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Wire tapping
D.L.E. (kl)
Small wire intercept receiver - wanted item

Der kleine Drahtlauschempfänger (the small wire intercept receiver), commonly abbreviated to D.L.E. (kl), or Drahtlauschempfänger (klein), was a battery-powered portable eavesdropping amplifier, used for monitoring analogue voice communication on fixed and field telephone lines, developed by Opta-Radio (dlj) 1 in Leipzig (Germany), for use by the German Army during WWII.

The device is housed in a metal enclosure that is small enough to be fitted to the webbing of a soldier. It was used in combination with a similar size battery unit that was connected to the power contact pins at the left side, via a special flat cable, with locking connectors at both ends.

The device was usually supplied in a leather carrying bag, complete with the matching battery unit, cables, webbing and accessories. It is powered by LT and HT batteries that installed in the matching battery case that is attached to the side of the amplifier, optionally via a cable.
D.L.E. (kl)

The amplifier causes a high-impedance load to the line, typically ≤ 150 kΩ. Assuming that the analogue telephone line has an impedance in the range 300 to 800 Ω, the impedance of the device is close to ∞ (infinite). As a result, users of the line will not hear the typical click when the line is being intercepted [3]. Knobs are available for adjusting the impedance and line balance. Alternatively, the device could be used with a fully undetectable current probe or a microphone.

  1. From 1941 onwards, the German Army used manufacturing codes on all products, rather than brand names. The code (dlj) was used for Opta-Radio in Leipzig [2].

D.L.E. (kl) D.L.E. (kl) Front panel Front panel Rear view With open rear lid Valves (tubes) Wehrmacht Abnahme Stempel (stamp)
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D.L.E. (kl)
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D.L.E. (kl)
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Front panel
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Front panel
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Rear view
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With open rear lid
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Valves (tubes)
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Wehrmacht Abnahme Stempel (stamp)

The diagram below shows a typical setup of the D.L.E. (kl) with the matching battery case. In the image, they are connected by means of a special rubber flatcable that has purpose-made bakelite connectors at either end. The connectors mate with the power terminals at the sides of the two cases. If the cable is omitted, the male terminals of the amplifier can be mated directly with the female terminals of the battery box. In that case, the battery is attached to the amplifier's side.

All controls are at the bakelite front panel of the amplifier, along with terminals for connection to the analogue telephone line (A/B) and ground (Earth). When connecting the D.L.E. (kl) to the line under investigation, the ON/OFF knob (left) is used to match the impedance in such a way that the device does not affect the line and will be difficult to detect. Furthermore, the line balance can be adjusted with the knob at the centre. At the right is the volume knob. The intercepted audio is delivered to a pair of headphones that is connected to the 3-pin female socket at the lower edge.
D.L.E. (kl) connected to the battery Power contacts at the left side Power contacts on the battery case Power cable Power connectors Power plugs
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D.L.E. (kl) connected to the battery
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Power contacts at the left side
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Power contacts on the battery case
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Power cable
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Power connectors
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Power plugs

Possible uses
The DLE (kl) is suitable for the following applications:

  • Wire tapping
    This is the basic configuration of the DLE (kl), in which the device is coupled directly to the line under investigation.

  • Undetectable interception
    In this case, the device is used in combination with Lauschzange 35, making it possible to tap the line without any physical connection [A p.4].

  • Room bugging
    By connecting an unpowered microphone directly to the input terminals of the DLE (kl), the device is turned into a highly sensitive microphone amplifier, making it possible to eavesdrop on a conversation in a room [A p.5].

Leather storage case Wired intercept receiver Battery case Power cable Headphones Mains power supply unit (PSU)
High impedance microphone
Current probe (Lauschzange 35)
Storage case
When unused, the D.L.E. (kl) was usually stored inside the leather carrying bag in which it was delivered, together with the battery case, operational instructions, wiring, tools, spare batteries, and other accessories.

At present, we do not have a photograph of the original carrying bag, so we are showing the image from the manual instead [A].

This is the actual amplifier that is connected to the (analogue)telephone line under investigation. It is described above.

The device is powered by an external DC source, with separate LT and HT voltages, such as the battery case shown below. The power source should be connected to the male socket at the side of the case.
D.L.E. (kl)

The batteries are housed in this case, that has the same size as the amplifier. The case has two hinged lids – one at the front and one at the rear – behind which the 3V cells should be installed.

The voltages are available on the female socket that is embedded in the side of the box. This female socket mates directly with the male terminals at the side of the amplifier.
Battery case with one lid open

Power cable
Although the battery case was normally attached directly to the amplifier, it could also be used in detached mode, by using the optional power cable shown in the image on the right, for connecting the battery box to the amplifier.

This configuration was particularly useful if the device has to be carried inconspiculously on the body, in which case the amplifier was carried in the left pocket of a soldier's trousers, and the battery case in the right one.
Power cable

The DLE (kl) was typically supplied with a pair of standard Wehrmacht headphones, such as the Doppelfernhörer Dfh a (4000 Ω) or the Dfh h (15,000 Ω), both of which should be connected to the outer pins of the headphones socket at the front panel (20 mm distance).

