Inside Abbey Road - The RS56 Universal Tone Control Unit

6th June 2019
Technical Engineer and Microphone Custodian Lester Smith take us through the iconic RS56 Universal Tone Control Unit. Commonly known as The Curve Bender, it was first designed in 1952 by our own engineer Mike Batchelor who later became the Chief Engineer at Abbey Road Studios.

It was the world’s first parametric equaliser and completely passive in design; it required no power. It was initially intended for the disc cutting rooms but sometimes made its way into the recording studios, if special permission was granted. Often used on Beatles recording and mixing sessions as their style became more experimental, the unit was tremendously more powerfully than the comparatively primitive REDD mixing console EQ.

Inside the RS56

"The RS56 has three similar sections covering the bass, treble and top ranges of the audible frequency spectrum. Each section has three controls:

1) A stepped control giving a 10dB boost or loss in 2 dB steps centred around a ZERO position

2) A ‘shaping’ control with 6 adjustable curve shapes – Low End, Blunt, Medium Blunt, Medium Sharp, Sharp and High End.

3) A selection of 12 different frequencies covering the full range from 32 Hz to 16,400 Hertz

Interestingly, all the chosen frequencies are based on musical divisions - exact octaves apart in the Bass and Treble frequencies and half octaves apart in the Top range. It is claimed that there are 14 million different combinations of the controls and it was nicknamed The Curve Bender because that is exactly what it did - it had a reputation for its “mangle” sound, which was actually against EMI’s original “true fidelity” concept, hence it commonly not being allowed for use in the recording studios in the late 50s and early 60s.
The unit is extremely heavy, having 18 control knobs doing the switching in measured and repeatable steps. A little later on when Stereo became the popular medium, it was upgraded with a second series of identical rotary switches and controls. This became the Right Hand Channel and was cleverly built to be side by side with the first series, now the Left Channel. They could be separated from or interlocked together using rack work gearing i.e. both channels could be altered equally, or the Right Channel could be disengaged completely or set to a different position. Its knob was much smaller than the Left Side to prevent the operator getting confused whilst working.
Ten years later, Mike Batchelor, after he had completed the design, building and installation of our first transisterised recording desks for all three studios, went on to design our first all transistor Transfer Desks. These were for the six disc cutting rooms housed on the top floor and is where I started in 1970 as assistant to the Senior Engineer Len Page. Without going into too much detail, the desk contained limiters, compressors, filters, faders, and tone controls now completely redesigned as two separated units.
Waves Audio and Abbey Road Studios have faithfully recreated the unique magic of the RS56, using advanced circuit modeling techniques based on the original schematics. The result is an extraordinary equalizer that is as effective today as it was when it was created over half a century ago.