I included some details from my 1978 M.Sc dissertation in the distortion measurement design note, with some updates, but there have been a few requests for more details, so here is the full original version in a choice of two formats:
Word document, 1,130 KB.
PDF file, 1,245 KB.
Thanks to Dimitri Danyuk for help with the OCR conversion from the scanned original to a more readable form, and thanks to my supervisor Peter Williams at the University of Wales, who persuaded me that the simple test method used was worth investigating and developing.
A more recent development is a much easier method, but suitable only for a particular amplifier configuration, and used to test my mosfet designs. These are inverting amplifiers with very high feedback loop gain and a unity gain low impedance output available from the input stage. This makes distortion extraction very easy, even when using music signals and speaker loads, as described HERE..
The Electronics M.Sc course in the Physics department at the University of Wales included a 9 month course of lectures and written exams, followed by a 3 month project presented as a dissertation. I only had financial support for the first 9 months, so the practical work had to be done quickly, and was finished in less than six weeks. I had hoped to do more extensive investigations of amplifier distortion, together with development of a new idea for a feedforward output stage, but that remained in a folder for nearly 20 years before I actually made a working feedforward amplifier, which was described in my April 1998 Electronics World article.