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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?

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  • \$\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
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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.

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  • \$\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

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