I had a terrible and constant low frequency whistle noise coming out of my bass pedalboard; even if the guitar is completely turned down. I solved my problem by plugging individual power adapters into problematic pedals.
Detecting the Source of the Noise
Rig hum can have many reasons. Three most significant reasons are; guitar pickup hum, ground loops and power issues.
My bass has split coil pickups, so guitar pickups couldn’t be the source of the noise. Besides; the hum was still there even when I turned the guitar down. So, the bass wasn’t guilty.
Ground loop could have been the reason; however, the hum started after I added some new pedals (not before).
Therefore, I suspected that there was a power issue. The fact that my pedals wouldn’t start up before blinking on and off for a while reinforced that idea.
Detecting Guilty Pedals
For this test, I removed each and every pedal from my pedalboard, and plugged my guitar in directly. No noise / hum at all. Good.
When I power any of those pedals individually via my Joyo Power Supply 2, they add up a little hum. Powering all of them end up producing a powerful hum.
Solving the Problem
I figured out that there was nothing wrong with the pedals. The only problem was; the power supply didn’t agree with the power demands of the pedals.
I disconnected those pedals from the power supply completely, and powered them through their original individual power adapters.
Wham! The hum disappeared. My rig went dead silent.
I figured that I needed a larger multi outlet for my pedalboard now; but it is a small price to pay for rig silence.
Using batteries instead of power adapters is also an agreeable solution, but I don’t want to worry about batteries going dead in the middle of some gig.