Electronic – Series regulator noise levels

The 60Vdc input from wains transformer is Valid .This means a 60 volt switching waveform and not a 360Vdc switching waveform.Linear post reg is good ,if there are concerns about HF rejection then extra LC filter before the reg is good.The coils and caops for this are not a drama at an average current of 5A .Synchronous will as orthodoxely implemented will be much more noisey at HF .If you still want synch then run the bottom switch in "FIODE " mode .This means that the switching regime makes the fet behave like a diode but with a lower volt drop that is dictated by RDS on .I think that a schottky diode rated at say 100V will be more sensible.Now deal with your turn on by whatever means because turnon into a conducting diode makes lots of HF and VHF noise which is more difficult to filter because capacitors look inductive and inductors look capacitive.I have used a S trap buck for Variable volts off inputs up to 820VDC so 60 wont be a problem . The S trap is a second cousin of a valley switch. Use lots of MLC caps because they are better at HF ripple suppression than ALIMINIUM ELECTROLYTIC CAPS.Sure you need thousands of microfarad on buck input due to 50/60Hz power but your output wont need much electro .Most importantly the ceremics deal with the switchmode ripple .Your design should be doable in SMD.

To clarify, we want to find a good, clean, low-noise power supply for work with our circuits which require +/-12VDC and 500 mA current. We're just not sure what the simplest, most straightforward approach is.

Okay!

The simplest is a linear power supply: IEC mains filter if required, transformer, rectifier bridge, caps, and a regulator. Really oldskool.

For your measurement stuff (and microphone preamp) the best is to have low interwinding capacitance in the transformer which reduces 50Hz leakage current through your grounds. For this the best is split bobbin EI core, which conveniently is also the cheapest... Toroid has less magnetic flux leakage but if you shove the power supply further away on your lab bench from the sensitive bits then flux leakage does not matter.

OK, you say you don't want to work with mains, but it isn't a problem if you do it right, make sure there is no easily exposed copper at high voltage. If you use a PCB mount transformer, just tape a nice thick piece of photocopier transparent on the back of the PCB. I always do this. The point is that component pins do not puncture it because it is thick. So if a finger wanders in the hot zone... it is safe. If you use a chassis mount transformer, use heat shrink. Or put the thing in a box.

"Low noise" power supplies are not necessarily worth the hassle either. For example, if you use a good old LM317 you can expect a few mV ripple on the output, at frequencies where your opamps will have 80dB PSRR or more...

Since you mention TPS7A47: this is a very low noise regulator... but if the load is opamps, ripple rejection matters more than noise. Opamps have high PSRR: 1µV noise or 100µV noise on the supply matters little if the opamp has, say, 60dB PSRR, which is a factor of 1000.

Usually you will have a few critical bits of the circuit which require a very clean supply but draw low current (like the microphone preamp) and other parts of the circuit which will work fine with a less clean supply but drive high current (like the loudspeaker driver).

You could build a very complicated, low noise power supply like you suggest. But it would still have a non-zero output impedance, if you include the wires and everything. High currents drawn from the power amplifier would thus influence its output voltage.

A much simpler option is just stick some RC filters with large capacitors on the supply of the really sensitive circuits. Or a local regulator. If your microphone preamp draws 10mA, just put a 1000µF low-ESR cap and a 100R resistor. You lose one volt... it costs 1€...

Please understand that the whole point of modern fancy regulators like LT3045 is that they are fast. This means they work with low output capacitance, which allows a small footprint, low height solution which you do not care about. This also means they have good HF PSRR which you do care about. However you can achieve the same with a passive filter. These regulators are not magical, just another engineering tradeoff (space, cost, height, etc).

Hm... I kinda went on a tangent here...