On the Noise performance of Low Noise Input Stages

Low noise input stages are widely used in audio equipment, in particular for Moving Magnet (MM) and Moving Coil (MC) cartridges. An existing quality MM preamplifier can be upgraded to the sensitivity required for a MC input by adding another gain stage (a pre-pre or head amp) (HA). An input transformer has obvious noise advantages but many disadvantages. Ovidiu Popa set out to analyze the well-known architecture consisting of a single JFET input stage followed by a gain block. The extensive noise analysis leads to some unexpected conclusions regarding the noise contribution of the input stage drain load type and its insensitivity to the gain block voltage noise.