[LAU] Hum pickup in DI boxes

Fons Adriaensen fons at linuxaudio.org
Tue Jun 8 22:09:36 CEST 2021

On Tue, Jun 08, 2021 at 03:54:31PM -0400, Brandon Hale wrote:

> > Don't do this - ever. You could easily destroy your mic input(s) if the laptop
> > power supply isn't as isolated as it should be (quite common), or the laptop's
> > headphone output if there is phantom power on the mic input.
> That was my bad in assuming he's not using the microphone inputs, but the
> Behringer's aux inputs that have quarter inch jacks.

Wouldn't make much difference. If the laptop is mains powered and happens
not to be grounded you could easily have a hundred volts on the laptop's
'ground'. High impedance, almost 'static' and hence not lethal to humans,
but more than enough to kill an audio input. Watched it happen more than
once, the more spectacular case was when a certain university professor
deciced to cut of the ground connection of his laptop's power supply in
order to get rid of the hum. He didn't do that again.

Anyway, without transformer isolation and separate grounds you will never
get rid of the broadband interference generated by both the laptop itself
and its switching power supply.


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