It is well known that Class-A gives the best sound quality in audio power amplifiers but the efficiency is so low that it is impractical to build much more than around 20 watts per channel. The Class-A amplifier here seems to break this rule it can deliver 50 watts in Class-A using the same parts as other 15 watt Class-A amps. This breakthrough is achieved using Square-law curve devices in push-pull. The article gives the information to build your own efficient 50 watt Square-law Class-A amplifier as well as efficiency- and distortion calculation methods. Ian also gives all the information you need to build even bigger and better Class-A amplifiers and still be 'green'.
Square-Law class-A: a family of efficient class-A power amplifiers