every stage
- Hybrid circuitry, with valves (triodes) for signal processing and solid state (BJT and Mosfets) for the power stage
- Electrical values chosen by listening
- When we ear a pure tone, many studies have verified the creation of harmonics inside the ear and, specifically, in the cochlea. This is not a new discovery. First reports on this distortion are from Fletcher (yes, the famous of the Fletcher-Munson curve, in the ‘20s); more precise reports came from H.F. Olson (Acoustics, 1947) and many others later. It is interesting to observe that the ear generates really high levels of second harmonic: about 10% for pressure levels of 90dB (not 120dB, 90dB!). Higher order harmonics levels decrease with the order of harmonics. We can define a spectrum of the harmonic distortion of the ear. The shape of harmonic distribution is very important: there is high predominance of lower order harmonics, with a decreasing spectrum. This spectrum changes with the pressure level but this point is too complex to examine here. The key points are: 1) the high level of distortion the ear self generates; 2) the ear+brain system cancel those harmonics and the perception is that of an absolutely pure tone. In other words, the hearing system suppresses the harmonics generated by itself. Interesting, isn’t it? Even more interesting is this: the system suppresses the sound of “that” range of harmonics even if they are of “external “ origin, under the condition that the shape, the pattern, is maintained. Quite obvious: the system is programmed to cancel that shape of distortion and it cannot distinguish whether the origin of the distortion is internal or external (some interesting musical phenomena are related to this behavior, i.e. the missing fundamental note). If the harmonics differs from this pattern shape, the ear+brain system detects the harmonics as different tones. Based on this, we think that an amplifier generating a distortion spectrum similar to that of human ear will result extremely transparent and clean, even if its THD level is relatively high.
- The above mentioned mechanism is dependent on the pressure level. In synthesis, the higher the pressure level, the higher the harmonics distortion that our ear generates (and accepts). This brings us to prefer amplifiers with distortion monotonically increasing with the output power.
- The higher the pressure, the higher the order of harmonics that ear generates. This means that, at higher levels, an increased quantity of higher order harmonics can be accepted.
- The detectable distortion depends, among other things, on: 1) the ratio between the peak and the average value of the signal; 2) the duration of each signal peak. Studies have proved that distortion can even reach very high values and still remains inaudible if the peak length is very short.
- Masking is the well-known phenomenon where a low level tone in close proximity to a higher level one is not audible. This can limit the influence of IM distortion in an amplifier with a predominance of low order harmonics. This effect seems to be even higher if the distortion spectrum shape of the amplifier is close to the ear’s one.
- Feedback is the classical method used to reduce THD and upgrade circuit performances (THD, IMD, bandwidth, noise and more). Unfortunately feedback reduces the lower order harmonics (less harmful and more “benign” for the ear) much more than the higher order harmonics. More and better: high levels of feedback act as a generator and “multiplier” of higher order harmonics (harmonics that ear detects as “dissonant” and noisy). Even if these high order harmonics are under the audibility level, the mechanism generates a “noise floor” that seems to be absolutely undesirable for the human ear. All this suggest to avoid feedback (specially overall feedback) or to try to reduce its use to the minimum.
- Optimization of distortion for amplitude and frequency: THD does not need to be extremely low, but it must absolutely follow the human ear distortion shape. This means predominance of lower order harmonics, with their regular distribution in frequency (the higher the order, the lower the harmonic level, with a relation between harmonics similar to the human ear one). Furthermore, the distortion level must be monotonically increasing with power level. The amp should have a soft clipping and, if possible, the distortion spectrum must be similar to the ear spectrum even in this area (or to the higher level possible before losing the right shape).
- The use of Zero Overall Feedback and minimum level of local feedback, to minimize the above mentioned negative effect that this technique has on sound result. This is the best way to reach the desired distortion behavior.
- A good open loop BW
- A reasonable DF, like in best tube amplifiers: between 15 to 20, without looking for useless records. This point also contributes to provide articulation and harmonics richness in the low frequencies
- Total stability on every load
- Absence of protection, to avoid their negative effects on sound and dynamics
- Extreme care to the power supply
- The Zero Overall Feedback and the minimum use of local feedback, when absolutely indispensable, is the first solution to reach the desired distortion behavior.
- The use of the A class in all stages is the logical consequence: we need to have the maximum linearity without “tricks”.
- The hybrid solution is the next logical consequence: the triode is the best voltage amplifier and, mostly in a s.e. configuration, it offers a “natural” distortion shape, very close to the required one. Silicon devices (and specially Mosfets) are the best choice to handle power and low impedances; if properly used and driven, they can offer a good distortion shape too. In the topology adopted they also give the desired output impedance.
