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Covert
Recorders
Nagra
  
Nagra SN →
  
Nagra DSP-1
Nagra SNST playback unit

DSP-1 is a playback unit for the covert Nagra SNST miniature stereo tape recorder, introduced in 1977 by Nagra in Cheseaux-sur-Lausanne (Switzerland). The device contains a sound-expander that matches the sound-compressor of the SNST, plus a stereo amplifier with built-in speaker. The DSP-1 is also suitable for the covert Nagra SN tape recorder — the mono version of the SNST.

The DSP-1 is housed in a hamerite grey metal enclosure with an eloxed aluminium front panel. It measures 10 x 8 x 15 cm and weight xxx grams (without batteries). It is powered by eight 1.5V AA-size batteries (12V) that are installed behind a transparent panel at the rear side.

All controls and connections are at the front of the device, together with the speaker and the meter. The DSP-1 has a fixed-length audio cable for connection to the Nagra SNST recorder. It has a double 3 mm jack at the end (for two channels) which can be stored to the left of the speaker.
  
Nagra DSP-1

When connecting the Nagra SNST, the double jack should be removed from the storage socket and inserted into the SNST. When using the DSP-1 in combination with the mono variant (SN), only one of the two pins of the double jack is used. The device contains a sound-expander that converts the compressed sound from the SNST back to its original dynamic range. For use with devices that do not have a compressor (such as the Nagra SN), the expander can be disabled.

Nagra DSP-1
Front view
Close-up of the controls
Close-up of the meter and the connections
Audio connector
Removing the audio connector
DSP-1 connected to a Nagra SN
DSP-1 connection to a Nagra SN
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Nagra DSP-1
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Front view
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Close-up of the controls
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Close-up of the meter and the connections
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Audio connector
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Removing the audio connector
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DSP-1 connected to a Nagra SN
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DSP-1 connection to a Nagra SN

Features
The image below shows the front panel of the DSP-1, which holds all controls and connections. At the centre of the unit is a rather large loudspeaker that produces a clear and crisp sound. At the upper edge of the case are four selector switches. The leftmost one is the ON/OFF switch. It is also used to select between mono and stereo playback. The second switch is used to turn the audio expander on or off. The other two switches are for the loudspeaker and the low-pass filter. The audio level (volume) is controlled by two large knobs (left/right) at either side of the speaker.

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Most of the connections are located at the lower part of the front panel. There are two stereo headphone sockets that accept a 6 mm jack, plus a set of banana-type sockets at which the audio is available at line level. The function of the meter can be determined by a switch to its left. It can be used to check the battery voltage, or the audio expansion of each channel (left/right).

The audio input (from the Nagra SN) is located to the right of the meter. It consists of a short fixed cable with a two-pin rectanglular connector at the end. When not in use, the cable runs past two circular guides at the lower corners of the case, and the connector is stored in a socket to the left of the speaker. It has to be removed when connecting the DSP-1 to the Nagra SN/SNST. When the unit is in use, the empty storage socket can be used for connecting a pair of earphones.

Close-up of the controls
Close-up of the meter and the connections
Audio connector
Removing the audio connector
Battery holder at the rear side
Battery holder at the rear side
Batteries installed at the rear
Connecting the DSP-1 to a Nagra SN
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Close-up of the controls
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Close-up of the meter and the connections
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Audio connector
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Removing the audio connector
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Battery holder at the rear side
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Battery holder at the rear side
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Batteries installed at the rear
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Connecting the DSP-1 to a Nagra SN

Interior
The interior of the DSP-1 can be accessed by removing one bolt from the top of the unit and two of the rubber feet from the bottom. This allows the front panel to be removed from the case. The DSP-1 consists of three Printed Circuit Boards (PCBs) that are mounted to the front panel.

The image on the right shows the interior after it has been removed from the case. All controls, the speaker and the meter are mounted to the rear of the front panel by means of metal stubs.

The three PCBs are mounted together as a 'sandwich' of which the bottom board contains the purpose-built battery holder. The remaining two boards hold the electronic circuits. All wiring of the boards is at one side, so that they can be separated easily in case of service or repair.
  
DSP-1 removed from its case

DSP-1 removed from its case
DSP-1 removed from its case
DSP-1 interior
DSP-1 interior, looking at the battery holder
Internal wiring
Switches and battery holder
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DSP-1 removed from its case
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DSP-1 removed from its case
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DSP-1 interior
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DSP-1 interior, looking at the battery holder
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Internal wiring
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Switches and battery holder

Further information
DSP-1 connected to a Nagra SN

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Crypto Museum. Created: Monday 06 October 2014. Last changed: Wednesday, 20 October 2021 - 07:24 CET.
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