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Wired room monitoring bug

Projekt 31550-6 was a miniature audio frequency (AF) 1 covert listening device (bug), developed in 1977 by the state security service of the former DDR (East Germany), the Stasi. The device was intented 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 cannot be used over switched networks [1].

The device is built on a ceramic substrate and is housed in a 24-pin DIL IC socket, cast in white epoxy. Without the epoxy, it measures 32 x 20 x 5 mm and weights just 8 grams. At one side are two wires (red and blue) for connection to the remote listening post. The bug delivers its signal to these wires and is also powered via them.

At the other side are two or three wires to which an external microphone should be connected. The image on the right shows a typical 31550-6 with a 3-wire microphone connection. The red wire supplies 1.5V for an electret microphone.
Stasi audio bug 31550-6

The 31550-6 is built around the FSA-6 circuit and is powered by 2.5V. The numbers 44-38 refer to the circuit diagram, whilst 62 is an indication of the sensitivity (61 ... 63). At the bottom is the date code KD which translates to December 1978 [1]. The full circuit diagram is available below.

Note that the device was not connected directly to a suitable power source, but rather via the external black unit shown in the image on the right. It was located elsewhere in the house and was connected at the end of the cable, where it separated the audio signal from the power line.

The item at the right is the power block, that allows a suitable power source to be connected to the black junction box. It has the terminals of a standard 9V block battery at the top – for connection of a 3V battery pack – and a socket for connection of a mains adapter at the side.
Junction box and power block. Image kindly provided by Detlev Vreisleben [1].

Devices like the 31550-6 were heavily used by the Stasi for (permanent) monitoring of homes or offices, in which case they were commonly embedded in a wall, hidden behind the plastering. In many cases, miniature microphones from western manufacturers were used, such as the dynamic Sennheiser MM-301 that was available in 1971 for US$ 13.50 [3]. The 31550-6 shown here has an extra wire for powering an electret microphone, and is therefore perfectly suitable for use in combination with some of the sub-miniature elements from the American manufacturer Knowles.

  1. AF = Audio Frequency. German: NF = Niederfrequenz.

Stasi NF bug 31550-6 Rear view Stasi audio bug 31550-6 Rear side close-up Close-up of rear side Junction box and power block. Image kindly provided by Detlev Vreisleben [1]. Modified (restored) 31500-6 bug
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Stasi NF bug 31550-6
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Rear view
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Stasi audio bug 31550-6
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Rear side close-up
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Close-up of rear side
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Junction box and power block. Image kindly provided by Detlev Vreisleben [1].
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Modified (restored) 31500-6 bug

The diagram below shows a 31550-6 bug ready for use. The actual bug is visible at the bottom left and is marked 44-38 62 KD. At the right are two wires (red and blue) for connection of the 2.5V DC power input. A simple circuit is used to decouple the audio from the power line. At the top left is the microphone, which can be virtually any type of dynamic or electret element. It is shown here with a Shure MC-30, but in practice a Sennheiser MM-301 was commonly used.

Circuit diagram
Below is the circuit diagram of the FSA-6 circuit that is used at the heart of the 31550-6. It is basically a 4-stage audio amplifier built around four identical BCE108B 1 miniature transistors. Power (2.5V) should be applied to the red terminal at the right via an external 600 Ohm resistor. This voltage is lowered by the 12k resistor to approx. 1.5V for the first three stages and the (optional) electret microphone. The last stage (T4) injects the audio signal into the power line.

The circuit is tolerant to a 15V power supply but will be damaged when the polarity is reversed. It is built in surface mount technology (SMT) on a white ceramic substrate, which is then placed in a DIL 24 socket, before casting it in epoxy. In the above diagram, the red numbers correspond to the pins of the DIL 24 socket. The device should be connected via the 31530-1 unit shown here:

At the left is the 31550-6 bug with its red (+) and blue (-) wires which should be connected to the corresponding terminals of the T-shaped black 31530-1 junction box. At the bottom right is the power supply, which was usually a 31530-2 or 31530-3 unit. At the top right is the audio output which can be fed to a suitable audio amplifier for recording, monitoring or distribution.

  1. The BCE108B is an Eastern Block miniature version of the well-known BC108 transistor. It is basically a universal NPN transistor and is equivalent to the Western BC848 SMD transistor (in SOT23 enclosure).

When we received the 31550-6 device featured on this page, it was in non-working condition. As the circuit is entirely cast in a strong white epoxy resin, there was virtually no chance of getting access to the internal parts and, hence, the chances of getting it working again were slim to none.

When we applied 2.5V power to the appropriate terminals via an improvised coupling circuit — similar to the 31530-1 junction box shown in the previous section — and monitoring the audio output on an oscilloscope, we noticed that the circuit was exhibiting unwanted oscillations at two different frequencies: 20 Hz and 80 kHz.

As the input and output voltages were otherwise correct, it seemed likely that the oscillations were caused by a bad 47µF capacitor on the secondary power rail (1.5V). It is quite common for such capacitors to dry out after many years.
Shure MC-30 microphone with two extra capacitors

Luckily, the secondary power line is available externally, as it is also used for powering an electret microphone. Connecting two external capacitors of 47µF and 22nF between the 1.5V line and ground, was enough to get rid of both oscillations and make the bug fully functional again. As it is likely that other surviving bugs of this type will exhibit the same anomaly after all these years, here is how we fixed it for this one. The two capacitors that were added externally are at the left.

Modified (restored) 31500-6 bug Close-up of the extra capacitors Shure MC-30 microphone with two extra capacitors Power/audio coupling circuit
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Modified (restored) 31500-6 bug
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Close-up of the extra capacitors
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Shure MC-30 microphone with two extra capacitors
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Power/audio coupling circuit

The 31550-6 is housed inside the framework of a grey plastic DIL 24 IC socket, of which some pins are used for the wiring. The diagram below shows the assignment of the active pins when looking at the device from the top (text side). All other pins are uncnonnected. Although it is not possible to access the solder joints (these are cast in epoxy resin), they can be reached from the top, by pusing a thin sharp measuring pen into the soft silicone paste that covers the holes.

  • Voltage
    2 — 3V DC
  • Maximum
  • Current
    1.1mA — 2.0mA
  • Frequency
    120 Hz - 10 kHz
  • S/N
  • Gain
    60dB — 69dB
  • Impedance
    ≥ 40 kΩ
  • Microphone
    Dynamic or electret
  • Load
    600 Ω
  • Temperature
    -15°C — +45°C
  • Dimensions
    DIL 24
  1. FSA-6 circuit diagram and technical description 1
    MfS, July 1977. 18 pages.

  2. Kennblatt Gerät 31550-401 1
    Datasheet (German). MfS, 5 August 1980. 3 pages.

  3. Technologische Vorschrift und Prufvorschrift fur Gerät 31550-401 1
    Test requirements and technical description (German). MfS, 5 August 1980. 3 pages.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

  1. Detlev Vreisleben, 31550-6 and technical documentation
    Personal correspondence, June 2018.

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

  3. Sennheiser Electronic Corpororation, Consumer Net Price List
    1 June 1971. Retrieved June 2018. page 6.
  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: Saturday 16 June 2018. Last changed: Sunday, 15 July 2018 - 20:08 CET.
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