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).
The OBA/9 system evolved in the 1950s to replace OBA/8, with a smaller footprint and expanded facilities to meet the demands of outside broadcast engineers as the needs of broadcast expanded.
The whole system was designed to be ‘portable’ of a fashion; consisting of an MX/29 passive mixer, OBA/9 valved amplifiers with high gain and power supply, along with monitoring amplifier and distribution facilities. Ancillary items included a loudspeaker LSu/11 and communication units CMU/9 and CMU/10
The ‘Kingswood Warren’ system was originally custom built in the 1960s by R&D at Kingswood Warren for their loudspeaker testing facility.
Selected members of the audio fraternity were invited to listen to such famous BBC loudspeaker designs as the LS5/8 and LS3/5 monitors from behind a curtain, after drinks and canapes, as part of the design and commissioning process.
After the R&D establishment was relocated, the Kingswood Warren was re-purposed by EVE’s Radiophonic Studio as a unique mixing system, primarily for mastering use by Eve Mastering. Sadly the field telephone is missing, but everything else remains.
The Kingswood Warren is unique in being entirely modular. A vertical modular mixer!
Everything is patchable, with particular attention being paid to impedance matching between building blocks across the system to maintain level topology. The entire system operates at 600 Ohm.
Whilst the original design did not require dedicated amplification being purely passive, the Kingswood Warren is now powered by 1950s ultra linear BBC Type B valve line bridging amplifiers, with the ability to switch via cross patching to BBC Type B microphone amplifiers, decade attenuators and external amplifiers if and where desired.
The mixing console is based on a passive constant impedance design, having the unusual feature of mechanically ganged faders, making it ideal for stereo work. This is achieved through an amazing precision engineered cog system using machined brass teeth.
Special thanks to Chris Chambers, Alistair Biggar and Phil Macbean without whom the Kingswood Warren would never have been possible.
One of two ‘Foreign’ instruments heavily modified by the Radiophonic Workshop during the early years; the other being an acoustic six stringed guitar.
Each string of the zither was wound with a custom electromagnetic pickup system for pseudo stereo effect.
A BBC Monograph written as early as 1963 refers to these instruments in the inventory of Workshop II, along with many others, some of which have survived and are now held in deep storage by the Science Museum.