The Hum Stopped When You Unplugged the Laptop Charger

#sergueyasaelshinder #music #audio #recording

You set up to record a guitar part.

Amp in the corner,
laptop on the desk,
audio interface between them.

Before you play a note
there is a hum
in the headphones.

Low.
Steady.
With a buzz on top of it.

You swap the cable.

Still there.

You turn the gain down.

It gets quieter
with everything else.

Then the laptop battery warning comes up,
you pull the charger
to move it,
and the hum stops.

Plug it back in.

Hum.

That is a ground loop.

Each piece of mains powered gear
is connected to earth
through its own plug.

The amp through one socket.

The laptop,
through its charger,
through another.

And the cables between them
connect those earths again,
through the screen of the cable.

Now there is a loop.

A small current
flows around it,
and on an unbalanced guitar cable
the screen is part of the signal path.

So the current
becomes part of the sound.

It runs at the mains frequency,
fifty times a second in Britain and Europe,
sixty in North America.

That is the hum.

The buzz on top
is its harmonics,
a hundred,
a hundred and fifty,
and up.

The battery test
is the useful part.

On battery,
the laptop has no earth
except through your cables.

The loop is broken.

So you know where it is.

Now fix it properly.

Put everything on one power strip
from one wall socket,
so there is one path to earth,
not two.

Use balanced connections
where the gear has them.

Put a DI box in the line,
and try its ground lift switch,
which breaks the screen
on the signal side only.

There are isolators
for USB as well.

One thing you never do.

Never remove the earth
from a mains plug.

That earth is what stops
a faulty amp
from making its metal case live,
with your hands on the strings.

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