[LAU] Hum pickup in DI boxes

David Kastrup dak at gnu.org
Tue Jun 8 23:05:20 CEST 2021

Fons Adriaensen <fons at linuxaudio.org> writes:

> 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.

Puts me back when a physics professor in a lecture did not get a
volunteer for entering a Faraday cage connected to a band generator
producing impressive sparks and then went in himself.

That was at a time when Lavalier microphones were not yet customarily

Static electricity, sure, and the assistant stopped the band generator
almost immediately again.  It didn't help getting volunteers...

> 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.

David Kastrup

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