I am trying to build a power supply unit that powers eight 9V guitar pedals (usually pedals consume 10-100mA). So, I thought I'd buy 8 cheap 9V-1A SMPS adapters and probably fix some bypass capacitors at the output to remove the ripples. This would make a very powerful isolated power supply unit with eight independent 9V-1A ports.
But this didn't work as significant humming noise was heard when connected to pedal and to amp.
I compared the performance by powering the pedal with a linear power supply which consists of a single transformer and eight LM7809 regulators at parallel outputs.
Result: Linear power supply gave very less humming noise than the cheap SMPS.
A probable problem could be that the switching frequency of the cheap SMPS is not high enough, even though I am not sure of it.
What Can I do to remove the humming noise from the cheap SMPS unit?