The phantom supplies were usually voltage doublers off the AC and then series regulated…. but nowadays there’s a very much better way: Go on ebay and buy a DC to DC converter board for just a very few pounds. I have to admit that I now use these boards myself on new equipment to achieve the 48V phantom power. They are reliable, quiet and extremely efficient. Just connect one to the +15V rail and adjust the output to 48V….. it really is that easy!