AES Digital Audio--Converting Balanced to/from Unbalanced
January 10, 2006
According to AES standards, the AES3 professional digital audio format can have two different kinds of electrical-physical connections. One is balanced 110 ohm using shielded twisted pair cable with XLR connectors (AES3) . The other is unbalanced 75 ohm using coaxial cable with BNC connectors (AES3id). Either type of connection carries the same AES3 formatted signal.
In many facilities both types of connections are used. To connect from 110 ohm balanced to 75 unbalanced, the best engineering practice is to use a device with a built-in transformer with a turns ratio of 1.21:1 followed by a resistor network (attenuator or pad). There are many commercial units available, both in-line and panel mounted. When using in-line units, make sure that the BNC connector that you use for your type of coax cable will fit inside the in-line housing on the mating BNC side. In addition to transforming the impedance from 110 ohms to 75 ohms, the transformer offers isolation between the two pieces of gear and also lowers the output voltage a bit. If the nominal AES3 balanced signal is 4 volts, then the output from the transformer is 3.3 volts. However this is still too high for the nominal 1 volt of the AES3id signal. That's where the resistor network (pad) comes in, to attenuate the level further to meet AES3id specs. When connecting from 75 ohm unbalanced to 110 ohm balanced, all you need is a transformer with a 1:1.21 turns ratio, with appropriate connectors. Again, in the commercial world, both in-line and panel mounted units are available. With this type of transformer, the nominal 1 volt AES3id signal will be raised to 1.21 volts. While this value falls short of the nominal level of AES3, it is well above the minimum 0.2 volts needed for an AES3 receiver to properly decode the signal.
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