I want to make an external power supply for various pieces of "vintage" audio gear that run on +-18V, +48V, -9V and so on.

So the plan is to put several off-the-shelf AC/DC switchers (like Meanwell LPC-35-1050) into an external enclosure with some filtering. Then there would be a CPC connector with a length of multi-conductor cable to each piece of gear which would have 2-3 regulators to make the desired voltages.

But I'm not familiar with the noise characteristics of SMPS. The following circuit is my first attempt at a post-switcher filter / shutdown control. It consists a ballast cap, common mode choke LC filter and remote control power switch that doubles as a capacitance multiplier filter.

enter image description here

So the main question is whether or not the filter needs to get low frequencies. Currently this circuit assumes the switching noise will be in the 50 kHz + range. But is that really true? Can SMPS generate low-frequency noise?

More generally, does anyone see a problem with this design? Again this is for pro-audio gear using convential analog designs like for op amps and discrete "vintage" circuits so the supply needs to be very low noise.


1 Answer 1


In general an SMPS generates a lot of switching noise. However, most SMPSes use a switching frequency above 50 kHz as a lower frequency would be inefficient.

An SMPS can generate lower frequency components depending on its design and in particular the amount of current it has to supply. Many SMPSes switch continuously at the same frequency. However some SMPSes go into a pulsed mode (PFM: pulsed frequency modulation) when the load draws only a low current. This is done to improve efficiency at low loads. The pulsed frequency can be in the kHz range, so every 100 us - 1ms there will be a short burst of 100 kHz. This you might want to avoid in your application. If the PFM mode of your SMPS is a problem then you might be able to work around it by drawing more power from it.

Another way to suppress the switching noise is to use linear regulators whith high supression in the desired frequency range.

  • \$\begingroup\$ Thanks. The switcher I just bought for tinkering was $12 USD so hopefully it is not too sophisticated to use a "pulsed" mode. Still not clear about designing LC filters in this situation though. Should the choke and cap just be as large as economically / physically possible or is there some objectives to the design process like minimizing resonant peaks and such? \$\endgroup\$
    – squarewav
    Commented Jul 15, 2015 at 1:05
  • \$\begingroup\$ Also, can I use a choke described as an "AC line filter" for a filter like this which would be mostly DC? \$\endgroup\$
    – squarewav
    Commented Jul 15, 2015 at 1:40
  • \$\begingroup\$ Yes, a choke should work, with a capacitor behind it. Then you can use this instead of an LC filter (choke + C will behave in the same wey I guess). For the C I would use an electrolytic like 100 uF in parallel with a small ceramic cap like 100 nF. That should provide enough filtering. If you have an oscilloscope available you can check the effect of the filter and see what works best. \$\endgroup\$ Commented Jul 15, 2015 at 7:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.