Total Harmonic Distortion Analyzers

(Link to HP, Tektronix, Krohn-Hite, Heathkit and  Boonton Analyzers)
Audio Magazine (sorry, they are out of biz, that's why I produce it below) article on a high performance DIY THD analyzer  

Build a High Performance THD Analyzer Part I  730kB

Part II (1.2mB)

Part III (Includes Correction) 730kB