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Nagra DSP-1
Play-back amplifier for Nagra SN

DSP-1 was a small battery-powered audio amplifier that was developed in the late 1970s by Nagra in Cheseaux-sur-Lausanne (Switzerland), especially for the play-back function of the Nagra SN/SNST subminiature tape recorder. Although the device was designed as an add-on for the stereo version of the recorder (the SNST), it could also be used with the mono version (SN).
The image on the right shows a typical DSP-1 amplifier. It is housed in a hamerite grey metal case with an eloxed front panel and measures just 10 x 8 x 15 cm. It is powered by eight 1.5V AA-size batteries (i.e. 12V), making it the ideal companion for mobile and portable applications.

All controls and connections are at the front of the unit, together with the speaker and the meter. The DSP-1 has a fixed-length audio cable for connection with the Nagra SN 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. Like the Nagra SN, the DSP-1 can only be powered by batteries, which are installed behind a transparent cover at the rear of the unit.
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

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.

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

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

Further information

DSP-1 connected to a Nagra SN

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Crypto Museum. Created: Monday 06 October 2014. Last changed: Saturday, 13 May 2017 - 07:45 CET.
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