Bill Purvis <bill@xxxxxxxxx> 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 _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx https://lists.linuxaudio.org/listinfo/linux-audio-user
