Gramophone preamplifier noise calculations – the 3852 Hz rule revisited Vol 8
Marcel van de Gevel
The noise of an amplifier can be represented by an input noise voltage source and an input noise current source. But some design decisions that reduce noise voltage result in an increase of noise current, and vice versa. The trade-off is determined by the source impedance. It gets complicated when you are interested in the total noise across a wide band while the source impedance changes substantially across this band as with MM cartridge.The RIAA-correction and the frequency-dependent sensitivity of the ears of the listener further complicate matters. In an earlier article in Electronics World in 2003 Marcel has shown that under a couple of simplifying assumptions, optimising the RIAA- and A-weighted noise of a gramophone amplifier is equivalent to optimising its spot noise at 3852 Hz. This article builds on that work, and the 3852 Hz-rule has been extended to other noise weightings and to gramophone amplifiers applying a relatively low load resistance to a moving-magnet cartridge. Comparing the results to a simulation with a more accurate cartridge model including loading shows that the rule is accurate enough for noise calculations.