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Thread: How do I get rid of a 'hum' between to analog telephones?

  1. #1
    Flashaholic skillet's Avatar
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    Default How do I get rid of a 'hum' between to analog telephones?

    I wasn't sure what title to list to get hits, so we will try this one first.. OK, my problem and question.

    I have a simple telephone to telephone system set up to communicate with another room. I am using a 9VDC A/C adapter in series with the +9v connected to the green wire (phone 1) and the Gnd connected to the red wire (phone 2) with remaining green wire(phone 2) and red wire (phone 1) connected together.. I'm sure whoever helps understands what I am saying, if not just google "connecting 2 phones together"..

    Anyways, to the question, I have a terrible hum on the line. Works great, except for the hum... I know a battery would be better, but I would rather use the A/C adapter.. I assume the hum is coming from the A/C adapter circuitry...

    What do I need to do to get rid of the hum?

    Thanks
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    Retired Administrator Norm's Avatar
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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    Try a regulated wall wart. The hum is from the poorly built adapter your currently using.
    I have a similar set up between two houses, it's worked without problem for the last 7 years running from a regulated supply.
    You may find a high value electrolytic across the supply helps.

    See if there is anything here to help.

    Norm

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    Flashaholic skillet's Avatar
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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    I figured they were all regulated. I'll investigate that further. Any other suggestions?
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    Retired Administrator Norm's Avatar
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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    The majority of wall warts aren't regulated. Measure the open Voltage, unregulated will be well above the quoted output.

    Norm

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    Flashaholic skillet's Avatar
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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    Was a limiting resistor necessary when using the regulated AC adapter?
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    Flashaholic* Got Lumens?'s Avatar
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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    You need a filtered AC to DC/DC converter. The simpliest way to get rid of it is to use rechargeable batteries that put out true DC current, not modulated DC found within most typical wall plug adapters. A simple 9V standard alkaline battery will do the trick, or 2-3 lions in series. Closed circut V of a standard american telephone wire(loop) is 25VDC. Open circut V is 48V, ringing is 90VAC. Telephone exchanges are constant on battery backed up by AC rectifiers. Typical setup use 24 2.2V very large batteries connected in series. Been there, done that. Good luck, let us know your results.

  7. #7
    Retired Administrator Norm's Avatar
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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    Quote Originally Posted by skillet View Post
    Was a limiting resistor necessary when using the regulated AC adapter?
    No

    For speech (disregarding Ring) most phones will work well below the normal line voltage, older phones (carbon mike and dynamic ear piece) will work down to 1.5 Volts, modern phones will require higher Volts. The set up here is running off a 12 Volt regulated wall wart.

    A very basic set up They show a 300OHM resistor which shouldn't really be necessary.

    Norm

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    Flashaholic* Got Lumens?'s Avatar
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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    As Norm may already know, if you have just two old style carbon mike handsets, you dont even need the phone part at all, just the handsets . Used these while an eletricians apprentice while wiring large commercial buildings.

  9. #9

    Default Re: How do I get rid of a 'hum' between to analog telephones?

    Are you humming while on the phone? If so try stopping.....that should do the trick.

    hahahah.....sorry...couldn't resist

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    Default Re: How do I get rid of a 'hum' between to analog telephones?

    Quote Originally Posted by jvc55349 View Post
    Are you humming while on the phone? If so try stopping.....that should do the trick.
    hahahah.....sorry...couldn't resist
    In that case it's not the humming, it's that fact you don't know the tune . . .

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