For long time Metrum Acoustics has used hybrid (triode) amps for pleasure listening and critical evaluations. What we did not like was the fact that triodes suffer aging problems. In simulation software, we started to simulate triodes with fets instead. The good thing with triodes is their low gain and a lot of bandwidth without any need for feedback. As a result we realized a full class A high voltage fet circuirty having the right gain ,very good open loop bandwidth and low noise figures. A similarity is the high voltage of 350 Volts to get this circuit running. As a result an amazing linearity without any feedback was realized. From this point the circuit is driving a class A/B output section (10 watts in class A). Some little feedback is necessary to get the right gain but also improves the damping factor at the same time . Compared to our old hybrids, there's no sonic difference, just less noise and lower distortion.
After two years of prototyping and testing, we've now turned this into a commercial product. The circuits are quite overdimensioned to be sure that no errors occurs. Still we added a lot of protection to avoid any damage to your loudspeakers in case of malfunction. A soft start procedure is added to charge the amazing capacity of 176000µF. This huge capacity is available during complex and dynamic transients. As known hybrids combine tube sound and amazing control as realized by transistor designs. Here you have the solution. The "Forte" power amplifier will serve you without problems as known with normal hybrids but still having the holographic sound as known from good tube designs. The Forte is an addition to our awarded Adagio digital amplifier.
Srajan Ebean of 6moons about the Forte
With the Forte, that cat is out of the bag now. This Dutch designer's inventiveness isn't limited to digital. When he attacks a different component category, he applies just as much out-of-the-box thinking as he's known for with R2R converters, split processing with error-forward correction and variable reference voltage for volume to adjust circuit gain directly rather than attenuate a fixed signal. Next time you think of tube hybrid amps whilst surveying the compact scene of Unison Research & Co., be sure to include Metrum Acoustics. They'll get you there without the noise and aging of glowing bits. What a perfectly executed trick!