[Phil Allison[_4_]]

Mike Rivers wrote:

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I said that to check that, she could try connecting a different power
supply.


There isn't anything in a COMMERCIAL, APPROVED switching mode power
supply that can cause hum. They're surprisingly clean.


** Ones that come with your shiny new audio toy is likely to be hum free, but ones you buy on-line or pick off a shelf in a blister pak can be pretty poor.

Also, in order to comply with EMC regulations, nearly all external SMPSs have a ceramic cap bridging the internal high frequency transformer to the shield or ground on the DC output lead. Only transformer coupling eliminates the resulting high pitched buzz from other audio gear.

Finding a
suitable substitute power supply, unless she'd bench-equipped like Phil
and me and maybe you is likely to be difficult. You need the right
voltage, pretty close to the right current, and the correct size plug
wired with the proper polarity.


** I would never ask a customer to do what GM has asked the OP to do - partly because linear ( regulated or not) external DC PSUs are banned from sale here in Australia. They fall foul of minimum energy efficiency regulations.



...... Phil