Transistorized audio-frequency telephone line amplifier, probably designed and made at the Post Office Research Centre at Dollis Hill, London, England, 1953.

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 one of the earliest exploratory designs of transistorised audio-frequency telephone line amplifier and was probably made in 1953.