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DC filtering... What the...

Discussion in 'Effects [BG]' started by vin*tone, Apr 1, 2009.


  1. I've been using a 9v SMPS for a while for all my pedals but recently got hold of an EHX Holy Stain which HATES it. I've done a bit of a search on this and it seems to be quite a common thing with not only the Stain but the Grail as well. If you're not using the EHX PS you get a pretty loud digital mosquito whine.

    I lent it to a guitarist to try out, warning him about the excess noise. He plugs it in, daisy chains it up to his Boss TU2 tuner DC out and....... SILENCE!

    I get really excited about this- I've made a few pedals and one of the tricks used to overcome a poorly filtered PS is to bridge across the terminals with a fat-assed capacitor. This shorts out any AC signal and leaves you with clean(er) DC voltage. I'm thinking the TU2 has a filter cap which is doing the filtering for the Stain. So I go home, plug the Stain into my TU2 and..... "eeeeeeeeeeeeeeeeeeeeeee". :)mad:)

    Plan B- I reach into the bag of tricks, pull out a 9V plug, 1000uf electrolytic cap and attach it to the daisy chain. It's better (about 50%) reduction, but still there.

    Plan C- added the uPOG into the chain- success, silence.

    I'm thinking it would be really handy to develop my little 'Plan B' DC filter to be added to a daisy chain (if there isn't such a thing already) to filter bad PS's. $1 worth of components could make more expensive filtered supplies unnecessary. Can anyone shed a bit more light on the circuit bits of the TU2 and POG that would be 'cleaning' the DC?
     
  2. Kinda fun having a public discussion with myself BTW.
     
  3. Seems like you might already know this, but the high pitched whine is the switch mode power supplies fault. The quick fix for this would be to go to a linear power supply, or find a switch mode that switches at a non-audible frequency.

    Re the filtering - a big cap is fine for getting rid of AC line frequency hum, but not so good at getting rid of high frequency noise (in practice), especially stuff with square edges. Try paralleling a large cap (1000uF), a medium sized cap (1uF) and a small cap (1nF) to see if that improves things.
     
  4. dannybuoy

    dannybuoy

    Aug 3, 2005
    England
    My EHX SMMH is quiet when run on a separate supply (the one that came with it), but introduces a high pitched whine into the signal when used on my 9v daisy chain. The whine is definitely coming from my pedal and not the power supply, as the pitch changes as I change modes or delay time, so I reckon it's some kind of digital clock noise coming from the electronics. Probably the same deal with your Holy Stain.
     

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