by Luca Chiomenti

Integrated amplifiers and power amplifiers, like all Riviera equipment, share the same basic principles based on the technical characteristics that an amplifier designed ‘for’ and ‘dedicated to’ the human ear should have. Only those measurements that have demonstrated an effective correlation with listening feedback have been optimised, without seeking useless technical results.
We summarise here the main theoretical points, already set out in the general section, from which the practical choices made on these devices are derived.

• Optimisation of amplitude and frequency distortion: THD must be minimized, but without aiming at just 0.0000… instead, it must absolutely have an optimised trend with respect to the ear’s distortion. This means: predominance of low-order harmonics with a regular frequency distribution (the level of the harmonics must decrease as the order of the harmonic itself increases, as happens in the ear). Furthermore, the level of harmonic distortion must be monotonically increasing as a function of output power. The amplifier must exhibit soft clipping and even in this area the distortion spectrum should maintain a shape as close as possible to that of the ear’s distortion.
• Zero Overall Feedback and as little local feedback as possible to minimise the negative effects this technique brings to the sound result. I consider this to be the best way to achieve the desired behaviour in terms of distortion.
• A good bandwidth even in open loop.
• A reasonable damping factor (Damping Factor): between 15 and 30, as in the best valve amplifiers, without chasing unnecessary levels. In my opinion, this characteristic contributes to restoring articulation and harmonic richness in the low frequency region.
• Total stability on any load.
• Absence of active protection, to eliminate the negative effects that these devices often have on the sound result and dynamics in particular.
• All our products use oversized linear, not switching, power supplies but the key point is this: since the power supply is also part of the signal, it must be designed like ad audio circuit. In fact, our power supplies were developed with the musical result in mind, using unusual and elegant solutions.

These theoretical points take in the design of the individual parts. As you will see, very often a choice turns out to be the direct consequence of previous choices.

• Zero Overall Feedback and the use of the minimum possible local feedback where strictly necessary. This is the basic point from which we started to obtain the desired behaviour in terms of distortion as well as many other parameters. This basic choice implies a serie of further decisions that become almost consequential in view of a reference result.
• The use of Class A in all stages is the first consequence: it is necessary to have the maximum intrinsic linearity in the amplification stages, since we decided not to resort to the ‘trick’ of feedback to linearise non-linear stages… Class A is the guarantee of the maximum possible intrinsic linearity.
• The hybrid solution is the next logical consequence. The triode is still the best voltage amplifier in existence and, especially in the single ended configuration, offers a form of ‘natural’ distortion that is extremely close to the desired one. This is why it was chosen as the voltage amplifier of choice. Solid-state devices (especially MOSFets) are the best solution when dealing with high power and low impedance and, if used correctly and properly driven, can provide excellent behaviour in terms of distortion. In the circuit configuration adopted, they also provide the desired output impedance.
• In our opinion, active protections are often detrimental to the sonic result. Therefore, there are no active protections: only safety fuses on the power supplies. The consequence of this choice is the need for really generous dimensioning, and not only in appearance (some components simply ‘big’ like certain capacitors the size of beer cans) but in the really significant points: power supply as a whole, transformers, power devices, heatsinks. A further consequence, secondary but not negligible, is the need for adequate mechanical dimensioning.
• Going into more detail about the design of the power supply section, in general we always have 2 transformers and at least 5 separate power supplies (in some cases more). The valve section power supply has a pigreco filter and is stabilised (with a MOSFet). A pigreco filter is also used in the power supply section of the power stage (an uncommon choice) and in some cases these filters are inductive (an even less common choice). Another absolutely uncommon choice is the use of distributed capacitances, a technique that I have been using with satisfaction since the early 1990s: instead of just two “big-and-slow” capacitors physically distant from thestage, I prefer many “small-and-fast” capacitors, with the latter placed very close to the power devices. This solution guarantees very high peak current availability, with a speed that is simply not achievable when there are long cable lines between the capacitors and the power devices involved.
• An extended and careful focus on both measurements and listening, with an optimisation that means a continuous transition from the design phase and measurement bench to the listening room and back again.

We have already highlighted the need for an adequate mechanical structure with appropriate dimensions to achieve our desired sonic results; since we believe that mechanical behaviour also has an important influence on the sonic result, we have dedicated the utmost care to our enclosure designs. The mechanical construction of our enclosures are built to laboratory instrument grade standards while the industrial design makes it immediately clear that these are entirely Made in Italy creations.

It is important to emphasise once again that when we refer to listening evaluations and optimisation of the sound result, we never focus on just one aspect of the result (the best resolution, the highest dynamics, the greatest impact, the deepest bass, the fastest speed, and so on). If you take one aspect to extremes, you do so to the detriment of the overall balance, losing the harmony between the various components, and you end up losing the music and musicality. Our research aims at achieving the best balance between all reproduction parameters, as each one is important in music.

The aim of each Riviera product is to reproduce music as naturally as possible, as in a live concert.