[LAU] Hum pickup in DI boxes

Chris Caudle 6807.chris at pop.powweb.com
Tue Jun 8 20:52:59 CEST 2021

On Tue, June 8, 2021 6:43 am, Bill Purvis wrote:
> Stagg dual DI box.

Is it this device?

I don't see any specifications, so I'll just guess that it is a pair of
low cost transformers inside the case.

David Kastrup's advice about verifying the adapters cables is probably the
first place to start.

> to get a lot of hum pickup on that, which I think is down to the
> proximity to the laptops.

The only mains frequency noise around a laptop would be the power supply,
which you should be able to move farther away since it is on a cord, and
probably the LCD screen refresh rate is running at 60Hz.
If it is a proximity problem because the DI transformer is not shielded,
you should be able to tell by using a longer cable between the laptop and
the DI box and just moving the box farther away.  Put the AC power supply
farther away, or just remove it temporarily and run on battery, and if
that is not the problem put the box farther away from the laptop screen
(or change the power settings to only power off the screen when you close
the laptop, not put the entire laptop to sleep, and just close the lid).

> keyboard and bass and I've had no problems with that.

Bass should be easy because it does not have a connection to AC power
supply, and most keyboards are low enough power that they have an isolated
power supply with only two pin power entry, no third earth pin, so they
are effectively floating sources and so pretty easy as well.  Computers
are almost all high enough power that they have a connection to safety
earth, which you can get rid of temporarily for troubleshooting by running
on battery power.

> 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.

Have you made sure that the volume controls on the laptop are at maximum
so you can optimize the gain structure?  If you have not, you may be able
to make the hum less noticeable by turning down the gain at the mixer and
turning up the output from the laptop.

> I wondered if the matching transformers in the DI box
> are acting as pickups for the RF noise generated by the laptops.

If you are seeing mostly 60Hz noise probably not RF, but could still be
magnetically coupled from the laptop or the laptop power supply.

Chris Caudle

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