Balanced-Unbalanced Converter For Audio Work


If you work in the
professional audio field, you need to use balanced lines for long signal
runs to prevent hum and noise pick-up. This Balanced/Unbalanced
Converter is really two projects in one. It can convert an unbalanced
input to balanced outputs and vice versa.

Specifications:

  • Signal to noise ratio: -100dB with respect to 1V output, 4.7kW input load.
  • Frequency response: -3dB at 2Hz and 200kHz.
  • Total harmonic distortion: less than .001% from 20Hz to 20kHz with a 1V input.
  • Signal handling: supply dependent; requires 30VDC or ±15V for 9V RMS signal handling.

Professional audio gear invariably has balanced inputs and outputs.
However, what if you want to connect standard audio equipment that has
unbalanced outputs to equipment that has balanced inputs? Alternatively,
what if you want to connect a balanced output signal to an unbalanced
input? Either way this Balanced/Unbalanced Converter project can do the
job.

The reason professional audio equipment utilises balanced inputs and
outputs is quite simple. It’s done so that audio connections can be
made over quite long distances without adding extra noise to the signal.
These balanced connections use 3-pin XLR plugs and sockets and screened twin-core cable.

Block Diagram

Block diagram shows the basic arrangement. Basically, the audio
output signal is coupled to two separate amplifiers and these drive the
two signal leads in the cable in anti-phase (ie, the signals have
opposite phases). In this case, Amplifier 1 has an output signal that’s
in phase with the input, while Amplifier 2 has an output that’s opposite
in phase with the input.

Circuit looks like:

The output impedance of each amplifier is the same and the twin-core
cable carries the signal to the equipment at the other end. However, in
some cheaper balanced line drivers, one core does not carry any signal
but is grounded instead. So in this case, Amplifier 2 is left out and
the left hand side of resistor R2 is grounded.

In operation, there will be some noise and hum pickup over the
length of the cable even though the cable is shielded. However, because
the cores in the cable are close together, any signal that is picked up
will be common to both.

Parts Layout

At the receiving end, the signal in each of the two cores is
subtracted to produce the original audio signal. At the same time, this
also removes most of the noise and hum that was picked up in the leads,
since the same noise signal is present in both.

Circuit diagram:

Balanced/Unbalanced Converter

Balanced/Unbalanced Converter

If one of the cores is grounded, as in the cheaper type of balanced
driver, then the signal level after subtraction will be the same as the
signal in the main core. Alternatively, if anti-phase signals are
applied to both cores, the subtraction process produces an audio signal
level that’s twice the level in the individual cores.

Source: Silicon Chip 09 June 2008


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