I'm building a guitar pedal which uses a series of relays to switch various signals. The relays are currently being powered by a cheap 5V switchmode supply and the audio path is inducting high frequency noise (109Khz) which I assume is from the power supply.

I cant do anything about the power supply - it will always be a switchmode wart though what should I do about replacing this noise? Will an electrolytic + voltage reg filter the noise?

  • \$\begingroup\$ electrolytics won't filter the noise that well (bulk storage). A PI filter using CM and diff inductors with capacitors differentially and common would help. Stear the "noise" to earth \$\endgroup\$ – JonRB Dec 3 '14 at 11:00
  • \$\begingroup\$ Please see these answers: electronics.stackexchange.com/questions/36701/… electronics.stackexchange.com/questions/92692/… \$\endgroup\$ – akellyirl Dec 3 '14 at 11:52
  • \$\begingroup\$ Are you using mechanical relays, or Solid State relays? Is the noise constant, or just "clicking" when the relays open/close? What have you got that's detecting noise at 109KHz? I'd normally suggest a low pass filter on the audio (say 30KHz) but some audiophiles complain these "distort" the phase / harmonics / something like that. \$\endgroup\$ – Alan Campbell Dec 3 '14 at 12:36
  • \$\begingroup\$ Are you sure the interference is coming from the supply? My favorite power supply noise "hammer" is the (transistor) capacitor multiplier. en.wikipedia.org/wiki/Capacitance_multiplier \$\endgroup\$ – George Herold Dec 3 '14 at 14:04
  • \$\begingroup\$ 109Khz, it seems just the normal switch power supply's frequency. Try use LDOs to do post filter. \$\endgroup\$ – diverger Dec 3 '14 at 16:00

If you need clean 5V, you could start with, say, 9V from a switch-mode supply and then drop it down to 5V with a linear regulator. A good old 7805 three-terminal regulator (with capacitors) might help.

But first, do make sure that the noise really is coming from the supply. Use batteries, or a lab-type bench PSU, as a temporary substitute for the switch-mode.

  • \$\begingroup\$ Installed a regulator today but only getting 4.3V out of it from a 5.2V supply - specs say min 7 :/ Will give batteries a shot - don't have a bench supply \$\endgroup\$ – Alex Turner Dec 4 '14 at 9:48
  • \$\begingroup\$ @AlexTurner That's why I suggested a 9V switch-mode. The regulator needs a couple of volts to work with. You won't be able to use a linear regulator if you expect the input and output voltages to be the same. Can you borrow a bench supply? At a local Hackspace? \$\endgroup\$ – John Honniball Dec 5 '14 at 19:32
  • \$\begingroup\$ Wish I had local hackspaces! Not enough proper technies in Sydney - may invest in a bench supply \$\endgroup\$ – Alex Turner Dec 6 '14 at 5:31
  • \$\begingroup\$ @AlexTurner A quick search showed up Robots and Dinosaurs, in Meadowbank: robodino.org Also, this Wiki link: hackerspaces.org/wiki/Sydney Any good? \$\endgroup\$ – John Honniball Dec 6 '14 at 8:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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