A cube-law power amplifier - Class A at 1/4 of the dissipation

A Cube-law audio power amplifier – class-A at ¼ the idle dissipation           Vol 8

Ian Hegglun

Ian’s article in Volume 1 discussed the design of a square-law amplifier, that is, and amplifier that deliberately uses (output) devices with a square law transfer characteristic, to construct an amplifier that operates in class-A yet requires much less bias current and therefore has much less dissipation than an equivalent traditional class-A amplifier. Not one to rest on his laurels, Ian has extended the concept to using cube law devices. These new breed of amplifiers show class-A operation with only ¼ the dissipation, genuine ‘green class-A’! The new 100W amplifiers operate in class-A for most of the power range, but the power supply and heatsink requirements are like those for a typical 100W class-AB amplifier. Better yet, it is also possible to bias this amplifier in class AB, for even better efficiency; in this configuration, the virtual absence of switching and cross-over distortion will result in a class AB amplifier that is as clean as a class A amplifier! Options for moderate feedback are provided, and the proposed PCB also contains a distortion measurement and indication circuit as well as a clipping indicator. The free PCBs with this article are all gone; additional boards are available from Ian’s partner in this project Mason Green at www.greenchild.us