Simple Phone Line Tap

Overview

This is a simple audio interface circuit which will allow an Olympus digital voice recorder (with the "voice activation" option) to automatically record any audio on a standard analog telephone line.

The audio interface circuit can be made from the parts found in old modems and cordless phones.  The main component in the interface circuit is a 600 ohm to 600 ohm isolation transformer.  An isolation transformer needs to be used because telephone lines are balanced (they have no reference ground) and in order to properly attach to them, you need an isolation transformer to provide a reference ground on your equipment side.

If you've ever heard a loud "hum" on a telephone call, it's because somewhere along the path the telephone line became unbalanced, and this reduced the ability of the line to reject any electrical interference.

The audio is coupled into the interface circuit via two series, non-polarized 0.1 µF capacitors (with at least a 100 volt rating) and two series 4.7 kohm resistors.  The capacitors block the -48 VDC on the phone line and prevent the target phone from going "off hook."  The resistors help to bump up the input impedance of the circuit a bit, and also provide high-pass filtering for any signals below 300 Hz.

The audio interface needs to provide a high impedance in order to prevent the phone line from being loaded down to a point in which the audio quality is reduced.  A proper high-impedance (10 megaohm or higher) interface using FETs is needed only if you're tapping a data line or trying to avoid a TSCM sweep.

On the secondary side of the isolation transformer, two back-to-back 1N4728 3.3 volt Zener diodes clamp the 90 volt ring signal (or any other high voltage spikes) to something the digital voice recorder can handle.

The final audio output is taken via the wiper on a 10 kohm potentiometer which goes to the "microphone" input on an Olympus digital voice recorder.

You may need to bypass the audio input to the recorder with a 1,000 pF capacitor if you receive any interference from AM radio stations.

Note that tapping an analog voice signal provide via a DSL line requires the use of a low-pass filter ahead of this interface circuit.  These DSL filters are commercially available.

It's also possible to inject audio into a phone line by just operating the circuit in reverse, injecting your audio in via the volume potentiometer.










Pictures & Construction Notes

Overview of the circuit board from an old U.S. Robotics modem.

The circled component is the 600 ohm to 600 ohm isolation transformer.  These transformers will all be physically different, but will have similar operating characteristics.

Remove the transformer by using a heat gun on the bottom side of the circuit board.

Close up view of the isolation transformer's pins.

Only four of the pins are used, two for the primary side and two for the secondary side.

For additional circuit protection, an optional spark gap can be added across the primary of the transformer.  The spark gap provides "front line" defense against any high voltage spikes being coupled into the transformer.

The circuit board in the above picture is from an old cordless phone.  This board has an enclosed spark gap (circled) installed across the incoming phone line.

Completed telephone audio interface board.

The 1/8-inch output plug is from an old pair of headphones.  Only the tip and ring of the microphone input plug are used.

Close up view of the telephone audio interface board.

The phone line connects on the right-side.  Polarity is not important.

The audio output to the digital voice recorder is on the left-side.

The two blue things are the 1N4728 Zener diodes.

The green thing is the 10 kohm volume potentiometer.

The spark gap is mounted underneath the isolation transformer.

Installing the tip and ring phone line connection leads.  Alligator clips are used here, but JS Popper clips are the best.

Note the two (optional) ferrite beads on the connecting leads to knock down any RF interference.

The telephone audio interface board is attached to the back of an Olympus VN-3200PC digital voice recorder using a couple of rubber bands.

Configuring the Olympus VN-3200PC digital voice recorder.

The highest quality ("HQ") audio settings should be used.  This will give you only eight hours of record time per folder.

The microphone sensitivity should be set to conference ("Hi").  If you get any audio clipping the volume potentiometer can't handle, switch to the dictation ("Lo") microphone sensitivity setting.  Shorter telephone local loops tend to have "hotter" audio.

Enable the voice activation ("VCVA") feature in the menu and you may wish to also disable the recording LED and system beep via the submenu.

After starting recording (pressing the "REC" button), you can adjust the level necessary for triggering the voice activation by using the left and right arrow buttons.  This is shown via the "block" along the bottom of the LCD screen.  A setting of "10" seems to work quite well.  Avoid too low of a setting, or all you'll record is line noise.

The recorder will run for about three days on a fresh set of high-quality alkaline "AAA" batteries.

Example Keptel SNI-4600 telephone network interface box used in Ameritech/SBC/AT&T serviced areas.

If you only have a Phillips screwdriver, you can only install the tap in the "Customer Access" side of the network interface box.

You can slide those little interface modules the phone line screws into up and down, but they are a pain to put back in place.

If you have access to the proper "tamper-proof hex" security bit, you can install the tap in the "Tel. Co. Access Only" side of the network interface box.

This will also be the quickest way as you can just hook the alligator clips to the lightning protection block screw terminals.