In this a low noise power supply for both the microphone amplifier and the high power output stage.
 

Circuit

function

 The approximately 16Vrms from the transformer are RF-filtered and rectified in the usual way. The filter at the input reduces interference from neighbour cables and devices. After further filtering, an integrated voltage regulator in a standard circuit is employed. The cut-off frequency of the intermediate filter is above the value required for audio, but clearly below the maximum of the regulator. The maximum control frequency is assumed to be 500 kHz - typically, 78-series types can handle up to  1 MHz under a moderate load. 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 15 V generated in this way is forwarded symmetrically to the power amplifier.

In parallel it is followed by a similarly constructed stage for the preamplifier. This is sufficiently well decoupled using inductors and capacitors.

The measured ripple voltage is only 3–4 mV and mostly noise-free.

Hint from the forum: The reverse-bias diodes at the second stage might introduce some noise bypassing the regulator. One reason could be the capacity of the charge zone in the semiconductor crystal. A parallel 1nF in front of the regulator or afterwards might help.

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Power Amplifier