Connecting a Telephone to Your Soundcard




Introduction: Connecting a Telephone to Your Soundcard

About: Writer, engineer, techie. I've been using computers since the original Apple II in 1978 and have always been interested in technical topics. Check out my articles on They include how-to...

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Note - You will need basic electronic skills to build the project in this tutorial. Use this information at your own risk; do not complain if it doesn't work for you. Only use as prescribed.

Telephone technology is pretty amazing. It’s been around for over a century and is extremely backwards compatible. The 1/4" headphone jacks used on musical instruments and high quality audio equipment are identical to the original telephone switchboard plugs and jacks from over a century ago. It’s amazing that just two wires can be used for bi-directional voice (and data) communications and that a century old telephone can be hooked up to modern phone lines.

A hybrid circuit converts the two wire telephone (tip and ring) into the four wires needed for separate audio in / audio out. Okay, you can get away with three wires since you can reuse the ground.

Step 1: Background

This circuit is based on an ancient Usenet post archived at

Make sure you read that page completely before trying this circuit, some phones are finicky and some tweaking may be required to get this to work.

All you need is a couple of components – a 600 to 600 ohm transformer with a center tap, a 150 ohm resistor (brown green brown), a modular jack, and two 3.5 mm. mini jacks. I mounted my circuit in a wall mount modular jack box.

Step 2: FREE Transformers

There’s probably a source for free transformers sitting in your junk box. If you’ve got high speed Internet, phone line modems are obsolete. Most modem cards have transformers which are used in the same exact manner as this circuit.

Step 3: Assembly

If you wish you can use short cables with mini plugs instead of jacks and plug the plugs directly into your soundcard. It looks neater if you have jacks mounted on your project box and use a couple of 3.5 mm male-to-male cables to hook up the circuit to your computer.

As with my other homebuilt projects it’s nice to use colored loose leaf labels to color code the jacks – green for line in and blue for line out.

It’s pretty trivial point-to-point soldering to connect the transformer and resistor to the jacks. Heat shrink tubing does make it look neater and helps avoid accidental shorts. I glued the transformer into the box but didn’t bother with making a cover. A piece of cardboard makes an adequate cover, plastic would be better.

Obviously there’s no way for a normal sound card to generate enough current to ring the phone – the only purpose for this circuit is to permit the phone’s handset to talk and listen.

It’s a pretty cool circuit and I was amused to use an antique telephone to talk through a VoIP link to someone half way around the world.

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    7 Discussions

    Dr Destructo
    Dr Destructo

    3 months ago

    There's no way that this could work, the hybrid circuit is right, but your SLIC does not provide phantom power, so the phone will not work. well actually there's a small amout of power out through the mic jack, it might be enough to run an an old non-electronic phone badly.


    Question 1 year ago

    I don’t have a modem card. What is the ratio of the transformer?

    Hi Philip, just sent you a private message, but forgot to say is there any chance of some better pictures of the wiring at all?also, i'm probably wrong but, surly there should be 4 cables coming from the phone connection for the speaker and the mic not just two? (the red and green in pictures)


    Reply 1 year ago

    Only 2. If their is 4, than you have two lines.


    Question 1 year ago

    How many turns are on the primary and secondary???


    8 years ago on Step 3

    Old modem card? Check.

    Now all I need is an old phone and some people to talk to!