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3.6 - How do I interconnect balanced and unbalanced components?




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This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.

3.6 - How do I interconnect balanced and unbalanced components?

First, let's define what the terms mean. The simplest audio
circuit uses a single wire to carry the signal; the return path,
which is needed for current to flow in the wire, is provided
through a ground connection, usually through a shield around the
wire. This system, called unbalanced transmission, is very
susceptible to hum pickup and cannot be used for low level
signals, like audio, for more than a few feet. Balanced
transmission occurs when two separate and symmetrical wires are
used to carry the signal. A balanced input is sensitive only to
voltage that appears between the two input terminals; signals
from one terminal to ground are canceled by the circuit.

The simplest way to connect between balanced and unbalanced
equipment is to use a transformer. The signals are magnetically
coupled through the core of the transformer and either side may
be balanced or unbalanced. Good transformers are expensive,
however, and there are cheaper methods that can be used in some
instances.

An unbalanced output can be connected to a balanced input. For
instance, from the unbalanced output of a CD player, connect the
center pin to pin 2 of the balanced XLR input connector, and the
ground to pins 1 and 3. To connect the balanced output of something
to an unbalanced input requires different techniques depending on
whether the output is active balanced (each side has a signal with
respect to ground) or floating balanced (for instance, the secondary
of a transformer with no center-tap connection). If it's an active
balanced output, you can simply use half of it; connect pin 2 to the
unbalanced input, and pin 1 to ground, leaving pin 3 floating. If
this doesn't work (no or very weak signal) connect pin 3 of the
output to pin 1 and ground and leave pin 2 connected to the
unbalanced input center pin. Some active balanced outputs,
particularly microphones, use the balanced circuit to cancel
distortion, so this hookup may result in higher distortion than if a
proper balanced-to-unbalanced converter such as a differential stage
or a transformer were used. [David]


 

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