The Distortion Magnifier
The Distortion Magnifier (DM) provides a way of measuring very low levels of THD and IM distortions. A semi-kit containing the PCB and some selected parts is available from Pilgham audio at www.pilghamaudio.com. From the Pilgham Audio home page, just go to DIY > DIY support > Cordell DM.
The most straightforward use of the Distortion Magnifier is with a conventional THD analyzer. The DM is fed the source sinusoid and the output of the amplifier under test. These input and output signals of the amplifier under test are scaled to be of equal amplitude, adjusted for exactly opposite phase, and subtracted. A selectable amount of the signal from the AUT is then added back to the result so that there is some known value of the fundamental for the subsequent THD analyzer to lock onto. Typically this process results in a relative magnification of the distortion by a factor of ten or one-hundred.
Features and issues
A complete description and construction project for the Distortion Magnifier can be found in Vol. 0 of Linear Audio (www.linearaudio.net). The DM and its use are also described in Chapters 21 and 22 of Designing Audio Power Amplifiers.