noise of any switching PSU can be attenuate if I put linear regulator before output?

[sghosh]

Friend tolds me that noise of any switching PSU can be attenuate if I put linear regulator before output?

For example, if I wanting to power +-12V opamp for amplifier, I can uses SMPS say with noisy 15V output and then from SMPS output feed LM7812, LM7912.

Output from LM7812, LM7912 will now have very very low noise compared to what having at their input?

If this is true, this amazing as no need to use transformer anymore.

Is this really correct as no more heavy PSU using transformer for Class A, B amplifier then?

 

[keith1200rs]

Yes, it works. Another approach is simply to add some extra filtering (L & C) to a switcher.

Keith.

 

[mtwieg]

It will generally work for differential noise, but not common mode noise or emi (which may not be a big problem). You should at least try some passive filtering before resorting to linear regulators.

 

[dick_freebird]

You want to look at high frequency PSRR (or gain if no spec)
to get an idea of how effective this will be at each harmonic.
You probably want a ferrite bead before the LDO and its input
filter to knock off the switching edge spikes that blow through
the inductor winding capacitance, which the LDO will have no
prayer of tracking (especially a very capacitive PMOS LDO).

Some of the RFIC companies have started putting out LDOs
with specifically high, high frequency PSRR performance.