Maybe unlucky or something wrong with the wiring. However, these modules are not "perfect" regarding PSRR and there is no way to get rid of it. You can see it in Amir's measurements as well. But, at the level where it is inaudible.@boXem | audio I recently had an amp with a Hypex NC122MP module in house for testing. It emitted an odd hum with a bit of mains (50 Hz) mixed with three peaks at 550, 650 and 700 Hz. Wasn't very loud, but always present, as long as the SMPS was powered - also in standby. Ever noticed this with your Hypex modules? Did I just get unlucky, or is this a common thing?