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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?

Alex Turner
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  • 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 –  Dec 03 '14 at 11:00
  • Please see these answers: http://electronics.stackexchange.com/questions/36701/power-supply-noise-in-audio http://electronics.stackexchange.com/questions/92692/smoothing-and-decoupling-power-source-input/101436#101436 – akellyirl Dec 03 '14 at 11:52
  • 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. – Alan Campbell Dec 03 '14 at 12:36
  • Are you sure the interference is coming from the supply? My favorite power supply noise "hammer" is the (transistor) capacitor multiplier. http://en.wikipedia.org/wiki/Capacitance_multiplier – George Herold Dec 03 '14 at 14:04
  • 109Khz, it seems just the normal switch power supply's frequency. Try use LDOs to do post filter. – diverger Dec 03 '14 at 16:00
  • @Diverger, aren't LDOs usually designed to filter 60 Hz ripple? I thought the high-frequency rejection was normally pretty bad. – Adam Haun Dec 03 '14 at 16:09
  • @AdamHaun: There indeed some LDOs designed to be post DC/DC regulator and ripple filtering, such like tps7a4901 and tps7a3001. – diverger Dec 03 '14 at 16:13
  • Hey all - thanks for all the helpful info - went out today and bought an electrolytic and a 7805 LR - not much luck unfortunately - will try GeorgeHerold's suggestion tomorrow!

    @AlanCampbell - 109Khz is coming from the DSO and want to keep the audio pure - just switched. They are mechanical relays but the noise is definitely HF coming from the SMPS

     – Alex Turner Dec 04 '14 at 09:47

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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.

John Honniball
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  • 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 – Alex Turner Dec 04 '14 at 09:48
  • @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? – John Honniball Dec 05 '14 at 19:32
  • Wish I had local hackspaces! Not enough proper technies in Sydney - may invest in a bench supply – Alex Turner Dec 06 '14 at 05:31
  • @AlexTurner A quick search showed up Robots and Dinosaurs, in Meadowbank: http://robodino.org/ Also, this Wiki link: http://hackerspaces.org/wiki/Sydney Any good? – John Honniball Dec 06 '14 at 08:44