"polarity protection" diodes - more useful than you might think

Posted by HamishR on Nov 24, 2018; 3:25am
URL: http://guitar-fx-layouts.238.s1.nabble.com/polarity-protection-diodes-more-useful-than-you-might-think-tp44848.html

I sometimes drop into my local music store with a newly built pedal as I have friends there who want to try things.  One thing I noticed when I first got into building pedals was that sometimes my pedals would make whining noises at the shop when plugged into cheap power adaptors in a room with fluorescent lighting, lots of amps and gear.  So much so that the pedals were unusable.  Weird, because it never happened at home.

The remedy is simple.  Just stick a 5817 or 4001 or whatever between the 9V into the amp and the rest of the circuit and the noise disappears.  So it would seem that as well as potentially protecting your pedal that diode is preventing noise from ruining your day.

I am no expert - I know next to nothing about electronics, but have built more than my fair share of tube amps and FX pedals.  So this is purely anecdotal.  I have no idea why this would work!  But I never hear this point being discussed.  The diode is only ever referred to as "polarity protection" in the sense that it may blow if the wrong power is applied.  But I have found it is essential in some pedals to avoid noise issues from (possibly poorly filtered) power supplies.  A 100ohm resistor can help in some cases too.

I'm posting this just in case it helps some of you guys with this issue.  I hope it helps someone!