Persistent hum on my connected phone

Started by Jeff205, February 22, 2011, 04:11:07 PM

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Thanks for your suggestions:

It is an old Sony powerless phone (and single line phone).

Reversing the polarity of the power supply (plugging it in upside down) made no difference.

I've switched to the cordless phone so cannot try pulling the Ethernet cable. But I did move the Obi close to my router and tried a 12" Ethernet cable, which did not help.


Quote from: MichaelW on April 25, 2013, 07:05:21 AM

It is an old Sony powerless phone (and single line phone).

Unpowered Sony. No battery even? That sounds rare.

I am suspecting that somehow the hum is being capacitively coupled to the phone by your hand. If that is guess correct, then grounding yourself would cut the hum. If that is the case, it sounds like a quirk of the phone.


Batteryless phone is a Sony IT-B5. Has 10-number memory, even without a battery. Solid and dependable, I don't recall it ever causing hum before.


I would still try the "shielded" (metal shield visible at the ends) vs "unshielded" (clear plastic head) Ethernet cable. The majority of Ethernet cables people run into are unshielded.


Quote from: Ostracus on April 25, 2013, 08:17:42 PM
I would still try the "shielded" (metal shield visible at the ends) vs "unshielded" (clear plastic head) Ethernet cable. The majority of Ethernet cables people run into are unshielded.

Thanks I will look for one of those.


Quote from: Jeff205 on March 09, 2011, 12:38:47 PM
Even when I break dial tone, the hum is still there, but only when 2 lines are plugged in to the phone.  My home phone service is actually VOIP provided by my cable provider, so even going to the box outside would not work (plus it's only when 2 lines are connected to the phone).  It does not matter which line I remove, the Obi or my home service, and the hum goes away.  I tried 2 and 4 wire lines as well with no success.

And do you think we both received a bad power supply?

Still can't figure this out!!!!

It sounds to me like the FXO line output in the Obi is sensitive to noise cross-talk in the phone from your PSTN line in to the Obi line in.  It may occur on two line phones that don't have very good isolation from line 1 to line 2.  That creates a huge connection from the Central Office to your phone's line 1 port to phone's line 2 port (from parasitic capacitance or some other kind of coupling), to the Obi.  Unplugging either line breaks the connection.  I think the Obi may have designed its FXO line output to be compatible with short, fully isolated (from earth) line connections and it may have noise or EMI susceptibility when coupled to long, non-isolate feeds as your PSTN line would be.  Poor longitudinal balance (as Michigan Telephone pointed out) on the Obi's line port could be a possible susceptibility.  Longitudinal balance isn't as big of a problem on short loops, such as the Obi would normally be feeding.  Unfortunately, if this is correct, then I can't really think of any good solution for it at the moment - except to use the Line port of an Obi110 and then bridge it to its phone port or even the other Obi's phone port using a SIP gateway, but I think that would be overkill.


Quote from: MichaelW on April 24, 2013, 10:05:50 AM

I can stop the hum, and stop the distortion my caller hears, but touching the metal rim around the Ethernet port on either adaptor.

Anybody have any ideas on this, please?
Maybe it's a cable/grounding problem.

Some info here:


I have an Obi200 (purchased late 2013 or early 2014, can't remember) that started exhibiting this problem with a new 2-line desk phone (AT&T) I installed last week.  My previous 2-line desk phone didn't exhibit this problem with my Obi200, or the Obi100 before that.

It was definitely a low-level, electrical hum.  And it happened only when I had 2 lines plugged in.  If I only plugged in my POTS line, no hum.  If I only plugged in the Obi line, no hum.  And it didn't matter which jack I used for which line ... when both lines were plugged in, in any configuration, it always created the hum.

So I went and found another powersupply to use with the Obi with the same output ... changed it ... and the problem is gone.  No hum.  So, clearly, the power supplies that Obi is using are susceptible to this.  I don't know what the replacement one came from - I've got a box of them I've collected over the years.  I just grabbed the first one I found with the same output, and problem solved!


This was talked about awhile back. The explanation is in the thread;wap2  In summary, the hum is related to how the phone lines are run in your house (which you cannot change) and the power adapter. The power adapter has a capacitor connecting ac line in to the output. If this cap is removed, the hum goes away. If you buy a different adapter and the hum goes away, then the value of that cap is smaller. If you were to plug the adapter into an isolation transformer, that would eliminate the problem also.

In the above mentioned post, I was a little more technical.


As soon as I saw your post, it peaked my interest. I too saw this issue with a new Obi 200 recently. I had a 2 line phone with 1 line connected to the Obi and the second connected to POTS. If I had only POTS connected or only the Obi connected to the phone the hum was negligible. There have been lots of discussions about hum with the Obi 110 which has its own POTS connection so everything added up to the power supply that came with the Obi 200. Most cheap adapters that come with everything these days are of the "switching" type instead of being linear devices, meaning the switching adapters do not have a transformer. That is one of the things that make them cheaper. Long story short, I purchased a 12V Linear adapter from Jameco and the problem goes away. But they cost $15 and are heavy so shipping can be steep.  Jameco part # 170245. YMMV


Some background on "switching" power supplies. The isolated versions, such as the wall adapters, have a transformer. The incoming line ac voltage is rectified to dc then switched at a high frequency which is transferred through the transformer which is then converted back to dc. "Linear" power supplies transfer the ac through the transformer with a frequency of 50/60Hz (depending in US or Europe). The switching power supplies can switch at 100KHz and up. The higher the frequency, the smaller the transformer.


That is good information and I stand corrected. I've heard rumors that some of the switching supplies could actually work but I'm not sure there is any way to know in advance which ones will work. It is true that the only linear supplies I've seen only work with 120v AC / 60hz due to the transformer.

*edited* I should have said the only ones I have seen with a transformer were strictly 120v / 60hz. I had no less than 3 of the "others" that I tried and none of them worked any better than the one that came with the Obi. Thanks