Landline Telephone Audio Recording Output Socket Adaptation Kit

I supply a kit of electronic parts to make this, and you can find that listed under Electronic Kit Of Parts at this link here:  The Useful Components Ebay Shop.

            Transformer, 3.5mm Socket and Wire

Officially speaking, any telephone so modified will no longer have the approval of your local copper line telecoms provider or a valid CE mark.

This is a very simple project that shows you how to safely link a standard landline telephone to the microphone input of a tape recorder or computer microphone input so that you can make good quality recordings of important telephone conversations. Traditionally this has been done using one of those stick-on magnetic pick-up coils which I don't like for several reasons, those being

a)  The sucker cup keeps falling off.
b)  The telephone handset is then chained to the recorder mic input, which is particular unhelpful if you're trying to connect to the microphone input down the back of your modern computer.
c)  The output from them is rather pitiful and is highly dependent on finding the right spot on the handset or base unit.  This is usually where you would like your ear to be, or involves standing up the base unit in a wobbly fashion.
d)  Some modern phones have practically no magnetic output.
e)  They pick up a lot of mains hum and other magnetic noise when you least want it.

You can, of course, spend several hundred pounds on a professional TBU, but you don't want that either.  You will still need to keep the phone base unit away from computer screens and mains transformers but this solution is much better than the open stick-on pickups.

Bill Of Materials

Electronic Parts Included in the Kit

Qty           Item             

1             Transformer LT700
1             3.5mm Stereo jack socket
1             50cm hookup wire


I supply a kit of the electronic parts in the list above, or you can buy your own.  You will need to solder the connections.  You can try to use 2A terminal strips to connect to the transformer and to make twisted wire connections around the jack socket terminals, but it's not really worth dealing with the poor connections that will inevitably ensure over time.    You'll need a bit of insulation tape and you may choose to use a hot glue gun to stop the parts moving.  You will also need a basic desk phone which is big enough to fit the parts into, and I've shown the adaptation procedure below using a cheap 6.99 item from Argos as an example.  You can click on the pictures to see a larger version.

Desk Phone
          from Argos in BoxBasic Unmodified Desk TelephoneArgos item label for basic desk phone

Background Information

To record both sides of a telephone conversation from a traditional landline, you only need to record
the signal coming into the receiver, or earpiece.  This is because some level of your own voice is deliberately allowed to leak electrically from the telephone's transmitter or microphone into the receiver.  The idea is that if you could only hear the caller at the far end and the receiver is sealed against your ear, it creates an odd effect because you can no longer hear yourself speaking in that ear.

Any modern landline telephone should do, providing that there's enough space inside for the transformer and somewhere to drill a hole in the case and mount the socket.  You might want to buy an new, cheap phone to do this with.  Line specifications for telephone systems vary somewhat around the world, but as we're only tapping into the receiver circuit, you don't need to worry about this.  It should work on any telephone worldwide.

Adapting Your Telephone

Open up the Handset

Here I've unscrewed my telephone handset and looked inside.  There's only one reason for this, and that is to identify the colour of the wires connected to the receiver.  In this case they are yellow and black.  While I was there, with the handset unplugged from the base, I also measured the d.c. resistance of the receiver and this measured 120.8 Ohms in this instance.  By comparison, the d.c. resistance of the transformer on the low impedance side is about 0.3 Ohms. 
You'll sometimes find that screws are hidden under stickers or small caps like this one.  Once the screw was removed from the handset, there were also plastic clips at the top and bottom which had to be eased apart quite carefully to avoid breaking them.  With the wire colours noted, you can now put the handset back together.

Cap Concealing the
            Handset ScrewHandset Screw RevealedWhole Handset Open

Handset Earpiece
            Resistance of Telephone Earpiece

Open up the Base Unit And Fit the Socket

The base unit can be opened up in a similar way, and you might need to temporarily unplug some wires, or in my case pull some filter toroids away from their hot-glue mount points to gain better access.  You will need to find somewhere to fit the parts, bearing in mind that it is best to have the transformer right next to the socket to avoid having long wires on the output side.  In my case, there was no flat side wall on which to mount the socket so I mounted it on the flat upper face of the base unit.  The hole required is 6 mm diameter.

