[yestertech]

The BBC took great care on all engineering matters back in the day. There were very few places hum could intrude in reality, as all external connections of any length were always done transformer balanced and at line level. Low-level signals like pickup cartridges and microphones were duly amplified within a unit before leaving for any form of rating and processing.Low level mic signals would be routed direct to an amplifier ( balanced, of course ) which in valve days would have likely been an AMC/5 of which there were many per studio.This remarkable 2 valve amplifier (ECC8x) had a noise floor within 1 db of theoretical maximum, even in the 50s !
Most HT units had some form of stabilisation ( usually neon ) and the later transistor ones had fully regulated split rail PSU's .In these, even the mains transformers were mu metal shielded as they were generally mounted in closer proximity to their amplifiers than in valve days.

The BBC were also very scrupulous about earthing. Mains and 'program' earth kept separate and large copper earth busbars would be found in the back of equipment racks.
Studios were well isolated, and had massive dual doors and double glazing. Acoustically, they nearly always seem to get the treatment just right for general purpose studios. Drama studios often had 'live and 'dead' areas within the same studio.

I recall several incidents where the relatively starchy continuity announcers in R3 and R4 had to apologise on air for a noisy amount of drilling which was taking place somewhere in the building at the vital moment. It took ages to find out exactly where and get it stopped.
Happy days indeed !

Andy