Design Flaw in the Super Pulse (SRT-SP) Covert Listening Device by the CIA

In 1973, the Dutch Radar laboratory (NRP) developed the SRT-SP, a covert device intended for clandestine listening applications by the US Central Intelligence Agency (CIA).

A SRT-SP consisted of the following parts:

• Regulator/Noise generator
• Video coder (Dirty Pulse audio masking scheme)
• Audio amplifier, described in this article
• Transmitter (RF unit)

SRT-SP Audio Amplifier with Automatic Gain Control (AGC) Reverse Engineered Schematic Diagram (Easy Chair Project):

Miniature audio amplifier integrates an audio pre-amplifier and an automatic gain control (AGC) circuit; however, a design flaw undermines its performance. Specifically, the second regulation stage (VT12) is incorrectly positioned prior to the AGC amplifier, rather than preceding the audio amplifier itself.

This misplacement results in a significant reduction in the potential regulation depth, as the second stage attenuates the AGC control level rather than the audio signal itself. Consequently, the effectiveness of the AGC mechanism is compromised, limiting the device’s ability to maintain optimal audio levels in fluctuating sound environments.

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