My friend, John Nunes, pointed me to a very interesting paper on the distortion of op-amps (more on this later).  I started poking around the site it resided on and found an excellent “Notable Site” for Clarisonus! This is the SG-Acoustics site, created by Samuel Groner, a masters student at the Zurich University of the Arts in Switzerland. As with many of the other Notable Sites, this one is a labor of love by a single creator, where he has posted information or research for the world to see, with no immediate commercial gain involved.  Samuel’s crisp, clean site gives his parts selection criteria, some mic amp and discrete op-amp circuits, test circuits, power supply designs, etc. Some Gerber files (for PC board layouts) are included.  All Samuel asks is that you read and agree to his “Terms of Use” and “Disclaimer”, basically asking permission for any commercial use of his designs or information.

The high-light of the site is Samuel Groner’s paper “Operational Amplifier Distortion”, published Sept. 1st, 2008. With excellent Swiss thoroughness, Samuel takes the op-amp measurement criteria established by Walt Jung in 1986 and extends it, both in methodology and in covering contemporary audio op-amps.  The full test methodology is given, which only starts with measurements taken by an Audio Precision System One. MATLAB is used to crunch and present the data. For anyone who has casually dropped an op-amp into an audio circuit without much thought, this paper is a wake-up call.  He presents distortion mechanisms most engineers have not thought of, including:

  • Transfer Linearity
  • Common-Mode Linearity
  • Output Linearity (effect of loading)
  • Input Impedance Linearity
  • High-Frequency Linearity (inverting and non-inverting)

Future tests envisioned will include PSRR non-linearity, input impedance non-linearity, and non-linear AC input bias currents. His test methods look solid. Anyone contemplating using op-amps in audio should read this paper!  The other data on SG-Acoustics is valuable, too.