I got both units (very similar in spec) from CPC in the UK (www.cpc.co.uk). They were about £8 each so no problem there. Inside there is a pair of audio transformers which isolate the supply from the output. The Impedance matching one has a few resistors in too but nothing more than that. My guess is that the Ground Loop version just has 1:1 windings of 20Kohms or so but the Speaker level impedence matching one has 4ohm primarys and 20K secondarys.