The first class D amp I was aware of was made by Infinity way back when (before they made speakers). The idea is nothing new and has been around for awhile. I think most car amps are Class D. It makes allot of since in auto and mobil applications. However, a liner power supply is still the way to go for the best sound and all designers know this. It does cost allot more and wastes energy and maybe it is getting harder to source really great transformers?
