Auxiliary power supply for the modular high end audio amplifier system

Project presentation

Abstract

The small auxiliary power supply presented here is part of my modular audio amplifier system. It can be used to power some circuits that require a clean power supply like operational amplifiers, and also can be used to boost the amplifier front end power supply rails above the main power supply rails. For boosted power supply rails, my full wave voltage doubler module may be a better option in some cases.

Design considerations

Power requirements for the auxiliary supply are pretty low and space available inside the chassis is also limited. This is why I settled for a 10VA transformer. The rectified voltage is then filtered using Pi filters and fed into linear regulators. The passive Pi filters ahead of the linear regulators may seem unnecessary, but considering the typical PSRR of a linear regulator, they really make sense. The 7815 regulator for example has a PSRR of roughly 75dB at 100Hz, which drops to 55dB at 10kHz and 45dB at 100kHz. The L-C filter improves PSRR from 1300Hz on and therefore help to maintain high PSRR across a wider frequency range.

Schematic

Note that the schematic does not show 100mΩ resistors in series with each inductor. This resistor is implemented on the PCB and serves to lower the Q factor of the filter, which otherwise would resonate without the extra series resistance.

Assembly drawing

Assembly in CAD

Assembly in reality

Summary

This power supply is versatile, small, reasonably low cost and powerful enough for many applications. Two of them easily fit inside the chassis of the amplifier.