I'm trying to build a low cost hifi (Hiraga Le Monstre) but I have two old computer PSUs at hand so I tried to use it as power source. The Monstre needs +12V and -12V, computer PSUs have both, but the negative output can only supply 0.8A or so, which is probably not enough.

My idea was this. PSUs are supposed to be galvanic isolated. Which means I should be able to connect one's GND to the other's 12V and use it as a floating common ground without burning something and getting -12V referenced to the common at the original GND. Seems like my plan fails somewhere. I was cautious and connected the two PSUs through a 630mA fuse, which burned when I turned them on. Both units worked independantly so they didn't break, only the fuse burned.


What was I missing? Or any ideas how to get it done safely?


1 Answer 1


Standard computer ATX power supplies are not isolated contrary to your understanding. The mains plug earth/ground is internally connected to black ground wire of the output side.

Therefore, if you connect two ATX computer power supplies to earthed mains sockets, the black ground wires are already referenced to same potential as they are connected together via the mains earth, so they cannot be connected in series to create +/- 12V supplies, or 24V supply.

  • \$\begingroup\$ I suspected something like this. What do you recommend? Is there a cheaper solution (I don't want get expensive stuff before confirming that the amp works at all) than getting a toroid transformator? Maybe I can use the PSUs in parallel to get like 1.6A on the negative line but it's class A, so it may still be less than required. \$\endgroup\$
    – balu
    Commented Feb 15, 2020 at 21:15
  • \$\begingroup\$ the easiest safe fix it to open one of the power-supplies and cut the connection between ground and 0V and make it instead connect between ground and +12V. doing this may require fitting insulating washers where the board mounts to the case etc. \$\endgroup\$ Commented Feb 15, 2020 at 22:28
  • \$\begingroup\$ I don't think it is safe to suggest modifying internals of mains powered power supplies to random tinkerers. Besides usually ATX power supplies connect the mains plug earth directly to the metal case, and the earthed metal case connects to PCB mains input filter via the mounting screw tab(s) only. Isolating all mounting tabs would disconnect the mains input filter too. Usually the output 0V is connected to the earthed metal case with one or two mounting tabs, so only those should be isolated, without modifying mains input filter. But the output 0V can be connected to earth some other ways too. \$\endgroup\$
    – Justme
    Commented Feb 15, 2020 at 22:48

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