Headphone outputs are a strange beast. Unbalanced, for sure (a single signal
lead for each channel, with a common ground return), but much lower impedance
than you would expect for an unbalanced line out.
I've struggled as well, trying to find a consistently reliable DI for this
application. There are several on the market which specifically address this
issue. Search for 'laptop interface' or 'A/V adapter' to find some of the ones
I've tried.
I've also resorted on occasion to abandoning the headphone jack altogether,
and using an external USB soundcard/interface instead, which provides proper
balanced outputs.
Rick Green
On Tue, 8 Jun 2021, Brandon Hale wrote:
Hello,
Have you tried plugging in the laptop without the DI in between? I think
laptop outputs are balanced, low-impedance, so you shouldn't need the DI in
between. Even if this is not true, it's at least worth a shot.
Brandon Hale
On 6/8/21 7:43 AM, Bill Purvis wrote:
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. We have
another identical DI box at the other end of the room which takes keyboard
and bass and I've had no
problems with that. Is there an alternative method we can try? We do 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. I wondered if the matching transformers in the DI box are
acting as pickups for the
RF noise generated by the laptops. The mixer on EQ display with RTA shows
noise across the audible
spectrum, but most of the audible sound is mains hum.
Bill
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