+5V to +/- 12V for op-amp: Switch mode then linear?

[snoopy33]

Hi,

Is a well implemented switch mode boost adequate for an op-amp or will it be too noisy, too much ripple etc and require a linear regulator?

So far my thoughts are to use a boost to take 5V to 15V and -15v and then to use a 7812 and 7912 to bring it back down - minus any artefacts.

Is this approach reasonable?

 

[FauxFrench]

It is a reasonable approach.
If you need linear ripple filters/regulators depends on what you use the OP-AMP(s) for. Non-critical use may not oblige you to use linear regulators as well. For high quality audio use I would definitely add the linear regulators.

 

[DonVK]

I do something similar to what you are proposing, on two different designs. One design uses a shunt regulator and other is a series regulator (78xx/79xx) after the SMPS. The post SMPS linear regulators remove the lower frequency bumps and ripples but not the higher HF components. The good news is that the residual HF does not seem to effect the op amp performance. I plan on adding an additional HF LC filter just out of curiosity when the parts arrive.

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[snoopy33]

Thanks, quality is uttermost important but I don't want to add an additional power supply (working from USB). I was tempted to do away with the linears but probably best not to.

 

[Nisbeth]

Definitely a reasonable approach, but again - the linear regs don't do much to remove the HF noise on the output of the switcher.

Adding a Pi-filter (C-R-C or C-L-C) on the input of the linear regulator should help a lot, just watch a) the voltage drop in the series resistor/inductor so the regulator doesn't drop out (because then it becomes noisy) and b) the maximum load capacitance allowed on the output of the switcher (so you don't damage anything).

I've done pretty much the same using ready-made DC-DC converter modules (and AC-DC PSU modules) and it seems to work well.

 

[OlegSh]

I second what Nisbeth says. I also used a ready made isolated DC-DC converter with +5V input and dual rail -15V and +15V output. I used LDO regs to regulate it down to -12V and +12V with Pi filters on the input and on the output of the converter and got really good results. The final design is shown here and the ready made PSU is here. I used the same PSU idea for the headphone amp (design is shown here) with very good result too.

Regards,
Oleg

 

[JPS64]

Input filter (normally suggested in DC/DC booster datasheet) + booster (@OlegSh ones not bad) + filter (then booster used: see datasheet for max. cap.) + low noise LDO.
I‘ve done one with @xrk971 following after LDO cap multiplier and CRC. Well, happy with the results.

JP

 

[snoopy33]

Looks good. Why's an input filter required pre boost? It's not amplifying the voltage so I wouldn't have thought noise would transfer through? The boost after all is a large LC.

Andrew