A modulated power supply comprises first and second coding units. The second coding unit is responsive to a feed-forward error signal representing the error between the coded output of the first coding unit and an input modulating signal. The second coding unit codes for the more limited range of the error signal, thereby improving resolution. The first coding unit can comprise M coding units arranged in parallel with one another. The M coding units are operated in a manner which increases the apparent number of quantisation levels. This is achieved by feeding a modified version of the input modulating signal to each of the M coding units or by offsetting by quantisation grids of the M coding units. The improvements are particularly useful for improving performance where the input modulating signal has a wide bandwidth.