This is a basic circuit dual power supply which fixed output voltage.This circuit used widely in general OCL power amplifiers.The output voltage calculated by:
Vo = 1.414 x Vac
When Vac = Secondary transformer voltage .
for exaple if transformer is 24-0-24 V the output voltage is
1.414 x 24 = 34V
The capacitor should have rated voltage greater than 34V.For diode rated voltage should be greater than 2×1.414xVac.source

15V dual Power Supply for Preamplifier based on 7815 and 7915 regulator.This circuit will not work with external DC power supplies.The rectifier is a “full-wave voltage doubler” type, and with an input of 16VAC will provide about +/-20V DC at a current of 100mAsource

A power supply that is suitable for most amps of 60W output. Can be adapted or modified to other voltages for more or less power.A power supply suitable for use with the 60W amplifier presented in the predeeding project is perfectly simple, and no great skill is required to build (or design) one. source