I'm in the process building a linear lab PSU, the fan which is supposed to cool the heatsink is causing 6 mV peak to peak noise at the output of PSU. It's a 4x4 cm brushless chinese fan with unknown brand.
I tested adding a wide range of capacitors from 0.1uF to 100uF, close to the fan circuit or on the supply and it didn't make any difference. also tried adding an LC filter (47uH with 220uF) with no luck.
Adding an overkill LC filter on the fan input smooths the output though, 100mH inductor with a 100uF capacitor is causing a huge drop in the current (fan voltage drops to ~4.5V).
The fan is being supplied from a different secondary than the main secondary:
The secondary that is supplying the fan is also used to supply the op amps in the PSU circuit and has a common ground with the main supply.
How do I eliminate the fan noise?