power supply for low noise amplifiers

power supply for low noise Class-D amplifiers and preamps
	48kHz Audio DSP	This project shows the low noise power supply for my class-D amplifier circuit with 2 paths for power stage and pre amplifier. Here a switch mode power supply is used as source. Circuit The +/24V from the SMP operate on an integrated voltage regulator in a standard circuit. The filter at the input of the SMP reduces interference from neighbour cables and devices. The cut-off frequency of the intermediate filter is above the value required for audio, but below the maximum of the regulator. The maximum control frequency is typically 1 MHz for these 78-series types. So 200kHz are considered. The reverse-bias diodes prevent the voltage regulator from damage if the voltage at the output becomes significantly higher than at the input, e.g. when the circuit is switched off. The 18 V generated in this way are forwarded to the power amplifier. In parallel it is followed by a similarly constructed power generation stage for the preamplifier. This part of the circuit is sufficiently well decoupled using inductors and capacitors. The measured ripple voltage is only 2 mV and mostly noise-free starting from 30kHz. Because reverse-bias diodes in the second stage introduced some noise bypassing the regulator, they had been removed. If in place, a parallel 1nF in front of the regulator or afterwards might help too. Read about the former power supply See the power amplifier page