8 Jun
2021
8 Jun
'21
4:48 p.m.
Bill Purvis <bill(a)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