In Linear Audio Vol 12, Vincent Thiernesse proposed Efficient, unity power-factor, low cost audio power amp supplies. The article discussed the concepts and a general imnplementation. Vincent also wrote about this in the AudioXpress June 2016 issue: You can diy an ecological power supply for power amplifiers. That article describes a practical implementation. The folks at AudioXpress kindly placed it online; it nicely complements the L|A article. Recommended reading.