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If you have totally isolated the transmitter, then I have to wonder if something else in your home is picking up the RF and rectifying it. Most DC supplies, like “wall-warts” and those inside of small appliances do not have bypass capacitors across the diodes, as the ones in your SSTran do. This is one place where rectification can occur.
It appears that as you get further away from the antenna, the re-radiated signal picked up on your house wiring starts to dominate over the direct signal from the antenna. Then, you start hearing the hum.
BTW I found out that if you leave jumper S1 in due to the need to control hum from your audio source (as explained above), that will still allow a significant amount of RF to be imposed on the ac line through your audio gear. This of course pertains to normal operation, i.e. when you have something connected to the audio input. To eliminate this RF path, the easiest way I can think of would be to obtain a fairly large ferrite toroid of #77 material and wrap the audio cable around it for as many turns as will fit. This would serve as a common mode choke for the RF, but would have no effect on the audio signal.
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