Audiotel wall probe kit
on a conversation it is not always necessary to use
an RF transmitter. If you have access to the building or an adjacent room,
it might also be possible to 'listen through the wall'. This can be done
with a contact microphone, or with a so-called wall probe microphone.
The image on the right shows an example of such a wall probe, that was
manufactured by Audiotel
in Corby (UK) in the late 1980s. The kit consists
of a highly sensitive amplifier
with a wide dynamic range, an extremely
sensitive electret microphone
in a metal enclosure and a pair of earphones
to prevent acoustic feedback.
The kit comes with a set of ultra thin metal tubes
that can be attached
to the front of the microphone. If the tube is entered into a small hole
in the wall, the eavesdropper can listen to any sound or
conversation in the adjacent room.
Over the years, Audiotel developed a range of highly sensitive amplifiers
for use with their wall probe microphones. The one featured on this page
is the VA-095, which was manufactured in 1989. It is so sensitive
that you can easily pick up whispering from the other side of the room.
In order to cope with the enormous dynamic range that is required for
this kind of bugging, the VA-095 amplifier has 6 sensitivity ranges
plus a separate volume setting (also ON/OFF switch).
A built-in automatic compressor/limiter
is used to suppress excessive sounds.
The standard microphone supplied with the kit is shown in the image
on the right. It consists of a
small metal cylinder of which the front
can be unscrewed. Inside the cylinder
is a sensitive electret microphone
with a three-wire cable to the VA-095 amplifier's
3.5 mm jack socket.
The front part of the microphone enclosure has a tiny hole through
which one of the supplied metal tubes
can be inserted. Once the tube
is inserted, the tip can be screwed back onto the microphone casing.
The wall probe microphone, with the selected tube, is now ready for use.
All controls are at the front panel of the VA-095 amplifier.
The ON/OFF switch is combined with the volume adjustment at
the centre. The microphone should be connected to the 3.5 mm jack socket
at the top left, whilst a suitable pair of headphones is connected to the
EAR socket at the bottom right. The GAIN control, to the left of the
volume knob, provides six sensitivity levels.
At the bottom right is a socket pair that can be used for the connection
of an external tape recorder. Such recorders were commonly used in the
1980s, and most of them used a combination of a 3.5 mm jack (for the audio
signal) and a 2.5 mm jack (for the remote control). The slide switch at
the top right controls the recorder. When set to RUN, recording is activated.
Setting the switch to VOX starts recording only when an actual conversation
is taking place.
Once a suitable tube is
mounted to the front of the microphone, it can be inserted into a hole
in, say, the wall or the ceiling.
Whenever possible, the investigator will try to use
existing holes, such as a keyhole, an existing borehole of a cable,
or a pipe of the central heating system.
The image on the right shows how the probe microphone, fitted with a short
tube, is inserted into the keyhole of a door. Once the microphone is
in place, it should be fixated, for example with a piece of duck tape.
The operator should also ensure that there is no acoustic leakage to
his own room, as this may lead to feedback noise (even when a pair of
headphones is used).
The microphone is then connected to the VA-095 amplifier, the headphones
are connected and the amplifier is switched on. If necessary,
an extension cable can be used for the microphone.
If it is not possible to use an existing hole, the investigator can use
a special drilling kit that enables him to drill a tiny hole through the wall
without leaving a mark at the other side. In order to cope with varying
thickness in walls, ceilings, etc. four different length tubes are
The metal housing of the VA-095 amplifier consists of two half case shells,
that are held together with a large bolt at the center of the top half.
After removing the large bolt with a screwdriver or a coin, the top half
can be removed, giving access to the interior and to the standard 9V battery.
The image on the right shows the interior of the VA-095.
The double-sided PCB has all its components on the top side and has
a cut-out for the 9V block battery in the rear corner.
The circuit is very well designed and built. Immediately behind the
microphone input is a ZN459 ultra low noise pre-amplifer, followed
by several LM358 operational amplifiers that form the automatic
fast-attack slow-release limiter.
Finally, the audio signal is amplified to earphone level using
a standard LM386 audio amplifier.
In the 1980s, tape recorders or cassette recorders were commonly used
for recording of conversations, intercepts and taps. As the length of the
tapes was limited to, say, 90 minutes, automatic voice operated circuits (VOX)
were used to record only the actual conversation.
In the VA-095, the VOX circuit consists of another LM358 which finally
drives a small Siemens relay. The latter was needed because it directly
controlled the motor of the connected tape recorder.
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© Crypto Museum. Created: Tuesday 21 May 2013. Last changed: Tuesday, 13 June 2017 - 06:55 CET.