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.

[Marky Mark @ Dec 19th 2005 2:47AM]

http://www.dynametric.com/

[A B L @ Dec 19th 2005 2:47AM]

Nice trick, but please couldn't someone sell this for $9.95 + S&H in a nice shiny box? Why is it so hard to get a device that record telephone conversations?

[Richard @ Dec 19th 2005 2:47AM]

I wish you guys would figure out some way to make "printer friendly" pages so I can print stuff like this out as a reference on my workbench as I make stuff. It's a pain in the butt having to constantly look back to my laptop or print out something that's awkaward.

[Brian Stuckey @ Dec 19th 2005 2:47AM]

Worth checking out: http://www.aapsonline.org/judicial/telephone.htm - requirements for recording telephone conversations in the States. "One party" means only one person has to know about the recodring. "Two party" means both people have to know about the recording device. Inter-state = much more confusing :)

[010111 @ Dec 19th 2005 2:47AM]

i need something like this that will work on the headset jacks vs. the wall jacks. since the wall jacks i have are digital. and thus do not work. whereas the narrower headset jacks... are in the analog stage and post-digital. any ideas?

[TAZ427 @ Dec 19th 2005 2:47AM]

Uhh... why don't you just get a phone with a mini-jack for headphones (most cordless phones have one built in if you bother to look), use a set of headphones when on a call you want to record. Then use a mini-Y audio splitter plug it into the phones mini-jack, plug the headset into one side, plug the mini audio cable into the other side (clip the line which goes to the audio in of the Telephone first) plug the other end into your favorite recording device.

[TAZ427 @ Dec 19th 2005 2:47AM]

010111 are your wall jacks digital (i.e. you got your phone plugged into a LAN) do you have VoIP service which goes through a converter box and then to your wall (the more comon VoIP setup for homes) which is analog.

Either way, my description of the simpler method for achiving what they described will work for you.

-TAZ

[John Bellone @ Dec 19th 2005 2:47AM]

Wow. Nice. Is it just me, or did you guys seriously just tell everyone how to make a phone tap? There's no way to turn this on or off? I mean, short of unplugging this thing, or making sure the software isn't recording.

[Justin Winokur @ Dec 19th 2005 2:47AM]

will it work with non-voip phones?

[Rod @ Dec 19th 2005 2:47AM]

Seems like a ton of hassle ... get a line splitter (available at any walmart) and get
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=43-228
Plkug it in to your sound card and you are reday to go.

[TAZ427 @ Dec 19th 2005 2:47AM]

Justin, Their method would work with any standard POTS (plain old telephone system) phone. Or a regular phone line that has a VoIP behind it, all your doing is amplifing pulling the audio signal off. Nothing really fancy about it.

What they describe would be closer to a phone tap as it can be easily concealled, what I described would be siting out in front of you for recording conversations as was described as the intent.

But for that matter, you can get this type of tap for about $5 (do a search on 'phone tap')

If you ask me this wasn't worthy of an Engadget Do It Yourself posting

TAZ

[Xumbi @ Dec 19th 2005 2:47AM]

I have one of those (that Rob linked to), the recordings it makes with my PC are terrible. I have a bunch of old modems laying around, so I think I'm going to give this a try!

[Jonathan @ Dec 19th 2005 2:47AM]

yea you can buy it there, but it's so much geekier to make it your self and gives you 1337 cred haha. I was messing around with trying to get the signal to my wireless infrared head set... this helps alot more than soldering direct to my headphone transmitter board. thnx.

[sakibomb @ Dec 19th 2005 2:47AM]

what's a good, cheap, low-powered recording device to couple with this? perhaps we need a DIY flash recorder posting...

i've got a box full of 28.8 pci modems in my garage. i bet all you DIY-haters would pay $20 for one of these on ebay.

[Emtea @ Dec 19th 2005 2:47AM]

I have a old creative Sound Blaster modem for a pc. The Transformer is marked K1. Do you think it will work?

[Kabuki @ Dec 19th 2005 2:47AM]

Yeah, Emtea, it should work fine. Hmm... Gotta go set this up on the neighbors'... Er... Use it for my Podcasts... Yeah... THAT's the ticket...

[Russ Turley @ Dec 19th 2005 2:47AM]

Michael Torino of the Digital Media Cast Experiment has the instructions on his web site to make a device for recording phone conversations. I built one with parts from Radio Shack and it sounds great.

http://dmcex.com/more/pdfs/osti_8-1000_plans.pdf

[Ryan @ Dec 19th 2005 2:47AM]

It would be nice to have some more explanation on how the circuit works / what that transformer actually does. I'd like to modify it to allow the computer to not just recieve, but also send audio siginals to the phone line.

[jon335 @ Dec 19th 2005 2:47AM]

Can you play audio through this?

[Joseph Palmer @ Dec 19th 2005 2:47AM]

Hmm. What happens when this is in circuit and the phone rings? I worry those 10uF 35V caps are not high enough voltage to stand off the ring signal in many locations. Also, there is no bleed resistors to balance the voltage on those caps, if there is more leakage on one than the other than much of the 48V DC loop voltage could end up on one cap.


Most modems use high voltage caps to prevent DC current from passing through the transformer, this cap should probably also be scavenged for the circuit. (You want to block the hook current through the transformer because otherwise it gets ever so slightly warm over time, which can screw up it's characteristics enough to need to re-sync the echo cancellation in a modem. Most modems use a semiconductor circuit to carry the off-hook current.)

I wouldn't leave this circuit connected to my audio rig in a thunderstorm either...

[Merlin @ Dec 19th 2005 2:47AM]

Joseph Palmer, Can you post a schematic for the circuit you're proposing?

