Bill Purvis <bill@xxxxxxxxx> writes: > On 08/06/2021 13:48, David Kastrup wrote: >> 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? > 3.5mm TRS plug both ends into a 3.5mm-1.4" TRS jack, That doesn't parse. What does "both ends" mean? What does "3.5mm-1.4" TRS jack" mean, apart from 1.4" probably being a typo for 1/4"? What kind of socket is there, what kind of plug? > into the (mono) DI box input. Any TRS plug into the mono DI box input is unpredictable since there are no guaranties which of RS will (if any) make a reliable connection to the T of the socket and you have no guarantee that R will not get shorted. > I've also tried a lead with 3.5mm TRS wired to a 1/4" mono jack plug > (effectively just the TS connections). That should work for the left channel. What does "wired" mean in this context? You need to make this connection with a shielded cable, with the T-T connection being the core of the cable and the S-S connection being the shield. If you just put in wires, that's where your noise injection will happen. > Positioning of the DI box affects matters, moving it well away from > the desk reduces the hum to negligible level, but this is not > acceptable for the regular usage. Usage is mono-only. Meaning you want to attach to both L and R of the laptop (creating a mid signal) or to just L? -- David Kastrup _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx https://lists.linuxaudio.org/listinfo/linux-audio-user
