I used with guitar pedals and all have loud 60 Hz hum and high pitched crackling noise.

High pitch hum through amp using this power supply.

The power supply, which is a switching power supply, radiates harmonics strong enough to produce a noticeable back ground high pitched whine that is picked up by the signal path and comes through the amp

I spent about a half hour trying to isolate the high pitched squeal emitting from my Blues Jr. tube amp.