What can the source of 100Hz noise be?
user16307 asks:  "but do you have any idea why there is a full wave rectifier needed in an industrial amplifier?"

Have you researched full-wave versus half-wave rectification?

Half-wave rectifier raw output:



Full-wave rectifier raw output:



From these images, it is clear that the output of the full-wave rectifier reduces the energy storage requirements of the power supply filtering circuit.