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Wired room monitoring bug
- under construction
Bremen 20 was the codename of a wired
covert listening device (bug),
developed around 1970 by the
state security service of the
former DDR (East Germany),
the Stasi.
The device was intended for room bugging and delivers an LF audio signal
onto a 2-wire line that also supplies the power.
The device requires a dedicated line and can not be used
over switched (telephone) networks [1].
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The image on the right shows the device, which measures just 23 x 13 x 8 mm
and weights no more than 4 grams. It consists of a three-stage audio amplifier
and is entirely cast in expoxy. The wiring is fixed and can not be replaced.
The device was often used in combination with an
MM-301
or an MM-26
miniature microphone, made by the West German company
Sennheiser. Such
microphones are extremely sensitive and provide good quality audio.
They were generally imported in East Germany under the pretention
that they were used for hearing aid production.
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The device was also available in a slightly larger package, in which case
it contained a built-in
Sennheiser MM-301 microphone.
Unlike some other bugs, Bremen 20 does not feature carrier modulation and,
hence, can not use a subscriber telephone line for transport of its intelligence.
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Instead, it had to be wired directly to an external listening post that could
be up to 500 m away, which was not always possible.
Bugs of this type are extremely difficult to find,
especially if they are extended with a probe.
In most cases, they can only be discovered by visual inspection.
The image on the right shows a wired Sennheiser MM-26 microphone
– made in Germany –
hidden by the
Ministerium für Staatssicherheit (MfS)
in the walls of the much-feared Bautzen II prison.
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We should like to thank Detlev Vreisleben in Germany for giving us access
to BREMEN 20 and its technical documentation, which was obtained from the
archives of the former Stasi in Berlin
[1][2].
Without his help, this page would not have existed.
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- Bremen 20-1
This version has a built-in
Sennheiser MM-301 microphone
and is fully cast in
epoxy. It measures 28 x 13 x 8 mm. The sound port of the microphone can be extended with a
hollow pipe, or probe.
- Bremen 20-2
This is the same device, but without the internal microphone.
It measures 23 x 13 x 8 mm and has a fixed shielded wire for the connection of
an external microphone, such as the MM-301,
MM-26 or similar.
The device described on this page, is of this type.
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The block diagram below shows how the bug works. At the left is the microphone. Depending
on the version, it is a built-in
Sennheiser MM-301, or an external one.
At the right is the output, which is also used as the power supply line.
The bug is connected via a twisted 2-wire line to an adapter which
is located at the listening post, somewhere outside the target area.
The adapter has a socket for a pair of
headphones or a recorder. It also supplies a 9V DC voltage to the bug.
Note that an internal diode protects the bug from being connected the wrong way around.
This way it was possible to connect two bugs anti-parallel (for example in two different
rooms at the target area), and monitor them with a single adapter. By reversing the
polarity of the two wires of the adapter, the alternate bug was selected.
If necessary, a reversing switch could be added.
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Input 2 — 4.5 kΩ
Level ≤ -75dB
Output 600 Ω
Supply 9V DC
Current 0.9 mA
Temperature -20° — +50°C
Line ≤ 2000Ω
Distance ~500 m
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Document from BStU archives, kindly supplied by Detlev Vreisleben [1].
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Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes
der ehemaligen Deutschen Demokratischen Republik
(DDR) —
Federal Commissioner for the Records of the
State Security Service
of the former German Democratic Republic (GDR) —
officially abbreviated to BStU.
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© Crypto Museum. Created: Wednesday 02 May 2018. Last changed: Monday, 24 September 2018 - 20:04 CET.
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![Microphones, hidden in the walls of the Stasi prison Bautzen II. Click for a closer look. Source: Gedenkstätte Bautzen [3].](img/bautzen2_large.jpg "Microphones, hidden in the walls of the Stasi prison Bautzen II. Click for a closer look. Source: Gedenkstätte Bautzen [3].")
