Transistorized audio-frequency telephone line amplifier, probably designed and made at the Post Office Research Centre at Dollis Hill, London, England, 1954.

Very soon after the development of successful transistors in 1947, the Post Office Research Station at Dollis Hill, London, were working on their use in telephone repeaters (amplifiers). This is a single transistor amplifier with a diode in order to protect the transistor against electronic surges. Seventeen of these were constructed in the latter part of 1954 and were put on field trial.