The device is also suitable for connection of a low-impedance (50 ω) in-ear speaker, that can be connected to the pins with 20 mm distance.

Power supply unit   NAG-6/30
In rare situations where the D.L.E. (kl) had to be operated for an extended period of time, it was possible to power it from the NAG-6/30 mains power supply unit (PSU) shown in the image on the right, instead of the regular batteries.

According to the manual, operation from the mains was an exception, and the NAG-6/30 PSU had to be ordered seperately from the Heeres-Zeugamt (Nachr.) in Berlin [A].
NAG-6/30 mains power supply unit (PSU)

Rather than for tapping a telephone line, the highly sensitive amplifier can also be used for bugging a room, by connecting a microphone directly to the input. This works best with small unpowered microphones that have an impedance in the range 200 - 800 Ω (@ 800 Hz).

Microphones with a lower impedance can also be used, if a suitable impedance transformer is inserted between microphone and amplifier.

No image available.

Current probe   Lauschzange 35
Rather than connecting the DLE (kl) directly to the telephone line, it was also possible to use the special current probe of the larger Lausch-Empfänger LE-35 (intercept receiver 35).

As this probe is not physically coupled to the line under investigation, it does not cause any load or disbalance whatsoever and, hence, will be completely undetectable.

Inside the device is a current transformer with a partly hinged O-shaped metal core, through which one of the wires of the intercepted line should be fed. The device measures 120 × 70 × 60 mm and weights 590 grams [B].

 Read the documentation

Battery case with one lid open Battery compartment Markings on the battery case Power cable Power plugs D.L.E. (kl) connected to the battery NAG-6/30 mains power supply unit (PSU) NAG-6/30 front panel
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Battery case with one lid open
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Battery compartment
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Markings on the battery case
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Power cable
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Power plugs
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D.L.E. (kl) connected to the battery
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NAG-6/30 mains power supply unit (PSU)
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NAG-6/30 front panel

The amplifier is built around two identical space charge RV2,4P45 valves (tubes). This allows the same number of batteries to be used for the filaments (LT) and the anodes (HT). The HT batteries are connected in series – giving a total of 24V – whilst the LT batteries are connected in parallel.

Although the batteries are similar, the filaments consume much more energy than the anodes. It was advised to swap the 3V LT and HT batteries regularly, to ensure that they lasted equally long.

A good impression of the interior of the device is given on Arthur Bauer's excellent website about German technology — cdvandt.org — as shown in the image on the right. [3]. The date stamp inside the device reveals that – although the device was built in 1942 – it was developed and (partly) built a year earlier, in April 1941 [3].

 More information (off-site)


Circuit diagram
The diagram below shows hows the DLE (kl) works. At the left is the line interface that consists of a balanced line circuit ending in a transformer (U1). The 100k potentiometers are used to adjust the required impedance, started with the highest possible impedance when the unit is switched ON in order to avoid clicks on the line. Another potentiometer is used to adjust the line balance.

D.L.E. (kl) circuit diagram

Behind the first transformer (U1) is a two-stage amplifier, built around two RV2,4P45 valves. A 2M potentiometer between the two stages acts as the volume adjustment. The output of the second stage is delivered to a pair of headphones at the right hand side via transformer U2.

Complete kit
Technical specifications
  • Gain
    ~ 10 Neper (≈ 87dB) 1
  • Frequency
    300 - 3000 Hz
  • Impedance
    4000 - 150 kΩ (depending on position of ON/OFF knob)
  • Resistance
    DC ∞ (infinite)
  • Output
    20 mm bus: 15 kΩ for Dfh f, or 4000 Ω for Dfh a headphones
  • Output
    12 mm bus: 50 Ω for earphone
  • Valves
    2 × RV 2,4 P 45
  • LT power
    8 × 3V (21 mm) parallel
  • HT power
    8 × 3V (21 mm) series (24V total)
  • Current
    LT: 110 mA, HT: 3-4 mA
  • Life
    LT: ~ 40 hours, HT: ~ 350 hours
  • Weight
    Amplifier: 1.3 kg, Battery case: 1.1 kg 2 , Complete set: 4.9 kg 3
  1. 1 Neper ≈ 8.686 dB.
  2. With batteries installed.
  3. Leather case with amplifier, battery case, spare batteries and accessories.

  1. Der kleine Drahtlauschempfänger
    D 1063/1. 1 February 1943.

  2. Die Lauschzange LZ 35
    Current probe LZ-35 (German)
    D 1060/3. Berlin, 11 March 1938.
  1. Günter Hütter, Information about D.L.E. (kl)
    Retrieved July 2017. Photographed with kind permission.

  2. Liste der Fertigungskennzeichen für Waffen, Munition und Gerät
    Manufacturing codes for weapons, ammunition and equipment (German).
    Berlin 1944. Reprint 1977, Karl R. Pawlas.

  3. Arthur Bauer, Drahtlauschempfanger (klein)
    Retrieved August 2017.
Further information
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Crypto Museum. Created: Wednesday 02 August 2017. Last changed: Friday, 01 September 2017 - 17:09 CET.
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