- Protection circuits damage sound. In Riviera thinking this is a fact. So… no protection circuits (only fuses on PSU rails). Consequently a real large dimensioning, not for appearance (coke-like capacitors…) but in the real key points (PSU, transformers, power devices). A secondary consequence is the need for an adequate mechanical dimensioning too.
- Accurate PSU design. 2 transformers and 5 separate PSU. The tube PSU is PI-filtered and stabilized (with a MOS, not a BJT); in the audio power PSU stage there is a PI filter too and the distributed capacitance: instead of 2 big-and-slow capacitors, many little-and-fast capacitors, with the last ones extremely close to each power device.
- There was a long and accurate musical and bench optimization (I do not know how to quantify or better explain this point)
- Mechanical and “physical” manufacturing with a quality level of a laboratory instrument and a made in Italy design, and this point is easy to see ;-)”
There is no real review without the music references. We all have our objective favorites, that clearly shows our intimate connection with the music. Still… A lot of people have a personal selection of a few songs, that are used strictly for the evaluation purposes.
Well, this works for some. Yours truly tend to enjoy the music each and every time, testing and reviewing intervals included! Even, if I’ll repeatedly hear some of them for hounded of times, plan is to enjoy the songs each and every time.
And with the upper echelon gear like Riviera Audio Labs AFM-50, such sonic explorations become a highly sweet endeavor.
There was a certain, underlying and invisible treat running across all of the listenings through the Riviera AFM-50. Somewhat vibrant-positive and highly engaging DNA…
Some of the high-end audio gear can pinpoint particular attributes and highlight only a portion of something special. AFM-50 seemed to be designed from the ground up to deliver the music in a balanced way, without focusing on any specific sound orientation.
Timeless and meditative John Coltrane – A Love Supreme certainly comes from some very different universe. I call it an altered sonic reality. There is so much hidden energy and complexity within this gem. Album also represents quite a challenge for any amplifier. A whole essay could be written about A Love Supreme implication, but let me focus on few attributes in connection with the Riviera Labs amplifiers.
When John’s saxophone blends with the Elvin Jones in the very beginning of the Pt. 2 Resolution, this movement alone de-mask any amplifier shortcomings. It’s not exactly easy to render the quartet’s pace and complex layering. Elvin Jones’ (demanding to reproduce) polyrhythmic drum playing extends across the complete album and even some of the most highly coveted amplifiers fail to convey his magnetism with full “bang”. As said before, some amps might isolate and perform a certain duty more than impressive, but when it comes to the harmonic, wholesome rendition there are really not so many amplifiers out there, that share this particular potency.
For me, A Love Supreme is up there with some of my favorite classical compositions. It might take a while to digest and appreciate it fully, but its sort of a “muss sein” repertoire for any music lover. I’ve marked down quite a few smileys within my listening notes in connection with AFM-50. If any given amplifier can render one of my “desert island” discs with such highly enough engaging factor, this alone is worthy of highlighting. And Riviera Labs AFM-50 hybrid power amplifiers passed this test highly convincingly.
The other timeless masterpiece, that I cannot get enough is Miles Davis – Sketches of Spain. Especially the opening track Concierto de Aranjuez (Adagio) radiates such a powerful, almost poignant message, that is hard to be unaffected even if I’ve heard this track most probably hundreds of times.
Again, the underlying complexity and a challenge formation of the brass instrument represent a step up a task to perform. Trying to portray the believable timbre while keeping rhythmic “Ordnung und Disziplin” is no slouch.
Here, the amp is challenged by the spectral shading, multiplex mirroring of quark like decays and delays, as well as the rendition of spot-on tone. Add ever-changing tempi and suddenly, what it might seem like a subtle, easy going track, instantly morphs in a demanding composition with many stringent attributes to match.
Across all of the Concierto de Aranjuez (Adagio) AFM-50s have managed to keep the “breath”. This translates into keeping the very energy of a song across the full duration. If amplifier fails in this regard, the sheer magic of Miles/Evans masterpiece is gone. Riviera Audio monoblocks passed the hard test of preserving the energetic essence along with the firm delivery of the multi-instrument kaleidoscope. Again, not an easy rendition.
AFM-50 monoblocks have kept the ebullience without losing the pellucidness. Usually, and too often, the details are masked within the warm hearth of the amplifier, but Riviera hybrid power amplifiers expertly blended the zing with translucency.
Across numerous-additional tracks Riviera Audio Labs AFM-50 have exhibited a warm, natural dark like nature, that is highly close to the gestalt of the real instruments and voices. When paired properly, the sense of dynamism was never absent and they’ve managed to paint the sonic canvas with a stand out gaiety and clarity.