[LAU] Hum pickup in DI boxes

Chris Caudle 6807.chris at pop.powweb.com
Tue Jun 8 21:42:45 CEST 2021

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

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