Looking into rear of the A590 unit:
u
4 1
3 2
u = keyway
1 = +5V
2 = ground
3 = power sense
4 = +12V
It IS the same as a CD32 PSU and also pin compatible with C-64's 1541-II diskdrive's PSU.
If you wanted to do some soldering there is a way to make the A590 draw it's power from the A500
http://members.iinet.net.au/~davem2/overclock/a500hd.html