[LAU] Hum pickup in DI boxes

[David Kastrup]

Bill Purvis <bill at billp.org> writes:

> 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,

That doesn't parse.  What does "both ends" mean?  What does "3.5mm-1.4"
TRS jack" mean, apart from 1.4" probably being a typo for 1/4"?  What
kind of socket is there, what kind of plug?

> into the (mono) DI box input.

Any TRS plug into the mono DI box input is unpredictable since there are
no guaranties which of RS will (if any) make a reliable connection to
the T of the socket and you have no guarantee that R will not get
shorted.

> I've also tried a lead with 3.5mm TRS wired to a 1/4" mono jack plug
> (effectively just the TS connections).

That should work for the left channel.  What does "wired" mean in this
context?  You need to make this connection with a shielded cable, with
the T-T connection being the core of the cable and the S-S connection
being the shield.  If you just put in wires, that's where your noise
injection will happen.

> Positioning of the DI box affects matters, moving it well away from
> the desk reduces the hum to negligible level, but this is not
> acceptable for the regular usage. Usage is mono-only.

Meaning you want to attach to both L and R of the laptop (creating a mid
signal) or to just L?

-- 
David Kastrup