[LAU] Hum pickup in DI boxes

[Bill Purvis]

On 08/06/2021 16:46, David Kastrup wrote:
> 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?
1.4"->1/4" typo!

It's a combined 3.5mm socket and 1/4" plug, both TRS.
> 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.
The 1/4" sockets o9n the D/I box are the standard TRS plastic sockets, 
with the R
wiper omitted. I did check!

>> 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.
I've been wiring thing for over 70 years. I know enough to use screened
wiring. In fact the cable came with the 3.5mm plug, I chopped the other end,
checked which one goes to the tip and wired that the T of the 1/4" jack.
>> 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?
Not really bothered which, but I guess it's usually the L. A mix would 
be nice,
but clearly needs more subtle treatment. No-one is going to know the 
difference
here. The setup makes no pretense of being stereo! Four channels out, 
all the same!

Bill

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