I'm still working on 60 Hz (and harmonics) interference here at Station 1093, North Bethesda, Maryland. I decided it would be useful to rig up a way to be able to move my H-field antenna and Amplifier 12 around the house and listen to the Channel A and B outputs using earphones to see if I could find a quiet spot in the house. (Sadly, all I've found so far is places that are more noisy than where I have the station set up.) I bought an In-Line 1/8" Phone Jack, P/N 274-0274, from Radio Shack, and made up a two-foot-long single-ended Ethernet cable according to the T568A wiring convention (
http://en.wikipedia.org/wiki/TIA/EIA-568). On the Amplifier 12 control connector, pin 4 (T568A blue wire) is the AMP-B output, pin 5 (T568A blue/white wire) is ground, and pin 7 (T568A brown/white wire) is the AMP-A output. I connected pin 7 (brown/white wire) to TIP on the phone jack, so AMP-A drives the left earphone, pin 4 (solid blue wire) to RING on the phone jack, so AMP-B drives the right earphone, and pin 5 (blue/white wire) to SLEEVE on the phone jack. See
http://www.connecthowto.com/20091227/connect-how-to-stereo-plug/ . I'm still operating tethered to a 9V wall-wart powering a 5V linear regulator that in turn powers Amplifer 12 through the mini USB connector, but the next step is to power the 5V regulator from a battery. The pot on Amplifier 12 works fine as a volume control, and with my earphones, I can get plenty of volume, probably enough to make a teenager cringe.
The attached photo shows the connections to the phone jack. I have a stereo cable plugged into the jack in the photo.
As Don points out in his message below, it may be more convenient to attach the phone jack to the test connectors, depending on what you have in your junk drawers. I happened to have a half-dozen Ethernet plugs, some Ethernet cable, and a crimping tool handy.
Of course, the bandwidth of this system is limited--given my age and exposure to rock concerts, airplane engines, small arms, and lawnmowers, maybe 10 kHz if I turn the volume up high enough.
Update 8/4/2014: I implemented the battery power option and went on walkabout through my house yesterday afternoon, and found a reasonably quiet spot in the northwest corner of the house on the lowest floor. It seems to be immune from the bathroom dimmer switch on the top floor, and the LED lights in the kitchen don't seem to interfere either. There's still something, though... But right now, at 5:20AM, my noise level is down to less than 60 mV pk-pk (as measured by the controller) and my gain settings are 16 x 4 x 40 = 2560.