I use a laptop live
for sequencing and soft synths, but I can hear an annoying whining sound
through the PA, which definitely isn’t our vocalist. The USB interface
I have works fine on my desktop studio computer, so what’s going on here
and how can I fix it?
Matt Calder, via email
If
PA whine caused by a laptop power supply isn’t eliminated by running
the laptop from battery power, connecting a two-channel transformer DI
box between your audio interface’s stereo output and the PA system could
do the trick.
SOS Editor In Chief Paul White replies:
There are
a couple of possible answers to this problem, one of which is that your
computer’s switch-mode power supply is breaking through onto the USB
ground, with the result that a digital background noise is being added
to your audio. This sound is quite different from analogue hum or hiss:
there’s a definite whining quality to the noise, sometimes with
a pulsing modulation added to it. An easy way to check whether the power
supply is causing the problem is to run the computer from battery power
alone, with the PSU unplugged, and see if the problem goes away. If it
does, your best solution is to run the computer from its PSU right up to
the point when the gig starts, then switch to battery power. You can
always plug in again at half time to recharge if you’re worried about
battery life.
If the problem is still there when
you’re running on battery power, a two-channel transformer DI box
between your audio interface’s stereo output and your sound system may
help, as that will provide complete ground isolation. Even so, this
isn’t a guaranteed fix, as some audio interfaces seem to be more prone
to USB whine than others, usually because of the way their internal
grounding is handled, and with some models we’ve found that nothing
really helps! If this turns out to be the case, you’ll need to look for
a new USB interface. When you do, take your computer with you to the
shop and insist on trying it before you buy. Fortunately, there are many
affordable two-channel USB audio interfaces out there that will do the
job perfectly well.
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