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Bremen 20
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].

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.
Stasi bug 'Bremen 20' with fixed wiring

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.

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.
Microphones, hidden in the walls of the Stasi prison Bautzen II. Click for a closer look. Source: Gedenkstätte Bautzen [3].

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]. Without his help, this page would not have existed.

Stasi bug 'Bremen 20' with fixed wiring
Close-up of the potted unit
Top view
Potted 'Bremen 20' bug
Microphone wiring
Bremen 20-1 (with internal microphone)
Bremen-20-1 compared to the size of a hand
1 / 7
Stasi bug 'Bremen 20' with fixed wiring
2 / 7
Close-up of the potted unit
3 / 7
Top view
4 / 7
Potted 'Bremen 20' bug
5 / 7
Microphone wiring
6 / 7
Bremen 20-1 (with internal microphone)
7 / 7
Bremen-20-1 compared to the size of a hand

  • 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.
Block diagram
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.

  • 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
  1. Kernblatt Gerät Bremen 20
    Device Bremen 20 datasheet (German).
    DDR, 9 February 1970. BStU, 5 pages marked BSTU 0064—0068. 1
  1. Document from BStU archives, kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, BREMEN 20, technical documentation
    Personal correspondence, April - May 2018.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. Gedenkstätte Bautzen, Image of Stasi microphone
    Stasi Prison Bautzen II Memorial. Retrieved May 2018.
  1. 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.

Further documentation
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© Crypto Museum. Created: Wednesday 02 May 2018. Last changed: Monday, 24 September 2018 - 20:04 CET.
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