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 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

  • Operates by subtracting input from scaled, phase-matched output of DUT
  • Useable with THD analyzer, or spectrum analyzer or other instruments
  • Greatly enhances the dynamic range of THD and spectrum analyzers
  • Coarse and fine amplitude and HF phase adjustment
  • LF phase adjustment is optional
  • Selectable magnification of 1X (bypass) 10X or 100X
  • Balanced differential inputs from DUT amplifier
  • Eliminates issue of distortion and noise inherent in the test source
  • It is as good as the op amps it uses
  • Avoids noise issues in source and in THD analyzer
  • Does not avoid noise introduced by DUT
  • DUT noise rejection is the job of the spectrum analyzer
  • Also valuable for use with PC-based THD and spectrum analyzers
  • Op amps are used: NE5532 or LM4562
  • Measurements below -140 dB are achievable

A complete description and construction project for the Distortion Magnifier can be found in Vol. 0 of Linear Audio ( The DM and its use are also described in Chapters 21 and 22 of Designing Audio Power Amplifiers.