+1.

See also: Magnetic Amplifier Control for Simple, Low-Cost, Secondary Regulation by Bob Mammano, Unitrode Corporation (now Texas Instruments).

https://www.ti.com/lit/ml/slup129/slup129.pdf

Are these still around though? Single-rail designs have been trickling down into lower and lower price points, meanwhile the cheapests power supplies never had independently regulated rails in the first place.