Early switched mode power supplies.

[ppppenguin]

I used to own a bench PSU made by IE (International Electronics?) type DS 50 50 2 (I think). It had conventional mains transformer, then switching pre-regulators running at low frequency (10s or 100s of Hz, variable, you could hear them change frequency as the load changed) followed by linear post regulators. It was dual output, 0-50V 2A. There was also a single output version. The BBC used a lot of these. I know it dates from before 1974 but have no evidence as to how much earlier.