I need to splice out the audio feed from the hand set line (4P4C jack) to a 3.5mm audio port on a computer to interoperate the call via speech recognition. I do not meddle around too much with analog signals, nor have I messed around with phone lines. I have found several pinout guides for the 4P4C connector, but not the technicalities I need to abide by. The primary guide I was referring to was: enter image description here

My overall plan was to cut a pre-existing handset coord and splice out the green and red lines. The mic lines black and yellow are not needed and will be left alone. The red wire will be tied to a potentiometer for varying volume and to satisfy the schematic (a \$500 \mbox{ } k\Omega\$ one is laying around). The audio will then be supplied to a 3.5mm jack red on the center ring with the green on the ground ring (the ring furthest from the tip). This will then be feed into the light blue jack on a computer.

Any advice? I would not like to plug this into my sound card and trash my audio in, or the whole sound card.

  • \$\begingroup\$ I'm a little confused by your post. You mention that you want to do speech recognition, and then say the mic lines black and yellow are not needed and will be left alone. How do you plan on sending audio to your computer without a microphone? Also (although it doesn't seem to be relevant, since I don't think you need the speaker if you are only sending audio to the computer), a 500kΩ resistor cannot be used in place of a 500Ω resistor -- the former is 1000 times more resistance and you wouldn't hear a thing. \$\endgroup\$
    – tcrosley
    Commented May 14, 2012 at 2:51
  • 2
    \$\begingroup\$ @tcrosley The mic is left alone because I only want to interoperate the incoming message. \$\endgroup\$ Commented May 14, 2012 at 2:56
  • \$\begingroup\$ Ok, you mean you want to do speech recognition on the audio coming from the other party. That wasn't real obvious (at least to me). \$\endgroup\$
    – tcrosley
    Commented May 14, 2012 at 2:58

2 Answers 2


If you intend to use the mic input mic input to your sound card, it will have a current source or pullup resistor to supply a bias to the microphone to which it was intended.

It might not be good for the speaker to have DC flowing through it.

If your phone is connected to the telco, be very careful. The speaker may run at quite a CM voltage and the DC offset might be -48V (depending which way round it is wired).

Telephones wires are very good at attracting lightning. During a storm high common mode voltages may be on the earpiece.

Stricly speaking you should use an audio transformer to couple the earpiece signal into the sound card, this provides DC isolation and some protection against lightning. Adding a TVS across the output of that transformer would be a good idea too.

The ac signal on the earpiece might be quite small. Only a few hundred mV. For a mic input, this will be fine (and can be potted down at the output of your transformer) but for line levels, which are usually 1Vrms it might be very quiet.

Not all phones work the same way, so measure the voltage first using a scope and judge the divsor as required.

The audio transformer should also be protected against DC usign a DC blocking (aka coupling) capacitor.

This circuit protects against discharge and DC currents, though use with caution as a mistake will cost you a PC!
enter image description here

  • \$\begingroup\$ Thank you for this reply, it was very informative. I'm glad there is a protective circuit that I can use. But I am confused on the phone line wires, does it matter which wire from the phone line out passes through the capacitor (C2)? And if you don't mind me asking, what are the two capacitors used for? I appreciate your help. \$\endgroup\$ Commented May 14, 2012 at 17:59
  • \$\begingroup\$ Good answer. Brief but thorough. \$\endgroup\$ Commented May 14, 2012 at 18:19

30 years ago I have transferred ZX SPECTRUM software over phone line and recorded them on audio cassette tapes via microphone input. If I remember well signal was 1000/2000 Hz, and it worked reasonably well. I didn't have any circuit. Handset speaker output was directly connected and the only protection was microphone potentiometer on the tape recorder. So, it is possible and proved in practice, at least for the mentioned frequency range.

EDIT: answer corrected thanks to tcrossley

  • \$\begingroup\$ There are several circuits around for interfacing a tape recorder directly to a phone line and capturing both sides of a conversation, but the OP doesn't want that -- he only wants to connect the speaker of the handset to the input of an audio card to capture just one side of the conversation. \$\endgroup\$
    – tcrosley
    Commented May 14, 2012 at 11:12
  • \$\begingroup\$ Huh, I see I wrote that phone line was directly connected and that's probably what triggered your comment. I was wrong. Now when I recall it I am sure I made wires connecting handset speaker instead of phone line wires into 3.5mm audio input. I will correct my answer. Thanks. I guess that 30 years have faded my memory a lot. That old tape recorder had 3.5mm audio input, and his sound card has the same 3.5mm audio input connector, so that should work. \$\endgroup\$
    – avra
    Commented May 14, 2012 at 12:22
  • \$\begingroup\$ I have tried connecting the circuit in to another computer with just the variable resistor. On this test I found that the line in and mic port did receive the sound, however the volume was very faint and sound was rather muffled. But more importantly, the phone tried to pick up when the cable was connected (even though the handset was left on the jack). Unplugging the jack from the computer made the phone operate properly. \$\endgroup\$ Commented May 14, 2012 at 18:08

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