[LAU] Hum pickup in DI boxes

Bill Purvis bill at billp.org
Tue Jun 8 17:01:44 CEST 2021

On 08/06/2021 13:48, David Kastrup wrote:
> Bill Purvis <bill at billp.org> writes:
>> Not strictly a Linux problem, but I hope for advice.
>> We're using a Behringer X32-Rack mixer for our church PA system. One
>> of the inputs that we need is
>> for people to plug in laptops - usually via the headphone socket into
>> a Stagg dual DI box. We seem
>> to get a lot of hum pickup on that, which I think is down to the
>> proximity to the laptops.
> Unlikely.  You are connecting the Lo-Z outputs via two XLR cables to the
> mixer, right?
> The main question is just _how_ you are connecting your laptop to the
> DI since the connectors are not identical and loads of adapters exist.
> The correct way would use a 3.5mm TRS connector into the laptop (it is
> important that it be TRS and not TRRS because laptops with combined
> microphone/headphone socket sometimes have the microphone on the second
> ring, sometimes on the shield and misusing the semi-floating microphone
> input as ground would cause serious hum) and two 6.3mm TS plugs into the
> DI (with S taken from the 3.5mm TRS plug, and T of one 6.3mm TS plug
> taken from T of the TRS plug, and T on the other taken from R of the TRS
> plug).  Sometimes one needs to piece a few adapters together to arrive
> at such a configuration, and it is important that those adapters don't
> just have the right plugs but also the correct wiring including what to
> use for shielding.
> So what adapters _are_ you using here?
3.5mm TRS plug both ends into a 3.5mm-1.4" TRS jack, into the (mono) DI 
box input.
I've also tried a lead with 3.5mm TRS wired to a 1/4" mono jack plug 
just the TS connections). Positioning of the DI box affects matters, 
moving it
well away from the desk reduces the hum to negligible level, but this is 
acceptable for the regular usage. Usage is mono-only.


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

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