[LAU] Hum pickup in DI boxes

[Will Godfrey]

On Tue, 8 Jun 2021 13:52:59 -0500
"Chris Caudle" <6807.chris at pop.powweb.com> 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/
>
>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.
>

A while ago I was surprised to find that some very cheap USB modules (mic in
phones out) to my great surprise *totally* isolated the audio from the USB. I
don't use the mic, just the stereo out (at 48k) Even though it's on a 3.5 TRS
the quality is good enough for all but the most demanding purposes. They go by
the name ugreen.

-- 
Will J Godfrey
https://willgodfrey.bandcamp.com/
http://yoshimi.github.io
Say you have a poem and I have a tune.
Exchange them and we can both have a poem, a tune, and a song.