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Obi110 and corded phone results in ground loop hum, how to get rid of it?

Started by TimSpencer, December 15, 2012, 08:46:46 PM

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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.

Lavarock7

Years ago I would have just headed down to Radio Shack and bought a cheap isolation transformer (120 v to 120 v). They are not that plentiful or cheap anymore.

My websites: Kona Coffee: http://itskona.com and Web Hosting: http://planetaloha.info<br />A simplified Voip explanation: http://voip.planet-aloha.com


sailing

AceE3

I made an assumption that a Lynksys or medical grade power supply would work based on other posts and an assumption on my part.

I don't have a clear vision of what your setup is but if your setup, without the Obi, doesn't have hum and then with the Obi inserted in the setup, does have hum, it probably is the Obi.

Another check would be to have the Obi setup. Unplug all phones. Then plug one corded phone in that does not use a power adapter. If the hum is present, it probably is the Obi. If you plug a single line phone directly into the Obi, I doubt there will be any hum. Another indication it probably is the Obi.

An isolation transformer, as mentioned on the previous two posts, would probably reduce or eliminate the hum. Unfortunately they aren't easy to come by and they aren't cheap.


Lavarock7

I searched yesterday before posting the isolation idea and saw that same ebay listing. It is a two wire (no grounding lug) and may work. The other idea I had was that in my case, I would investigate using two back to back transformers since the unit doesn't draw much current. Something like a step down and a step up transformer. It doubles the cost but still isolates the output. Regulation may not be great  as the 120 fluctuates.

Here is a possibility: alliedelec.com model Triad Magnetics N-68X
http://www.alliedelec.com/search/productdetail.aspx?SKU=70218526#tab=specs
a single one at $11 and you use the 120 v taps on each end. Then again, you have to assemble and put in a case.
My websites: Kona Coffee: http://itskona.com and Web Hosting: http://planetaloha.info<br />A simplified Voip explanation: http://voip.planet-aloha.com

sailing

Here are isolation transformers I would recommend, if you want to try this.

http://www.digikey.com/product-detail/en/171A/171AHM-ND/459365
Digikey part number is    171AHM-ND
The plug and outlet are already wired in.

http://www.alliedelec.com/search/productdetail.aspx?SKU=70218525
This isolation transformer requires wiring. It is a better choice then the other post recommends.

By the time you get done buying components to wire the one from Allied in a safe manner, the cost won't be that different. Considering the time and effort required to wire a transformer, I would go with the Digikey transformer.

Disclaimer: Although an isolation transformer should eliminate the hum problem, I have not tested it so I make no claims. I only post this to make it easy for someone to try this. If you do, please post your results.

Diana

There are a couple of things you should try before going out a purchasing extraneous devices:

(1)  Check the box where your old telephone cables enter your house and check to see if it is grounded to say the water pipe and/or a ground rod.  Is it the same grounding source as your electrical system?  Is the receptacle serving the OBi properly grounded?

(2) If you have additional/extra pairs of telephone cables running from jacks, try switching to another pair throughout.  There might be an "offending" pair between two jacks that is giving rise to this hum.  You could test one at a time to isolate which jack is the source of the hum by starting with one jack and testing the phone, while adding additional jacks.

(3) Since you have a Cordless Phone System, why worry about the corded phone......

sailing

I used a Lynksys adapter before the Obi. There was no hum with the same setup. Of course someone else's wiring could be a problem accentuating the problem but I believe the problem is the Obi.

Myself and others have focused on the power adapter because that is the only part that we can change or adjust but I believe the issue has to do with the way the audio is driven over the telephone wires. The original telephone systems used a transformer to couple the audio. This technique produced balanced lines which rejected noise that was injected into the line. With the latest electronics, a transformer was replaced with opamps that would try to mimic the noise reduction capability of a transformer. It is possible to achieve good noise reduction with electronics but the circuitry must be designed with this in mind. Since I have not taken apart an Obi, and I don't intend to, and without a circuit schematic, I am speculating that this is the case. Below are to reference links on the topic. The first one is very technical. They both sum up the issue very well.

http://sound.westhost.com/articles/balanced-2.htm#s1
http://yarchive.net/phone/60hz_hum.html