Hum in active speaker caused by floating flyback SMPS

[Pafi]

Primary to secondary capacitive coupling have 2 different component.

1st for common mode primary voltage, which is excited by 50 Hz and related to the strength of coupling of your transformer. It's not easy to decrease without ruining magnetic coupling, so in case of fly-back your hands are tied (unless you use earth connection...) But it's not very high, typically 100 pF.

2nd for switching signal. Since 1 end of the coil is connected to switching node, and the other to the relatively clean DC bus, you have the chance to match clean end close to the output side, and keep distance or the best: shield switched side form output. If you can do it, your effective transfer capacitance can be significantly lower, hence you can also decrease Y cap.

Shielding inside transformer:
1 layer of thin wire between switched node wires and parts to protect (secondary side, core...), and connected to a clean primary potential (DC rail). If secondary number of turns is not very low, then it can also act as a noise source also! Match phases in order to maintain low voltage diifference between adjacent wires!

Always be careful with creepage distances!

 

[29285]

If increasing the Y-cap increases hum, but removing does not remove hum, I suspect you did not remove the right cap - or you have overseen a second y-cap.

 

[29285]

A noise spectrum of 50Hz, 150Hz etc may be expected with a 50Hz transformer. SMPS typically do primary rectification thus produce 100Hz and harmonics. So what are we talking about?

 

[powerbob]

If there was a schematic with values plus pictures, this would be a good thread. Without there can only be a lot of guess work. Please post schematic and pictures.

 

[Pafi]

A noise spectrum of 50Hz, 150Hz etc may be expected with a 50Hz transformer. SMPS typically do primary rectification thus produce 100Hz and harmonics. So what are we talking about?

If Y capacitor is connected to Power Line Voltage (probably this is the case here), and that voltage is distorted symmetrically, and capacitive coupling to the resistive input forms a high pass filter with +6 dB/octave gain, odd harmonics are unavoidable.