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. The main purpose of this power supply is to boost the amplifier front end power supply rails above the main power supply rails. It also can be used to power operational amplifier circuits.

Design presentation

Design considerations

Power requirements for the auxiliary supply are pretty low and available space 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 1.3kHz on and therefore helps to maintain high PSRR across a wider frequency range. While floating regulated supplies on top of unregulated ones do not make much sense, it makes this high PSRR and tightly regulated auxiliary supply also suitable for dual use as OPamp power supply. Also, active regulation in this power range does not cost much and offers a lot of value for the money.

Schematic

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

Assembly

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.