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Protona Minifon Mi-51
Miniature wire recorder
 
The Minifon Mi 51 was the first of a successful series of portable minature wire recorders built by Monske & Co GmbH (later: Protona) in Germany. It was introduced in 1951 at a time when transistors were not available yet, and all electronics were valve-based.
 
Using the first generation of minature valves, this marvel of technology measures only 17 x 11 x 3.5 cm, small enough to fit in a pocket or a handbag. It didn't come cheap however. The price of an Mi-51 in 1951 was DM 680!

The Mi-51 is battery powered, making it a truely portable device. Bringing the device back to life today is rather difficult, however, as it requires three different voltages: 9V (for the motor), 30V (anode voltage for the valves) and 1.5V (for the filaments). It has a recording capacity of approx. 2.5 hours.
  
The Mi-51 is the protective case, lying on top of the accessories in the storage case.

The Mi-51 was invented by Willi Draheim, who started the development in 1948, just a few years after the end of WWII. The Mi-51 was in production from 1951 to 1955, when it was replaced by the P-55. Willi Draheim left the company in 1952, just before the Minifon was taken over by Protona.
 
The closed storage case. Here is a somewhat deteriorated condition. The opened storage case, showing its contents. The Mi-51 is in a further green case at the right. The Mi-51 is the protective case, lying on top of the accessories in the storage case. The Mi-51 in its protective green carrying case Typical view of the bar Mi-51 unit. A look inside the Mi-5 after the lid has been removed Close-up of the wire reel. As you can see, the wire is currently broken. The famous microphone that was disguised as a typical watch

 
Accessories
The Mi-51 usually came in a green or brown storage case, such as the one shown above. The case is made of artificial leather, and is not likely to have survived the years. The case shown here has deteriorated somewhat, especially at the edges. After opening the zipper, its contents are revealed. It came with a lot of accessories, such as microphones, earphones, wire spools and a power supply unit.
 
By far the most eye-catching accessory, is the microphone disguised as a typical wrist watch, shown in the image on the right. It is designed to be worn inconspiciously on the left arm, whilst the wire is hidden in the sleeve. The microphone is very sensitive and the user has to ensure that it is not brushed against his clothing.

The recorded audio could be played back over the supplied headphones that had the shape of a stethoscope.
  
Close-up of the watch-microphone on the wrist

 
The famous microphone that was disguised as a typical watch Close-up of the microphone that was disguised as a typical watch Close-up of the watch-microphone on the wrist This is how the watch-microphone was carried on the wrist The headphones in the shape of a stethoscope The 'standard' microphone supplied with the Mi-51. Attaches to clothing with a leather strap.

 
References
  1. Roland Schellin, Spion in der Tasche
    Detailed history of Protona and the Minifon recorders
    ISBN: 3-936012-00-8 (German)

  2. Protona GmbH, Bedienungsanteitung für das kleinste Tonaufnahme-
    und Wiedergabe-Gerät der Welt...

    User Manual for the smallest audio recording and play-back device in the world.
    Two-colour print (German)

  3. Protona GmbH, Minifon, Ihr sprechendes Notizbuch
    Sales brochure and list of accessories. Black print (German)

Further information

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