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Minifon Mi51 & P55 Miniature Wire Recorders

General Notes

The Minifon Mi51 wire recorder was introduced by Protona of West Germany in 1951 for covert surveillance use by government and other agencies. It used wire spools allowing 1 to 4 hours recording and cost around $US300. The case is cream coloured bakelite and the switches are push-pull types to allow easy use under clothing. The record/playback function is controlled by the type of plug used - a short plug is used for the microphone whilst the longer phone plug operates the changeover switch for playback. The wire spools for this model have a black and white centre which can be seen through a small window in the top, which enables the operator to know when the unit is running. The unit is very small (170 x 110 x 35mm), about the size of a video cassette. Three batteries were needed, a 9V heavy duty one for the motor and 1.5 and 30V A & B batteries for the 3 miniature valves used which consisted of 2 x DF67 and 1 x DL67 Philips types. A range of accessories was available which included a wrist watch microphone and body harness.

The reason for the large take-up spool on this model is to cater for wire lengths giving up to 4 hours recording time. The wire speed of around 24 cm/sec is controlled by the take-up spool, and to keep this fairly constant over the 3.5 kms of wire, the spool needs a large inside diameter. The need for careful winding of the human hair sized wire means that the recording head must be made to rise and fall to provide smooth layering on the take-up spool and during re-wind; hence the circular protrusion on the top cover.

Prototype black box The Mi51 inspired the Australian, Dr David Warren, to devise the first "Black Box" flight data recorder in the 1950s. By the time the prototype was built in 1956, seen right, the later P55 components may have been used as can be seen from the wire spools and recording head. Although tape recorders were available at this time, the magnetic tape would not be able to give as much recording time or to survive the amount of heat or G forces that could occur in a crash.

The Mi51 covert unit is often confused with the next model, the P55, introduced by Protona in 1955. The P55 came with a wider variety of accessories and had control buttons that were more convenient to use as it was not normally hidden under clothing. It had both spools the same size and would still allow up to 4 hours recording, depending on model. The P55 was later replaced by a transistorised magnetic tape version, the Attache model, once this technology was sufficiently advanced, though recording time was limited to 1 hour, much less than the wire based models. Protona was taken over by Telefunken about 1962 who continued with the Minifon brand until 1967.

An excellent book in German on the Minifon series of recorders has been written by Roland Schellin and is titled "Der Spion in der Tasche" (The Spy in the Pocket).


These units are sought after by anyone interested in spy items or audio recording history. Mi51s are considerably scarcer than the later P55 machines which on internet auctions fetch around $US100, whilst the Attache tape model would fetch less. There were several other units during the life of the Minifon. Units are more likely to be found in Germany but those imported by EMI in the UK and by Geiss in the US come onto the market quite often.

Watch microphone

Current Condition

My Mi51 was obtained by chance in an auction lot comprising of a number of items, so nothing is known of its history, other than it was sourced from the USA since it is marked by Geiss-America, the importers. It appears to be in original condition, with the usual signs of age, but no damage. There are no batteries, leather case, speaker or microphone. The input spool is fitted with wire for 1 hour's recording. No attempt has been made to test its operational condition, partly having no batteries and also due to the risk that mild corrosion on wire that has been spooled for many years could lead to breaks or wear on the recording head. I have not examined the underside of the chassis due to the difficulty of removal of the set from the case. The most likely cause of failure of these sets would be failure of the rubber drive belts over the years, and therefore few would probably work without restoration. I also have two examples of the later P55 wire recorder. One is the basic version of the P55 in original box with manual and speaker/mike and the other a more comprehensive set complete with attache case and several accessories including a watch mike.

Additional pictures of these recorders, including a circuit of the Mi51, can be seen in my Mi51 & P55 gallery.


Count since 28/6/07.
Page maintained by Simon Buxton VK2UA . Created: 28/03/07 Updated: 16/01/2010