[LAU] Hum pickup in DI boxes

[David Kastrup]

Bill Purvis <bill at billp.org> writes:

> Not strictly a Linux problem, but I hope for advice.
>
> We're using a Behringer X32-Rack mixer for our church PA system. One
> of the inputs that we need is
> for people to plug in laptops - usually via the headphone socket into
> a Stagg dual DI box. We seem
> to get a lot of hum pickup on that, which I think is down to the
> proximity to the laptops.

Unlikely.  You are connecting the Lo-Z outputs via two XLR cables to the
mixer, right?

The main question is just _how_ you are connecting your laptop to the
DI since the connectors are not identical and loads of adapters exist.

The correct way would use a 3.5mm TRS connector into the laptop (it is
important that it be TRS and not TRRS because laptops with combined
microphone/headphone socket sometimes have the microphone on the second
ring, sometimes on the shield and misusing the semi-floating microphone
input as ground would cause serious hum) and two 6.3mm TS plugs into the
DI (with S taken from the 3.5mm TRS plug, and T of one 6.3mm TS plug
taken from T of the TRS plug, and T on the other taken from R of the TRS
plug).  Sometimes one needs to piece a few adapters together to arrive
at such a configuration, and it is important that those adapters don't
just have the right plugs but also the correct wiring including what to
use for shielding.

So what adapters _are_ you using here?

> We have another identical DI box at the other end of the room which
> takes keyboard and bass and I've had no problems with that. Is there
> an alternative method we can try? We do raise the ground lift switch
> on the DI box, which reduces the hum somewhat but still get enough hum
> to be noticeable when nothing else is playing. I wondered if the
> matching transformers in the DI box are acting as pickups for the RF
> noise generated by the laptops. The mixer on EQ display with RTA shows
> noise across the audible spectrum, but most of the audible sound is
> mains hum.

-- 
David Kastrup