Telephone Line Recorder

To be able to make MP3’s of the various tones, I needed something to interface between the tone oscillators, the telephone line and the microphone input of my PC. Just holding a microphone up to a telephone earphone left much to be desired. I came up with this circuit:

This provides isolation between the telephone line and the microphone, sound limiting and a few other things.

Basic isolation is provided by the transformer, a 600/600 Ohm mini repeat coil . You can monitor the circuit by opening the switch or holding a telephone line by closing the switch. Telephone line current is limited by a 440 Ohm resistor (two each 220 Ohm half watt resistors in series) and line current is indicated by the LED’s.

Clicks are clipped to a low level by the 1N4004’s. A 0.022 capacitor provides RF suppression and some high frequency filtering. The 2.2K resistors and the 0.022 provide additional high frequency filtering. The 0.33 capacitor isolates the DC on the Mic Input (DC is used by electret microphones) and the 10K matches the impedance of the Mic Input. The Tip and Ring of the Mic Input are connected together. Capacitors are rated 250 VDC.

I used binding posts so I could provide universal connectivity. You can attach the wire ends of  Mod Cord to the binding posts if you are going to connect it to a telephone line.

The performance of the circuit is excellent and quality of the recordings is very good. There are many free recording programs to make MP3’s for PCs.

42TU-016 has been replaced by TY-145P from Jameco.com. Cost is about $3.