The OBA/8 system was introduced in 1938. Intended for portable use with war in Europe looming, it was more lightweight then predecessors and utilised a pentode valve for the first time in British amplifier design.
The whole system, designed for self operation, consisted of two OBA/8 amplifiers (one on standby), a passive MX/18 mixer, LSU/1 loudspeaker and comms.
Many units, particularly the MX/18 mixer, continued to find a use long after peace had been declared, in provincial studios and of course the Radiophonic Workshop.
Here is a picture of Daphne Oram with three such passive mixers, which utilised wire wound constant impedance attenuators (and an OBA/8 in the foreground).
