Filtering analog power supply
OK, let's go.

Most regulators have crummy HF PSRR, so we begin by adding a ferrite bead (I checked the ratings), Murata provides all spice models so they're already loaded into my simulator which is convenient. A cap to ground (1µF 100V MLCC) and a series resistor to prevent ringing. This gives a nice HF PSRR boost (see graph).



Next we have two choices. LM317 is out, as ratings will be exceeded at power-up when output voltage is zero (caps discharged) and input voltage is 50V. Thus we have to use LM317HV, or another HV regulator, or hack one together from some spare parts.



Red is LM317, black is roll-your-own. Top is PSRR (ferrite included), bottom is output impedance. Three traces for each (0mA, 20mA, 75mA output current).

DIY custom job wins on PSRR due to BD139 having low capacitance, also it has more feedback, but requires big output cap. Also it could be unstable if you screw it up. But let's be honest, LM317HV will be sufficient.

Note the ESR of output caps matter, I've specced cheapo Panasonic general purpose ones, plus a 100nF ceramic.

I'd use a switcher to get 15V from 50V, then LC filter (ferrite/resistor/cap) then LDO or LM317, that would do the job without a heat sink. If output impedance is not critical, a switcher followed by LC filter would be enough. Opamps work fine with a few ohms impedance in the supply, they usually have huge LF PSRR, it's the HF noise they have trouble with.