Hum voltage at the ouput of a rectifier, capacitively or inductively loaded, is generally too high. Auxilliary low-pass filters will then be connected at the output of the power supply. There is two main kinds of filters :
RC filters
LC filters
In all cases, filters are characterized by a rejection, denoted by
A, equal to :
Hum voltage at the output of the filter is equal to input hum voltage
multiplied by rejection. When cascaded filters are used, overall
rejection is approximately the product of individual rejections. It
worth be noted that rejection of a LC filter (second order) is much
higher than that of a RC filter (first order) for a similar LC-RC
product. RC filtergin is nevertheless very useful for feeding input
stages from an already LC filtered supply.