[LAU] Hum pickup in DI boxes

[Bill Purvis]

On 08/06/2021 19:52, Chris Caudle wrote:
> On Tue, June 8, 2021 6:43 am, Bill Purvis wrote:
>> Stagg dual DI box.
> Is it this device?
> https://staggmusic.com/en/products/view/SDIST-2-channel-passive-di-box-with-monostereo-switch/
That's the one!
> I don't see any specifications, so I'll just guess that it is a pair of
> low cost transformers inside the case.
That's what is inside.
> 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).
That does help. I'm investigating if I can place the DI box at the base 
of the desk
which is as  far as I can get and still have it within the confines of 
the desk.

> 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.
Slight problem there - the laptop that controls the mixer is so ancient 
the battery
is kaput! It will only work with the power cable in.
>> 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.
50 Hz here in the UK!
I don't see it much but it's the main sound source. The mixer RTA 
display shows
sound input across the spectrum.

I'm going to try a direct input, but need to make up a 3.5mm - XLR cable 
for that.
Something to try tomorrow!

Bill

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| Bill Purvis                            |
| email: bill at billp.org                  |
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