On Tue, June 8, 2021 6:43 am, Bill Purvis wrote: > a lot of hum pickup on that, which I think is down to the > proximity to the laptops. The advice from David Kastrup about verifying the wiring of your cables is a good place to start. But it is certainly likely that a low cost passive DI would have transformers with no or poor shielding. Proximty related problems should be really easy to test, though, get a longer cable and move the box farther away. > 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. Not likely to be RF pickup if it is mostly 60/120/180 Hz noise. Could be multiple things going on, so you will just have to be systematic and note which things help, and does it help all of the noise, just the mains hum, or just the higher frequency components of the noise. There are two main places you find 60Hz in a laptop, the power supply, and the screen refresh rate. If you don't have a longer cable for the laptop to DI box, you do have a cable between the power supply and the laptop, so start by moving that farther away from the DI box. Does it help or not? Unless your battery is worn out, you can also remove the AC power supply from the situation entirely, so there is no mains power involved. Does that completely get rid of the problem or not? Does it get rid of the 60Hz hum, but not higher frequency buzzes? To check screen refresh signals you can move the DI box farther away, change the orientation between the DI box and the laptop, and modify the laptop power settings so that when you close the lid it only turns off the display and doesn't put the entire laptop to sleep. Close the lid and see what happens to the noise when the screen goes off. One other thing to verify, and this really should be first: either get someplace quiet, or use some sensitive headphones, and make sure the noise isn't always present on the headphone outputs. On my laptop I hear all kinds of noise on the headphone output when I use Shure in-ear phones, which are both relatively sensitive and also block out external sound pretty well. There is no way I would want to take that signal and amplify it to play over a large sound system, I can hear varying noise levels as different applications start and stop, as the screen pattern changes, all kinds of things leak into the headphone signal at a low level. Mostly covered by the audio when playing, but on quiet sections or when nothing is playing it is defintely noticeable, so no magic DI box is going to get rid of something that is present in the source. -- Chris Caudle _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx https://lists.linuxaudio.org/listinfo/linux-audio-user
