[LAU] Hum pickup in DI boxes

Fons Adriaensen fons at linuxaudio.org
Tue Jun 8 21:39:30 CEST 2021

On Tue, Jun 08, 2021 at 08:21:20AM -0400, Brandon Hale wrote:

> Have you tried plugging in the laptop without the DI in between? I think
> laptop outputs are balanced, low-impedance, so you shouldn't need the DI in
> between. Even if this is not true, it's at least worth a shot.

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.

Connecting laptops to a PA or recording system can be one of the most frustrating
things you may come across as a sound engineer. In particular if the laptop's mains
power supply is being used.

You need a DI box designed for this task. Those made for guitar etc. usually
won't do. Transformer isolation and a 'ground lift' switch are a must. This
doesn't mean the DI-box has to be 'passive'. There are a few which have both
transformer isolation AND phantom powered active amplification.

The one I used when working at the  Casa della Musica in Parma was the Radial
JPC <https://www.radialeng.com/product/jpc>. It worked perfectly, and I can
really recommend it. Not the cheapest at around $200, but worth every penny.



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