Telephone Base Unit
            opened UpMoving One Filter ToroidHole Drilled Inside
            the Telephone Case

Hole on the Outside
            of the Telephone Base UnitJack Socket mounted Inside the PhoneJack Socket
            Fitted in the Top of the Telephone

Connect the Transformer to the Socket

I've put the transformer right next to the socket to avoid using any screened cable.  In this phone I've folded over the transformer mounting tabs in order to make it fit, and later glued it down with some hot glue.  I've connected initially just to the ground terminal on the side of the socket and the stereo tip terminal, and then bridged across to the ring connection in order to record the same signal on both channels.  The middle wire on the transformer winding is unused and you can just cut it off. 
The LT700 transformer has a high impedance side of about 1.2K Ohms.  This is pretty much ideal for connecting into tape recorder microphone inputs and computer microphone line inputs

Fitting the
            LT700 TransformerSoldering
            the LT700 to the Output Socket

Connect the Transformer into the Earpiece Circuit
Down in the base unit it should be possible to identify the same two coloured receiver wires connecting into the circuit board by some connector.  You want to cut into one of them, as I have done with the yellow wire and connect the two free ends to the side of the transformer which only has two wires on it.  This is the low impedance side.  Because it is the low impedance side, it has very little effect on the signal going to the receiver and to the rest of the telephone.  Electrically it is almost invisible, like connecting the two ends of the cut receiver wire back together again.  You should twist the wires around each other to stop it forming a large loop, then you can insulate both ends with tape and glue down the transformer and in my case, re-glue the toroid filter that I had to move.

            Wiring in the Base UnitYellow Earpiece Wires
            Earpeice Wires Connected

Tip and Ring Terminals BridgedTransformer Linked to EarpieceLink from Another Angle

Transformer and Toroid Glued Town


Put the telephone case loosely back together and connect a standard 3.5mm jack plug stereo screened line into the socket on the phone and most probably on the other end into a stereo microphone socket on your tape recorder, mp3 recorder or computer microphone input socket.  Press play and record simultaneously (:) or start the system recording.  Monitor on headphones or PC speakers.  Lift the telephone receiver.  You should hear the dial tone as usual in both the standard telephone receiver and over your recording system.  Screw the telephone base unit back together, and you can now dial out or receive calls as usual and record that fractious conversation with your mum, bank, phone scammers, estate agent, or Ebay customer support in ascending order of intransigence.  In the picture below I'm recording dialling tone, the recording level control is at about 4 out of 10 and there's a full 0dB on the recording level meter.  Connecting a pair of powered PC speakers or headphones into the headphone output allows you to monitor the recording, or allows someone else to listen in.  You can do this via the computer also, if you can figure out the signal routing.

Telephone Modified
          with Recording SocketRecording Conversation From Telephone Landline

More Background Information

I think that the quality available is good, though the near-end voice is somewhat high-pass filtered and this will depend mostly on the phone used.  Because we've used an isolating transformer there are no ground loops to cause additional hum.  I've found that it's best to earth the recording device.  I find I'm much more inclined to record interesting conversations with irritating callers or institutions if it is more convenient to do so, and it can be incredibly useful to have a full record of such interchanges. 

Audio Samples

Here I am talking to myself on the telephone.

Tape Recorder Output Socket Test 1


Q: I want to record conversations on my mobile phone.  How can I do that?
A:  As far as I know, /all/ old Nokia phones could do that when we were still banging the rocks together and dinosaurs roamed the earth in 2002.  I suggest reading the manual or getting an old Nokia and swapping out the SIM for such occasions. 

Q:  I want to make a full Telebalance Unit, a TBU, to put callers on air on my student radio station and talk back to the caller from the studio microphone.  Can I do something like that in a similar way without spending a small fortune?
A:  Yes, yes you can, but I've not designed or documented it properly yet.

Q:  Why do you need a transformer?
A:  The earpiece lines are not electrically isolated from the telephone line which will be at some different voltage compared to your recording device.  Attempting to connect directly to the phone receiver is a very bad idea as there will be all sorts of hum and worse, it can cause currents to flow from the phone line into local mains earth.  This will upset the balance and d.c. state of the line, and is likely to annoy the phone company.

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