Telephone Recording Using Transformer

Discussion in 'The Projects Forum' started by PartyLine4, Apr 2, 2013.

  1. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
    I have been trying to make a proper circuit for recording certain phone calls on my old Bell model 500 phones.

    (refer to the included diagram for the rest of this post)

    I hooked the red wire of the phone line in series with one T1 as in the diagram, and the audio output went to the other. I was amazed when I hooked it up to my GE tape recorder to find that it worked pretty well!

    The second example was conducted with a transformer ( T2) that was much larger and had over 200Ω across one of its coils. The speaker's amplitude and the recorder's amplitude were much lower than Ex.1's when energized.

    The question(s) I have are these:

    Could Ex.1 damage the phone equipment in any way?

    Can using a transformer that is NOT an isolation transformer on this type of circuit cause damage to the telephone equipment?

    Be serious about it and don't say YES without explaining why! ;)
  2. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
    I actually ran out to the shop and wound some .28mm wire 118 turns for each coil around a 5mm iron nail.

    Seems to work just as well as the first example!

    The wire actually came from Example one's transformer :p

    I think I was just worried about if the 1:1 ratio meant that the recorder could be sending voltage back into the telephone line.

    It would seem like you could play audio files back over the telephone line?

    I will have to try that tomorrow!
  3. Externet

    AAC Fanatic!

    Nov 29, 2005
    That is the correct way ! to extract or inject audio from/to telephone lines. A transformer in series to only one wire.

    It is a little better using an audio output transformer instead of a power transformer, but both work well. The low impedance winding (thicker wire) is the one to be connected in series to telco line. The higher impedance winding to the audio equipment.

    That is an isolation transformer. There s no direct metallic coupling between windings. Can do no harm to telco lines.

    1:1 is not the best convenient choice but works. Try 1:10
    Winding on a nail works perfectly well too, and also safe to telco.
  4. PartyLine4

    Thread Starter New Member

    Jan 12, 2013
    Oh ok! Thanks for the info! I noticed the inevitable " Local voice " being louder than the distant one.

    And apparently there is no way to remove this.

    You can ALSO play audio files back over the phone line! Probably how they did it back when the Mamma Bell was around!

    Thanks for your reply!