• OBI 202 VOIP interface has HUM on phone line

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    Background:  I have an OBI 202 – 2 line VOIP (ATA) internet phone interface.  I have had it for over a year with NO problems.  I had an old AT&T 1872, 2 line phone, but it was having issues. HOWEVER there was NO HUM heard on that phone.   I bought a new AT&T ML 17929 2-line phone.  It cost around $63 on Amazon. It is a very nice phone (features fit my needs) and the price was good compared to other options (which were more than twice the price).

    There is a hum (like a 60 or 120 hertz hum) on the phone whenever that line is connected to the phone.  If I turn my volume low it is not too loud, but callers can hear it on their phones.  Note that the hum appears on BOTH lines (the other line is connected to a standard (POTS) AT&T landline).  If I remove the phone line connect to the OBI the hum is gone (even from the AT&T line).  It is not the specific phone (I got a replacement and it has the same problem).  It could be the power connector (which is a typical 120v AC to 12v DC unit).  It also could be the design of that phone (does not have adequate filtering).

    Is there any way to filter the power going into the plug (I have tried different outlets) or the phone line between the OBI and the phone?

    • This topic was modified 2 weeks, 2 days ago by Drac144.
    Viewing 3 reply threads
    • #2480813

      Phone line filters may help when connected to the phone systems.

      All wiring coming in from the street could be ancient old and connections rotten to the inside of your home, also the internal wire needs to be checked by the provider. Please kindly and firmly make it an issue indeed because a bad connection causes all manner crazy irritation to certain folks.

      Maybe your OBI box has faults which OBiTalk can check.

      All choices can be effective in defeating the humming noise.


    • #2480887

      A few things you can try.

      Verify both the ATA and the phone are not within 3 feet of anything that uses WiFi.

      Verify the phone cord between the ATA and the phone does not run “parallel” to a power cord, surge strip, etc. If it does, re-position it as close as possible to 90° to prevent it from picking up the 60 Hz power hum via electromagnetic induction.

      Replace the existing phone cord between the ATA and the phone with one as short as possible.

      If the phone cord is already short enough, replace it with one containing a “ferrite core” or purchase one of the snap-on style cores and add it to the cord.

      Place a DSL filter between the ATA and the phone.

      Note, it’ll need to be one specifically designed to filter 2-line phones (i.e. 4 pins instead of just 2 in both the plug that you’d connect to the ATA and the jack you’d plug the phone cord into.)

    • #2480963

      Thanks for the replies.  I have eliminated all options except using the ferrite cores or DSL filter – which I thought were for RFI.  I assumed AC hum is probably from a power supply without adequate filtering. I did not think ferrite cores would help with that.  If I am wrong, please let me know.

      Please note that I have used the ATA with a different phone for over a year with no issues.  The problems started when I got a new phone.  I tried a different power cord (with built in power supply – 120vac to 12 vdc) for the ATA and had the same problem. At this point it is either something inside the ATA or it is the phone.  Again, I tried TWO different phones (same model) and both had the problem.  I am expecting a new ATA in two days (a different manufacturer).  That should determine if it the ATA or the phone.  My educated guess is that it is the phone. I think it is assuming a POTS line with pure DC current so no need for any filtering and thus the AC hum from the imperfect ATA power supply (or the unit itself) is causing the phone to hum.  A new ATA is WAY cheaper than a different phone so I am hoping it is the ATA.  If it is the phone it will be around $100 more to return this new phone and get a better phone (which may or may not have the same problem) as opposed to $36 for the new ATA.  I got all this on Amazon so returns are easy and free.

    • #2480994

      While ferrite cores and DSL filter are designed to stop RFI, they’ll sometimes also stop 60 Hz AC power hum.

      BTW, in your initial post you indicated the ATA is only being used for one of your 2 lines with the second line being an AT&T POTS landline.

      Check the plug on the phone cord between the ATA and the phone.

      If it has all 4 wires, the hum could be coming from the unused “outer pair” in the ATA jack which are meant to support a second ATA line.


      If it has 4 wires, try replacing it with a cord that only has 2 wires and see if that eliminates the hum.

      FYI, most “cheap” phone cords or those that come with FAX machines typically only have 2 wires.

      • #2481041


        As I said I used this config with my previous phone.  I am aware of the 2 wire/4 wire issue.  The ATA has 2 phone jacks. One (port 1) has 4 wires (2 lines) the other (port 2) only has one line.  So if I plug a 4 wire cable into port 1 and plug it into line 1 of my 2-line phone both lines from the ATA are now connected to the phone.  My config has a 4 wire cable from ATA port 2 to line 1 on the phone and 4 wire cable from the phone jack in the wall (POTS) going to line 2.  I have also tried a 2 wire cable from the ATA port 2 to the phone line 1 with the same results (hum).

        I suspect if I connected a good DC 12V power supply to the ATA the hum would go away. I have the power supply but not a cable with the proper connector.  The new ATA, which is now supposed to arrive on Saturday, has a USB cable with a micro USB connector for powering the ATA (which uses 5V at 1 amp). I can try plugging it into my computer’s USB port to see if that has any effect (if the included power supply causes hum).  I suspect that the computer has a much cleaner 5 V DC output than whatever little plug/transformer comes with the new ATA. I will check out the DSL filter option just in case the new ATA also has hum.  Thanks for your continuing help.



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