Application Note 07

Using the Luxlink^® AT/AR-1001 for Balanced or Unbalanced Audio Applications

In audio applications the installer usually encounters two types of signal wiring schemes. One, using shielded cable is unbalanced where the center conductor of the cable carries the actual audio signal and an outside braided shield acts as the signal return as well as a means to block extraneous signals from interfering with the audio. This scheme is simple but often suffers the problem of hum inducement if the shields are not terminated properly. The second scheme is to use a balanced configuration where the audio is transmitted as a differential signal through two wires. Any extraneous signal coupled to this type of arrangement winds up being present on both conductors and tends to cancel itself out at the terminated end. While this scheme is more interference and hum resistant than the unbalanced mode, it requires that both signal leads float from any shield. If a shield is present, it is not connected to any of the signal leads.

The AT/AR-1001 fiber optic audio transmission system is designed to transmit either balanced or unbalanced line-level audio signals from point-to-point. Since the input and output port of each unit is transformer coupled, the use of balanced or unbalanced audio signals is simple as this application note will show.

Figure 1 shows the unbalanced input configuration. In this example position 5 of the terminal block is used for the audio, position 4 is jumped to position 3 and used for the audio return and/or shield. Note that position 3 is also common to the housing thereby making the entire installation part of the shield.

Figure 2 shows the balanced configuration. Here positions 5 and 4 are used for the audio and position 3 is used for a shield if present. In this case there is no signal connection to the housing and the audio leads truly float.

In both cases the interface is a 600-ohm transformer winding so it is important that only audio without any DC component be used.