This is an old thread but the issues with 60Hz hum were never completely answered. I had the chance to investigate this issue and thought others would be interested. I am keeping this as short and non-technical as possible. The FXO and FXS ports are controlled by integrated circuits (IC) from Silicon Labs. (I have an ob110.) The specifications of these ICs show good suppression down to 200Hz. Below 200Hz there is no specification. I would assume that at 60Hz, the suppression is much less so if your power lines can inject enough hum into your phone lines, it will be heard. The choice of these ICs were not a bad one. I would guess, that to have designed these devices with the same suppression that the old phones had, would have raised the price. Other analog telephone adapters (ata) I have used did not have this problem. I don't have any of these units to compare what they did to eliminate this problem but I suspect they used transformer coupling.
Running telephone wires all over the house means they run past power lines. Capacitive coupling will cause the injection of the 60Hz current. To actually hear the hum, the injected noise on the phone line needs a path back to earth ground which is through the power supply (wall wart). The power supply has current leaking through it continuously. It is only about 10 microamps but if this leakage is eliminated, there will be no hum.
Comparing a few wall warts, they all had about the same leakage current. Inside, there is a capacitor connected across the transformer. This is where all the leakage is from. There are agencies that specify the amount of electromagnet interference (emi) allowed. All electronics must meet these requirements. This cap is there for this reason. With this cap removed, my phone lines are dead quite. Don't even think about opening one of these wall warts unless you know what you are doing.
On a previous post, I had said if you tie the 12V negative output from the power supply to earth ground through a 0.1uf capacitor, the hum would be reduced. It will be but I will recommend against this unless you put voltage transient protection across this capacitor to earth ground and the phone line inputs to earth ground. You would have to know what you are doing to rig something like this up. The reason I don't recommend tying only the 12V negative to earth ground, is that under the right conditions, you could cause a failure of the phone port. Just like the 60Hz hum is injected into the phone line, if a transient, lets say due to a lightning strike on the power lines not far from your house, will induce a large voltage that will also couple into your phone line. Over 200V could be induced on the phone input to the Obi. If the negative output of the power supply is being held at earth ground, the voltage rating of the ICs will be exceeded and you will get a failure.
If you have a wall wart with the proper power supply and a ground plug on it, don't use it. The ground plug ties the 12V negative to earth ground creating the same problem described in the previous paragraph.
A non-intrusive way to eliminate this leakage current is to use an isolation transformer. The transformer rating should be 15VA or higher.
Most don't have this problem. You need to drive the phone lines in your house from the Obi, you probably have many extensions and the phone lines run past the power lines numerous times.
