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How-To Build a Telephone Recording Circuit from an Old Modem


We started out with an old modem and ended up with a useful circuit to convert phone audio to regular line audio. This little hack is useful for those of us wanting to record Voip telephone calls for podcasts with good quality audio. Keep in mind that recording telephone calls without notifying all parties involved is against the law in many parts of the world.

What you'll need:
- an old modem with an audio transformer
- a soldering iron
- a throughhole circuit board
- a computer with mini jack audio in (and a Griffin iMic for older macs)
- an extra telephone cable

First carefully remove the following parts from the old modem:
- a 1:1 audio transformer: usually a big blocky chunk with 4 pins
- two 10K ohm resistors
- two diodes (regular ol’ signal diodes)
- two 10uF capacitors or equivalent
- two female phone jack sockets, circuit mount
- a female mini jack socket, circuit mount (or a male cable if you don’t have a socket available)

The transformer is usually marked T1 or something similar on the circuit board of the modem:


Grab the phone jacks so you won’t have to cut open your phone or phone cable. You can also use a female mini jack


Now on your circuit board start by placing the transformer and the two phone sockets and the audio socket. Solder them to the board.



Once the parts are soldered down and glued down with a dab of super glue, cut the circuit board down to size with a file or rotary tool:


Next, following this schematic, wire up the resistors, capacitors, and diodes (click for larger size):


Phone socket 1 and phone socket 2 are opposite each other on our circuit to help when plugging in, so we must tie across cables to connect them. This allows the phone signal to travel “through” our device. These two cables on our circuit are red and blue. (Note that in this image we are missing a connection between the transformer and the mini jack sleeve.)


Finish up your circuit and test it by connecting it to the phone line you wish to record. On the other end connect a second phone cable and plug it into the phone. This circuit can be placed anywhere on the phone line, before or after the phone. Now connect a mini jack audio cable from the circuit to the line in on your audio card or recording device.



Test the audio in on your computer or audio recording device. We recorded our phone conversation using the free and open source Audacity (mac/linux/win compatible):


Why not do this all in software such as Gizmo or a Skype recorder you ask? Well, we already have unlimited Voip using analog telephone lines here at Engadget headquarters. We were pretty dang tired of hearing those tiresome soft phone echos and low quality audio recordings. Now go forth and record thee some phone conversations for thy podcasts.

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