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Wehrmacht Wire tapping Eavesdropping
Small wire intercept receiver
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.
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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 same size
battery unit that is connected to the power contact pins at the left side,
or via a special flat cable with lockable 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.
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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.
During WWII, the D.L.E. (kl) was used by the German Army at the Eastern
Front, to eavesdrop on the telephone lines of he Russian Army. It was also
used by the various German intelligence and security services, such as the
Geheime Staatspolizei (Gestapo) — the secret state police.
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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].
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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.
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The DLE (kl) is suitable for the following applications:
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- 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].
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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].
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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.
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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.
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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.
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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.
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Power supply unit
NAG-6/30
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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].
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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.
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Current probe
Lauschzange 35
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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
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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.
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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)
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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.
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.
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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
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1 Neper ≈ 8.686 dB.
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With batteries installed.
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Leather case with amplifier, battery case, spare batteries and accessories.
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- Günter Hütter, Information about D.L.E. (kl)
Retrieved July 2017. Photographed with kind permission.
- Liste der Fertigungskennzeichen für Waffen, Munition und Gerät
Manufacturing codes for weapons, ammunition and equipment (German).
Berlin 1944. Reprint 1977, Karl R. Pawlas.
- Arthur Bauer, Drahtlauschempfanger (klein)
Retrieved August 2017.
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© Crypto Museum. Created: Wednesday 02 August 2017. Last changed: Wednesday, 03 March 2021 - 17:38 CET.
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