ABSTRACT
This chapter describes a phenomenon stumbled across when checking for capacitor distortion. An ideal capacitor is a perfectly linear component, but some real types of capacitor create easily measurable distortion when they have a significant signal voltage across them. A rather more serious problem is presented by the use of non-electrolytic capacitors to define a time-constant, as in RIAA equalisation or filters. The capacitor has by definition a large signal voltage across it at some frequencies, and some sorts of non-electrolytic capacitor, notably those with a polyester dielectric, can generate quite large amounts of distortion. Whenever a capacitor sample was re-measured, its distortion was lower than the time before. Plotting distortion against time revealed that under the test conditions the distortion dropped, at first quickly and then at a slower rate. Linearised polyester capacitors will never be as sweetly distortion-free as polypropylene, and the linearising effect is only semi-permanent.
