Power amplifiers mono — Riviera Labs AFM100 Special Edition

The AFM100 Special Edition is a mono amplifier built in Italy by Riviera Audio Labs, sold as a pair and presented as a Class A reference design. It follows a hybrid layout in which valves handle voltage amplification and solid-state devices deliver the power, and the whole circuit runs with no overall negative feedback. European pricing quoted for the pair is EUR 87,000, and two finishes are offered, anthracite and champagne.

Key specs that matter

Each chassis delivers 100 W into 8 ohms in pure Class A, and the rating doubles to 200 W into a 4 ohm load. Gain is fixed at 22 dB. Input sensitivity is given at two points, 220 mV rms for 1 W into 8 ohms and 2.2 V rms for the full 100 W, so a source or preamplifier with a couple of volts of output drives the amplifier to its rated level. Input impedance is 47 kilohms. One unbalanced RCA socket and one XLR socket are fitted, and the XLR input is described as true balanced rather than a converted single-ended connection. Each unit measures 26 cm wide, 58.5 cm deep and 27.5 cm high and weighs 35 kg.

Engineering and use context

The whole voltage gain stage uses a single ECC82, also sold as 12AU7, operated as a triode in Class A. The output section then relies on MOSFETs, with 20 solid-state output devices per channel, each rated to dissipate 150 W. That large reserve of dissipation is what lets the amplifier stay in Class A at its full rated output, and it explains why each chassis is so heavy. A custom-wound transformer of generous size feeds the circuit, and the parts sit in a monolithic chassis intended to keep mechanical stability high. Because the design runs without global feedback, each stage has to be linear in its own right instead of correcting errors around the whole loop. Class A operation at this output level means the amplifiers draw substantial current from the mains even with no signal, so each chassis needs free air around it and a solid, level surface able to carry its weight.

Pairing

Riviera builds its own valve preamplifiers with transformer-coupled balanced outputs, and one of them connected through the true balanced XLR input keeps the signal path balanced from preamplifier to power stage. Two chassis are needed for stereo. The low-impedance rating makes loudspeakers with lower impedance a practical load, and the relatively high gain suits sources with modest output.

Alternatives

The Riviera AFM25 is a smaller hybrid Class A monoblock by Riviera with a rating of 25 W into 8 ohms in a 14.4 kg chassis.