I have a small preamp that I use to feed a tiny poweramp+12″ for the purposes of monitoring my guitar while I am recording D.I. tracks. It is just a simple distortion unit with a fixed-value ‘tone stack’. I made a few adjustments to the capacitor values to tame the low end slop while I was recording solos yesterday and as a result it developed a nasty low-frequency hum. After an a period of fruitless of debugging, isolation, ferrites and RC filtering attempts, I started swapping out components to see if any of them were defective.
I didn’t make much progress until I began swapping out various capacitors at the input. The values didn’t seem to matter, but strangely the composition DID. Silver mica, ceramic (stacked and monolithic) and electrolytic capacitors hummed horribly, but when I swapped in an old Sprague metallized poly-film cap, that all went away and it was as quiet as a tiptoeing ninja in fuzzy slippers. I just chalked up the situation to my angering the gods of electricity somehow, BUT I will keep it in mind for the future.