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Sennheiser MM-61
Concealed microphone

MM-61 is a dynamic microphone, concealed as a foutain pen, manufactured from 1962 onwards by Sennheiser Electronic Laboratories in Wennebostel (Germany). The microphone was used for covertly recording of a conversation, and was used during the Cold War by both Western and Eastern intelligence agencies. For concealed recording, Sennheiser also developed the MM-23.

The MM-61 looks like a fountain pen from which the lower part, the nib, is missing [3]. Instead of the nib, it has a miniature coaxial socket for connection of the audio cable, as shown in the image. The audio cable is approx. 95 cm long and has a 2 mm jack plug at the other end, for connection to a recording device or transmitter.

The pen was normally carried in one of the brest pockets of a coat or shirt, which was modified in such a way that the cable could be led through an invisible hole, to the transmitter or recording device that was hidden elsewhere on the body.
  
Microphone concealed as a fountain pen made by Sennheiser

Although the MM-61 was originally intended for use in combination with dictation machines and the first generation of (large) hearing aids, it soon became a prime candidate for covert listening devices and recorders. When carried casually in the outer pocket of a coat, the microphone could barely be distinguised from a real fountain pen and would not raise any suspicion. Because it was overtly carried, the sound quality was superior compared to that of a concealed microphone [2].

The MM-61 was used by intelligence services and police investigators throughout Europe, but was also one of the favorites of the dreaded East-German intelligence service MfS (Stasi), where it was often carried by a unofficial employee on an infiltration mission or working under cover. [2]. The MM-61 was available in 1964 for a price of DM 46 1 and was in production until ~ 1975.

  1. List price in 1964: DM 46 (approx. EUR 23).

Fountain pen microphone in original box Microphone concealed as a fountain pen made by Sennheiser Microphone separated from the cable Close-up of the connector at the base of the fake fountain pen Tiny holes just beneath the golden clip The base of the fake fountain pen says: 'made in Germany' Top cap removed Close-up of the microphone sound port
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Fountain pen microphone in original box
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Microphone concealed as a fountain pen made by Sennheiser
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Microphone separated from the cable
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Close-up of the connector at the base of the fake fountain pen
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Tiny holes just beneath the golden clip
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The base of the fake fountain pen says: 'made in Germany'
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Top cap removed
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Close-up of the microphone sound port

Specifications
  • Frequency
    400 - 4000 Hz
  • Impedance
    500Ω
  • Connection
    Special coaxial socket
  • Cable
    Supplied, 95 cm, with 2mm jack plug
  • Size
    86 mm long, diameter 15 mm
  • Weight
    19 grams
  • Shape
    Fountain pen
  • Material
    Plastic, with metal clip
  • First sold
    1962
  • Last sold
    1975
References
  1. Radiomuseum, Mikrofon MM 61
    Retrieved April 2017.

  2. Museum in der Runden Ecke, Magnetisches Füllerhalter-Mikrofon MM 61
    (German) Retrieved April 2017.

  3. Wikipedia, Fountain pen
    Retrieved April 2017.
Further information
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Crypto Museum. Created: Tuesday 15 September 2015. Last changed: Tuesday, 13 June 2017 - 11:00 CET.
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