On 08/06/2021 16:46, David Kastrup wrote:
Bill Purvis <bill@xxxxxxxxx> writes:
On 08/06/2021 13:48, David Kastrup wrote:
Bill Purvis <bill@xxxxxxxxx> 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@xxxxxxxxx |
+----------------------------------------+
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