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Re: R-44 issue-geek reply

Subject: Re: R-44 issue-geek reply
From: "Avocet" madl74
Date: Fri Jun 22, 2012 1:38 pm ((PDT))
> I haven't read all this thread but I thought I'd give a geek answer.

Sources like mics, and the inputs of mixers or recorders can be
unbalanced, balanced or floating.

Unbalanced lines have one leg connected to ground. Any interference
pickup, including from mic power sources will be heard down the
unbalanced signal line.

Balanced lines work in exact push-pull, so interference is generally
cancelled out, including noise from phantom power. One line is
in-phase with positive air pressure referenced to ground and the other
is out of phase. Classically, balanced inputs and outputs go through
transformers with a centre tap ground connection, but balanced outputs
and inputs can also be transformerless.

Floating sources and floating inputs are also balanced lines but
without a ground reference. If both the source (mic) and the cables
remain floating, external interference should cancel out. With care a
floating mic can be used successfully into an unbalanced input
provided the signal return is independantly connected to the input
socket ground and not through the cable ground. Golden rule - only
ground once.

In other words a 3-connection cable is used. Most XLR mics are
floating. The mic return circuit must not be grounded at the mic end,
but the mic body should be grounded through the cable screen.

The configuration usually found in "affordable" recorders with
XLR inputs is often neither balanced nor floating, but independantly
grounded unbalanced line. They work when the mic is fully floating,
but can give problems with 2-wire unbalanced devices. The solution is
a transformer (see below)

What happens when you connect different types of device together?

Many mics are in effect floating, but powered mics can often have a
balanced output. Classical mics with transformers, like ribbon mics,
are often balanced with a centre tap connected to the cable ground.

XLR cables should be floating and can be used on any type of device.

With a balanced input, which is grounded at the mid point, an
unbalanced mic or device is connected to the in-phase input and
returned to ground and not to the out of phase input. The out of phase
input is not connected.

With a fully floating input an unbalanced mic goes to the in-phase
input and the out of phase input is grounded. The independantly
grounded screen is not connected to the signal path anywhere.

The best solution for long cables with an unbalanced two wire mic is
to use a transformer at the mic end giving a floating output which
will go anywhere. My cheap Brinibox mics are wired through 100metre
cables this way without any trace of hum.

Hope this makes sense.


David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce

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