Obi110 and corded phone results in ground loop hum, how to get rid of it?
TimSpencer:
When I plug in a corded phone(any brand, I have several in the house),
there is a distinct ground loop hum. It's not very loud, but still annoys
me and the person I'm calling.
The ground loop hum is not there when I connect the Obi110 to a
cordeless phone, however.
I have tried to plug the Obi110 , the router, and corded phone in other
parts of the house, on different outlets, and even outdoors. Nothing
seems to be able to get rid of this problem
Thanks in advance for any suggestions! :)
sailing:
I gave an explanation to this in another post. See http://www.obitalk.com/forum/index.php?topic=367.msg27061#msg27061 It is a technical explaination.
I thought I also posted two possible solutions. If I did, it was removed. I suspect Obihai considers it would void the warrantee and the customer may hurt themselves if they do something wrong.
There are two solutions. One is to find an adapter with low leakage currrent. (Did you read my other post first?) Wall supplies designed for medical electronics will have lower leakage. I did not get any since I wasn't about to spend money on getting a unit I did not know what the outcome would be. Others have found Lynksys wall adapters eliminated the hum. So if you have a 12V one, you can try it. Make sure it is a switching regulated one, not an analog one. (Again, did you read my other post first?)
The solution I use, is to attach a 0.1uf, 50V capacitor from the negative side of the output of the adapter to earth ground. Earth ground it that third pin on the outlet. Since my obi device is in the basement, I tie it to the metal conduit. So as to not have to cut the wire from the obi's adapter, I cut one off from an old adapter I had and soldered in the capacitor and a female jack for the other obi's adapter to plug into. This eliminated the hum. I bought an Obi110 over the Summer and it has been working great with this solution.
If you have no idea what I am talking about with the cap solution, I don't recommend you try this. If you pick up the ground from an outlet but you plug it into the wrong pin, you will probably damage the adapter and you will toast the obi device.
AceE3:
I'm having a similar problem. Seems to be an incompatibility with my Obi110 and 2 line corded phones (I've tried 2 different 2 line phones - an AAstra business phone, and an AT&T 2 line). Terrible hum on the line when Obi110 & POTS are feeding the phone. If I remove the POTS line from the phone, the hum disappears. However, I also tried the Obi connected to my NID (disconnecting from the telco's line) to feed all the jacks in the house - and the hum was there as well (despite the 2-line phone now only being fed by the single Obi110, without POTS).
The weird things is that I've been using a NetTalk Duo device up till now, connected to my line #2 on the phone, and absolutely no hum when POTS is connected to line #1. Mind you, that device is USB powered, not 12v.
I did try both a Linksys wall adapter; as well as a certified medical grade power adapter, none of them eliminated the hum.
TimSpencer:
Quote from: sailing on December 16, 2012, 05:29:28 am
I gave an explanation to this in another post. See http://www.obitalk.com/forum/index.php?topic=367.msg27061#msg27061 It is a technical explaination.
I thought I also posted two possible solutions. If I did, it was removed. I suspect Obihai considers it would void the warrantee and the customer may hurt themselves if they do something wrong.
thanks! I just read your other thread...
I'm just going to use the cordless phone for now. It's a Panasonic cordless(1 base plus 5 handsets)
Not really sure why the hum noise doesn't affect it at all. Perhaps the fact the cordless base unit
is also plugged into the AC makes a difference? If that's the case, I'll going to find the AC adapter
for my corded phone. It uses the AC adapter for the built in lighted LED clock only. Just hope
I can still find it in the basement. :)
sailing:
TimSpencer
The explanation as to why cordless phones may not have hum is actually in my original explanation. Stating it in less techie terms, you need a long wire run to get hum and that wiring has to run past other wires or plumbing. If you plugged your cordless phone into the wiring of your house as you would your corded phone, you would get hum. If you only plugged your cordless or wired phone directly into the Obi you would not get hum.
Having thought about this problem again, I suspect Obihai left out some suppression circuitry from the device to save on cost. Since I do not intend to open my Obi or Lynksys device to see if I can confirm this, this is only speculation on my part. I'm just throwing this out in case there is another techie who might know about the internal circuitry used.
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