Picture of Connecting a Telephone to Your Soundcard
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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.
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Step 1: Background

Picture of 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

Picture of 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.
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)
tyeo0983 years ago
Old modem card? Check.

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