I've had a problem with an unearthed Class II SMPS amp that would buzz when connected to an earthed audio source. The issue seems to be leakage current from the amp's ground to the earth through the aux cable shielding conductor. It is described more in depth here:

Ground Loop hum with Class I/II connections

I'm trying to find a solution to stop the buzz without changing the type of input (3.5mm aux jack) and came to think of these small audio isolation transformers.


They don't seem to do the job, as there is still leakage current to earth between primary and secondary somehow. I wonder how that would work as I thought whatever might be common mode to a transformer side couldn't make it across to the other winding.

Could someone explain what's happening, or if there is a better way to insulate the aux input to eliminate the buzz? I'm attaching the schematic of the previous amp, which in theory should have worked fine.

Cheers, A

Current diagram with transformer current diagram

Input section after the input isolation transformer input section without transformer

Original amp's input section previous amp input section

  • \$\begingroup\$ If the transformer does not help where you put it, the problem is elsewhere. You also don't show where you connected them and how. \$\endgroup\$
    – Justme
    Nov 16, 2022 at 15:25
  • \$\begingroup\$ Using a better 3.5mm Aux cable solved the issue. Even tho it appeared to work its shield conductor was around 8 Ohms which was enough to create the interference and probably a sign of it having degraded. I'm still unsure as to why the transformer solution wouldn't work, since it should create some sort of galvanic isolation between the two units. \$\endgroup\$
    – Alex P
    Nov 16, 2022 at 17:33
  • \$\begingroup\$ @AlexP did you perhaps accidentally wire the transformer like a common-mode choke, i.e. with its pins rotated 90° ? \$\endgroup\$
    – tobalt
    Nov 16, 2022 at 17:51
  • \$\begingroup\$ I did assume I've made that mistake, but checking with the multimeter it showed open circuit between what I've assumed was secondary and primary pins. \$\endgroup\$
    – Alex P
    Nov 17, 2022 at 19:32


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