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Placement of RFI filters in a linear power supply for HiFi audio

Wayne

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#1
In designing a linear power supply for a HiFi component is it desirable to place RFI filters before the transformer or downstream from the transformer, say after the bridge rectifier?

Would the transformer pass sufficient RF from the house power (mains) that the filters should be both before and after?

If the linear power supply incorporated an isolation transformer, would that make a difference?

Thanks
 

DonH56

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#2
RFI filters usually go right at the input connector before the transformer, and should be grounded to the chassis so there is no RF return path inside the box.

Yes, the transformer can pass RFI. Broadband filtering and decoupling after the rectifier should handle anything at that point.

An isolation transformer in addition to the power transformer? Not sure the point but it would have the same problem as any other power transformer/inductor; shunt parasitic capacitance among the coils can pass RF signals.

SMPS (switch mode power supplies) require very careful design and layout. Properly implemented, they can be cleaner than most linear supplies, and their high switching frequencies places their fundamental noise well above the audio band (our designs operate around 500 kHz and up). I have helped design a few, but we have a board team dedicated to them, and they typically pass their design and layout (layout is critically important) back to the regulator manufacturer for review before the final layout is completed.

HTH - Don
 
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