If there's a way to plug in all those components directly on the modem's board, it would probably be better since all the necessary circuits are already installed and have proper routing.

[Joseph Palmer @ Dec 19th 2005 2:47AM]

Better yet, I found a great webpage ^_^

http://www.tkk.fi/Misc/Electronics/circuits/teleinterface.html#networkint

[Protoflavin @ Dec 19th 2005 2:47AM]

If you're recording to a computer, you can save file space by sampling at 8,000 Hz...a phone won't give you better fidelity.

[Stephen @ Dec 19th 2005 2:47AM]

Guy's all I did was got a cable modem and a pair of headphones. I cut the modem jack off and the speakers on the headphones and then taped the wires together and plugged into my computers line in. Be aware this has been known to overpower a laptops soundcard and burn out a speaker cable.

[Elysian @ Dec 19th 2005 2:47AM]

A word of warning here. The standard POTS on-the-hook line voltage is approximately 48VDC measured tip to ring. When a handset is taken off the hook the voltage drops to around 8VDC due to the loop impedance. A ring signal is a 90Vrms sine wave at 20Hz applied over the on-the-hook voltage. The telephone company may also apply higher test voltages. Why is this important?
It means that upwards of 200V may be applied across the phone tap circuit here. The image above shows two 10uF 35V caps being used. Not only are these polarity sensitive, they have a maximum working voltage (as marked 35V DC) that should not be exceeded. They can leak, or even explode. Please, if anyone builds this circuit, use decent parts and get at least 200V caps. Given the signal levels and frequencies, 10uF is overkill. A 0.1uF 200V ceramic capacitor would be more than sufficient, as well as much safer.
A simple google search provides more information on POTS interfaces.

[Joe @ Dec 19th 2005 2:47AM]

When this circuit is in line with your phone, does it keep the phone off the hook?
If I hang my phone up.. does this circuit hangup too? What if we wanted to take this a step further and make it voice activated?

[Mark @ Dec 19th 2005 2:47AM]

1. Those are electrolytic polarized capacitors - very bad idea. Reverse them and they tend to explode.

2. Those are only rated for 35 volts .... anyone with POTS experience knows a ring signal could easily be over 200V, not to mention spikes and surges.

These things almost surely add up to an explosion at some point in time. What you need to use is a HV (high-voltage) ceramic capacitor rated for no less than 1000V.

Please, take my word for it. I know all too well what it takes to make a phone interface. I don't want to see any would be phreakers lose an eye over something as silly and obvious as this.

[Joseph Palmer @ Dec 19th 2005 2:47AM]

Hmm. What happens when this is in circuit and the phone rings? I worry those 10uF 35V caps are not high enough voltage to stand off the ring signal in many locations. Also, there is no bleed resistors to balance the voltage on those caps, if there is more leakage on one than the other than much of the 48V DC loop voltage could end up on one cap.


Most modems use high voltage caps to prevent DC current from passing through the transformer, this cap should probably also be scavenged for the circuit. (You want to block the hook current through the transformer because otherwise it gets ever so slightly warm over time, which can screw up it's characteristics enough to need to re-sync the echo cancellation in a modem. Most modems use a semiconductor circuit to carry the off-hook current.)

I wouldn't leave this circuit connected to my audio rig in a thunderstorm either...

[mike harves @ Dec 19th 2005 2:47AM]

That's overkill dude. All you need is an 1:1 isolation transformer from rat shack, it costs $3.

[jay kominek @ Dec 19th 2005 2:47AM]

i was fiddling with a live circuit once and (while leaning over it, looking at it), i inserted a polarized electrolytic cap backwards, exploding it instantly. i'm not sure about this losing an eye stuff, but i sure as hell wouldn't sign up to do it again. (fortunately my sense of smell returned after a few days.)
lesson: choose your capacitors wisely. and wear safety glasses when you're doing anything more dangerous than reading a book.

[Tenetri @ Dec 19th 2005 2:47AM]

HELP PLEASE ( sry for caps :) )
Hey, I'm building this circut right now. I have all the things saudered down to the board... but now I have no idea which side is positive and negative...

I need to know which pins are which P/N for the
-Telephone Jack
-Mini jack

Can't really continue until someone answers :P
Thanks!

[Vv @ Dec 19th 2005 2:47AM]

i built this device, and it seems to work but not that great, when recording on audacity it seems to pickup radio signals, and also the sound from the phone is very very "low." i just scavanged up parts from stuff i had laying around the house, would polarized and non polarized capitors make the diffrence? i used polarized 10uf 50vs. any other suggestions?

[John Feloukas @ Dec 19th 2005 2:47AM]

If you have a voice modem you can try this sotware

http://www.concelsys.com/voice_modem_software_call_recorder_logger_apr.htm

[Josephus Dy @ Dec 19th 2005 2:47AM]

i am building the circuit right now and connected a headphone. it is working great. i tried connecting a mic to the connector but it doesnt work. just checking, he he. do you have any idea how to modify the circuit such that i can connect a mic and the phone caller can hear my voice?

[fadumpt @ Dec 19th 2005 2:47AM]

tenetri: The minijack you can more or less figure out by looking really hard at his mini jack wiring scheme and putting 2 and 2 together. (sorry i'm trying to teach you instead of just give you answer, especially since my answer might be wrong)

the phone jack was a little harder to find a schematic on the connections, but looking at it when the clip on the bottom, pin 1 is ground, then recieve, then transmit, then power. I'm still kind of fuzzy on what's + and - in this project, but you only need the middle two pins.

[sean @ Dec 19th 2005 2:47AM]

Where are the Zener diodes?

what do they look like?

[hermawan @ Dec 22nd 2005 10:28PM]

thx.. this is not works.. still have noise to record